References for “The Asian Monsoon” book – Chapter 5
Adams, D. K. and A. C. Comrie (1997). "The North American monsoon." Bulletin of the American Meteorological Society, Boston, MA 78(10): 2197-2213.
The North American monsoon is an important feature of the atmospheric circulation over the continent, with a research literature that dates back almost 100 years. The authors review the wide range of past and current research dealing with the meteorological and climatological aspects of the North American monsoon, highlighting historical development and major research themes. The domain of the North American monsoon is large, extending over much of the western United States from its region of greatest influence in northwestern Mexico. Regarding the debate over moisture source regions and water vapor advection into southwestern North America, there is general agreement that the bulk of monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California, while the Gulf of Mexico may contribute some upper-level moisture (although mixing occurs over the Sierra Madre Occidental). Surges of low-level moisture from the Gulf of California are a significant part of intraseasonal monsoon variability, and they are associated with the configuration of upper-level midlatitude troughs and tropical easterly waves at the synoptic scale, as well as the presence of low-level jets, a thermal low, and associated dynamics (including the important effects of local topography) at the mesoscale. Seasonally, the gulf surges and the latitudinal position of the midtropospheric subtropical ridge over southwestern North America appear to be responsible for much spatial and temporal variability in precipitation. Interannual variability of the North American monsoon system is high, but it is not strongly linked to El Nino or other common sources of interannual circulation variability. Recent mesoscale field measurements gathered during the South-West Area Monsoon Project have highlighted the complex nature of the monsoon-related severe storm environment and associated difficulties in modeling and forecasting.
Ahlquist, J., V. Mehta, et al. (1990). "Intraseasonal monsoon fluctuations seen through 25 years of Indian radiosonde observations." Mausam, New Delhi, India 41(2): 273-278.
Radiosonde records for 12 major Indian cities have been checked and analyzed for intraseasonal activity in the monsoon. After forming time series with two observations per day, removing bad values, and filling data gaps with linear interpolation, the time series have been plotted with and without filtering and have been subjected to spectral analysis to reveal the nature of intraseasonal fluctuations and their interannual variability. Spectra were estimated from the time series using the maximum entropy method (MEM), which fits an autoregressive (AR) model to the time series. MEM spectra based on tenth order AR models show that most of the variance in monsoon weather comes from intraseasonal activity with periods longer than 10 days, but do not show separate peaks at 10-20 and/or 30-50 day time scales for the majority of summers.
Alekseeva, L. A., M. A. Petrosyants, et al. (1989). "Vertical structure of the atmosphere during different phases of the Indian summer monsoon." Meteorologiya i Gidrologiya, Moscow, Russia 5: 54-59.
On the basis of rawinsonde data from 28 Soviet scientific expeditions in the Indian Ocean, an analysis is made of the features of atmospheric circulation during different phases of the Indian summer monsoon. It is shown that there are substantial differences in the structure of air flows in the free atmosphere over the Indian Ocean during the break and normal monsoon periods. First of all it concerns a displacement of the quasi-stationary anticyclone over Tibet during the breaks which is accompanied by a pronounced weakening of the upper-tropospheric eastern jet stream. At the same time the intensity of the lower-tropospheric Findlater stream does not change so significantly during different monsoon phases, and the southern branch of ITCZ does not affect substantially the intraseasonal variability of monsoon precipitation.
Alekseyeva, L. I. (1989). "Cloudiness of the southern Intertropical Convergence Zone and activity of the Indian summer monsoon." Meteorologiya i Gidrologiya, Moscow, Russia 1989(3): 44-50.
On the basis of the set of daily satellite cloud images for June-Sept. 1967-1983, an analysis is made of the behavior of the southern Intertropical Convergence Zone (ITCZ) over the Indian Ocean during the following phases of the southwest monsoon: active, normal and break. It is shown that the cloudiness of the souhern ITCZ is not the principal indicator of activity of the Indian summer monsoon. It, therefore, cannot be used as a predictor in forecasting of monsoon precipitation. The influence of the southern hemisphere circulation regime on the variability of monsoon precipitation is considered. It is demonstrated that the basically pulsing character of precipitation of the steady monsoon is caused by the Northern Hemisphere circulation processes.
Alekseyeva, L. I., Y. Semenov, et al. (1989). "Vertical structure of the atmosphere during different phases of the Indian summer monsoon." Meteorologiya i Gidrologiya, Moscow, May(5): 54-59.
On the basis of rawinsonde data from 28 Soviet scientific expeditions in the Indian Ocean, an analysis is made of the features of atmospheric circulation during different phases of the Indian summer monsoon. It is shown that there are substantial differences in the structure of airflows in the free atmosphere over the Indian Ocean during the break and normal monsoon periods. There is a displacement of the quasi-stationary anticyclone over Tibet during the breaks, which is accompanied by a pronounced weakening of the upper tropospheric eastern jet stream. At the same time, the intensity of the lower tropospheric Findlater stream does not change so significantly during different monsoon phases, and the southern branch of ITCZ does not affect substantially the intraseasonal variability of monsoon precipitation.
Alekseyeva, L. I., Y. Semenov, et al. (1989). "Typical airflow patterns for different phases of the Indian summer monsoon." Meteorologiya i Gidrologiya, Moscow, Feb(2): 31-36.
On the basis of the ECMWF objective analyses of wind and daily precipitation amounts of 29 stations in central India, the phases of monsoon activity are detected, and composite charts are constructed which depict the averaged atmospheric circulation during the break, active, and normal monsoon periods. The structure and interaction of the basic circulation components of the southwestern monsoon are considered. It is shown that the Northern Hemisphere processes play the key role in the origin of the summer monsoon breaks over central India.
Alexander, G. (1978). "Fluctuations of monsoon activity." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 76-87.
To study the evolution of flow patterns associated with strong and break monsoons, the authors studied six cases each of such epochs during the period 1965-1973. Because of an interest in the departure of the flow patterns from the normal, the authors subtracted long-term climatological pentad means from the daily data and constructed anomaly wind, height, and thickness charts at standard levels. The charts show several contrasting features during and prior to the epochs. A few days before the onset of the strong monsoon epoch (usually in association with a depression), an E-W anomaly trough extends from southeast Asia to peninsular India in the lower and middle tropospheres. The trough over peninsular India shifts northward with the advance of the epoch. Westward movement of a cyclonic anomaly from across southeast Asia is also seen. In the upper troposphere, a warm anomaly ridge is seen to the northwest of India. Prior to the beginning of breaks, however, an anomaly ridge extends from peninsular India to Malaysia in the lower and middle tropospheres. This feature shows some northward shift with the advance of the epoch. In the upper troposphere, an anomaly trough is seen to the northwest of India.
Annamalai, H. and J. M. Slingo (2001). "Active/break cycles: diagnosis of the intraseasonal variability of the Asian Summer Monsoon." Climate Dynamics 18(1/2): 85-102.
In this study, various diagnostics have been applied to daily observed outgoing longwave radiation (OLR) and ECMWF ReAnalysis (ERA) products to provide a comprehensive description of the active/break cycles associated with the Asian Summer Monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability, 10-20 days and 30-60 days. Composite analysis of OLR based on filtered daily All-India rainfall (AIR) for the 40 day (30-60 days) intraseasonal mode indicates that during active phases, convection is significantly enhanced over the Indian continent, extending over the Bay of Bengal, Maritime continent and equatorial west Pacific, while convection is suppressed over the equatorial Indian Ocean and northwest tropical Pacific, resulting in a quadrapole' structure over the Asian monsoon domain. In response to this heating pattern, the large-scale Hadley (lateral) and the two east-west (transverse) tropical circulations are enhanced. There is also a significant impact on the extra-tropical circulation through excitation and propagation of Rossby waves. In contrast, the 15-day mode is more regional to the monsoon domain and has a prominent east-west orientation in convection. Only the local Hadley circulation over the monsoon region is modulated by this mode. The evolution of these two modes as revealed by POP (principal oscillation pattern) analysis, shows that the 40-day mode originates over the equatorial Indian Ocean. Once formed it has poleward propagation on either side of the equator, and eastward propagation into the equatorial west Pacific. From the equatorial west Pacific, northward propagation over the west Pacific and westward propagation into the Indian longitudes are prominent. The propagative features are complex and interactive and are responsible for the quadrapole' structure in convection seen from the composites. The interannual variability, assessed from the POP coefficient time series, indicates that the 40-day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic TCZ (tropical convergence zone). The POP analysis of the 15-day mode indicates that this event originates over the equatorial west Pacific, associated with westward propagating Rossby waves, amplifies over the northwest tropical Pacific and modulates both the continental and oceanic TCZs over Indian longitudes simultaneously. This mode is pronounced during the established phase of the monsoon. Due to the complexity in the propagational features of both the intraseasonal modes, it is concluded that understanding the subseasonal variability of one regional component of the Asian Summer Monsoon (ASM), requires understanding the entire ASM system.
Annamalai, H., J. M. Slingo, et al. (1999). "The mean evolution and variability of the Asian summer monsoon: comparison of ECMWF and NCEP-NCAR reanalyses." Monthly Weather Review, Boston, MA 127(6, Pt. 2): 1157-1186.
The behavior of the Asian summer monsoon is documented and compared using the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA) and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis. In terms of seasonal mean climatologies the results suggest that, in several respects, the ERA is superior to the NCEP-NCAR Reanalysis. The overall better simulation of the precipitation and hence the diabatic heating field over the monsoon domain in ERA means that the analyzed circulation is probably nearer reality. In terms of interannual variability, inconsistencies in the definition of weak and strong monsoon years based on typical monsoon indices such as All-India Rainfall (AIR) anomalies and the large-scale wind shear based dynamical monsoon index (DMI) still exist. Two dominant modes of interannual variability have been identified that together explain nearly 50% of the variance. Individually, they have many features in common with the composite flow patterns associated with weak and strong monsoons, when defined in terms of regional AIR anomalies and the large-scale DMI. The reanalyses also show a common dominant mode of intraseasonal variability that describes the latitudinal displacement of the tropical convergence zone from its oceanic-to-continental regime and essentially captures the low-frequency active/break cycles of the monsoon. The relationship between interannual and intraseasonal variability has been investigated by considering the probability density function (PDF) of the principal component of the dominant intraseasonal mode. Based on the DMI, there is an indication that in years with a weaker monsoon circulation, the PDF is skewed toward negative values (i.e., break conditions). Similarly, the PDFs for El Nino and La Nina years suggest that El Nino predisposes the system to more break spells, although the sample size may limit the statistical significance of the results.
Bagla, P. (2002). "Climate Forecasting: Drought Exposes Cracks in India's Monsoon Model." Science (Washington) 297(5585): 1265-1267.
The annual summer monsoon rains are vital to India's economy. But a drought this summer suggests that a homegrown prediction model might be all wet. India's first serious drought in 15 years is doing more than parching the soil and threatening the country's food supply. It has also stirred up a debate over the robustness of a homegrown climate forecasting model that badly missed predicting this summer's sharp decline in life-sustaining rains over much of the country. The summertime monsoon across India is among the toughest climatic phenomena to understand and predict because of the complex atmospheric conditions in the tropics.
Bannister, A. J. and K. J. Smith (1993). "The South Pacific and southeast Indian Ocean tropical cyclone season 1990-91." Australian Meteorological Magazine, Canberra, Australia 42(4): 175-182.
During the 1990-91 season a total of eleven tropical cyclones affected the South Pacific and southeast Indian Ocean basins, of which five reached hurricane intensity (ten-minute mean wind speed in excess of 120 km h super(-) super(1) ). There was a total of 64 cyclones days spanning the period late November to mid-May, including 16 hurricane days. Sina and Joy caused damage costing US$55 million, accounting for over 95 per cent of the total cyclone damage in the basin. Eight lives were lost; six of these were associated with Joy and two with Fifi. Key features of the broadscale circulation included: a more active than normal westerly monsoon in the Australian region; a weaker than normal South Pacific convergence zone (SPCZ) and an anomalously weak long wave trough over eastern Asia (with fewer cross-equatorial surges over western Indonesia and the Indian Ocean). The Madden-Julian oscillations (MJO) were weaker and less regularly defined than in the preceding two seasons. The Southern Oscillation Index (SOI) was generally negative but small. Sea-surface temperatures (SST) remained above average in the equatorial central Pacific and warm SST anomalies, although generally small, persisted in the tropical Indian Ocean.
Bao, Z., H.-R. Cho, et al. (2002). "Multi temporal scale variations of summer precipitation in wet seasons over China and their association with 500 mb geopotential height." Climate Research 20(2): 107-122.
Multi scale characteristics of summer precipitation variation were studied using rotated principle component analysis and wavelet analysis on 76 stations for the period 1968-1997. We found 2 significant precipitation periods in northern and southern China corresponding to the movement of the East Asian summer monsoon. The 2 precipitation periods are characterized by a continuous 3 d mean precipitation higher than 10 mm in northern China during early July to late August and higher than 20 mm in southern China during early May to late June. The precipitation amounts in the first half of August over northern China and second half of June over southern China are the major contributors to the total precipitation during the 2 rainy periods, respectively. In addition, a weekly time scale precipitation variation of about 7.2 d during the first half of August over northern China and an intraseasonal time scale of 25 d during the second half of June over southern China are evident during wet seasons. Case studies for 2 extreme wet seasons over northern China in 1994 and 1996 show that weekly time scale precipitation is associated with a 500 mb anomaly over the Sea of Japan during July to August. The 2 wet seasons over southern China in 1968 and 1994 show that intraseasonal time scale precipitation is closely associated with 500 mb geopotential height over the tropical western Pacific during May to June.
Basu, B. K. (2001). "Simulation of the Summer Monsoon over India in the Ensemble of Seasonal Simulations from the ECMWF Reanalyzed Data." Journal of Climate 14(7): 1440-1449.
The ensemble of seasonal (120 days) simulations of the Northern Hemispheric summer for the reanalysis period of the European Centre for Medium-Range Weather Forecasts has been examined to assess the extent to which the characteristic features of the Indian summer monsoon can be reproduced in these simulations. The present simulations could reproduce a better distribution of the seasonal average precipitation over India in comparison with the earlier Atmospheric Model Intercomparison Project simulations. The Interannual variation in the seasonal total of the spatially averaged precipitation over India has predictability. The 10-day-average precipitation values did not show any impact of the El Nino or La Nina events or any periodicity in the amount of precipitation. The intraseasonal variability did not produce any distinct pattern for 10-day-average rainfall during the excess or deficient years. The simulated patterns over India correspond to the weak phase of summer monsoon with excess precipitation over the northern part of the country and adjoining China. The cloud cover is less over the central parts of India and near-ground maximum temperature is higher. A simulated motion field reproduces the typical features of the Indian summer monsoon including the low-frequency seasonal migration of the Tibetan anticyclone at 200 hPa.
Basu, B. K. (2001). "Simulation of the summer monsoon over India in the ensemble of seasonal simulations from the ECMWF reanalyzed data." Journal of Climate, Boston, MA 14(7): 1440-1449.
The ensemble of seasonal (120 days) simulations of the Northern Hemispheric summer for the reanalysis period of the European Centre for Medium-Range Weather Forecasts has been examined to assess the extent to which the characteristic features of the Indian summer monsoon can be reproduced in these simulations. The present simulations could reproduce a better distribution of the seasonal average precipitation over India in comparison with the earlier Atmospheric Model Intercomparison Project simulations. The interannual variation in the seasonal total of the spatially averaged precipitation over India has predictability. The 10-day-average precipitation values did not show any impact of the El Nino or La Nina events or any periodicity in the amount of precipitation. The intraseasonal variability did not produce any distinct pattern for 10-day-average rainfall during the excess or deficient years. The simulated patterns over India correspond to the weak phase of summer monsoon with excess precipitation over the northeastern part of the country and adjoining China. The cloud cover is less over the central parts of India and near-ground maximum temperature is higher. A simulated motion field reproduces the typical features of the Indian summer monsoon including the low-frequency seasonal migration of the Tibetan anticyclone at 200 hPa.
Bate, P. W. (1997). "The tropical circulation in the Australian/Asian regionANovember 1996 to April 1997." Australian Meteorological Magazine, Canberra, Australia 46(3): 237-246.
A summary of the broadscale tropical circulation from 70 degrees E to 180 degrees , for the six months November 1996 to April 1997, is presented. The weak La Nina phase, which characterised 1996, declined and some early El Nino indicators emerged by the end of the season. These included the appearance of strong westerly equatorial wind anomalies near the date-line, increasing sea-surface temperature in the tropical Pacific and a fall of the Southern Oscillation Index to negative values. The monsoon in the summer hemisphere was of average development or better, with strongest anomalies in the southwest Pacific. The near-equatorial trough was active in the northwest Pacific near the date-line. Twenty-nine tropical cyclones were analysed, twenty in the Southern Hemisphere and nine in the north. Overall, tropical cyclone occurrences were slightly above long-term averages. Three full cycles of the 30 to 60-day intraseasonal oscillation were diagnosed.
Bhaskaran, B., R. G. Jones, et al. (1996). "Simulations of the Indian summer monsoon using a nested regional climate model: domain size experiments." Climate Dynamics, Berlin, Germany 12(9): 573-587.
Seasonal simulations of the Indian summer monsoon using a 50-km regional climate model (RCM) are described. Results from three versions of the RCM distinguished by different domain sizes are compared against those of the driving global general circulation model (AGCM). Precipitation over land is 20% larger in the RCMs due to stronger vertical motions arising from finer horizontal resolution. The resulting increase in condensational heating helps to intensify the monsoon trough relative to the AGCM. The RCM precipitation distributions show a strong orographically forced mesoscale component (similar in each version). This component is not present in the AGCM. The RCMs produce two qualitatively realistic intraseasonal oscillations (ISOs) associated respectively with monsoon depressions which propagate northwestward from the Bay of Bengal and repeated northward migrations of the regional tropical convergence zone. The RCM simulations are relatively insensitive to domain size in several respects: (1) the mean bias relative to the AGCM is similar for all three domains; (2) the variability simulated by the RCM is strongly correlated with that of the driving AGCM on both daily and seasonal time scales, even for the largest domain; (3) the mesoscale features and ISOs are not damped by the relative proximity of the lateral boundaries in the version with the smallest domain. Results (1) and (2) contrast strongly with a previous study for Europe carried out with the same model, probably due to inherent differences between mid-latitude and tropical dynamics.
Bhaskaran, B., J. F. B. Mitchell, et al. (1995). "Climatic response to the Indian subcontinent to doubled CO sub(2) concentrations." International Journal of Climatology, Chichester, UK 15(8): 873-892.
Results from the United Kingdom Meteorological Office (UKMO) coupled climate model have been analysed over the Indian subcontinent in order to validate the model's performance and to assess the changes in climate and its variability in a simulation with a 1 percent increase per year in CO sub(2) (compound). The model produces a reasonable simulation of present-day climate over the Indian subcontinent. At the time of CO sub(2) doubling, the model simulates temperature increases of the order of 1 K to 4 K over the Indian subcontinent during winter and monsoon seasons. The model-simulated monsoon circulation shifts by 10 degrees latitude towards the north and intensifies by approximately 10 percent in the warmer atmosphere. The interannual variability of monsoon onset dates and intraseasonal variability of monsoon rainfall are not significantly different when the CO sub(2) concentration doubles. However, the model simulates increased interannual variability of the monsoon rainfall and a greater number of heavy rainfall days during the monsoon.
Bhaskaran, B., J. M. Murphy, et al. (1998). "Intraseasonal oscillation in the Indian summer monsoon simulated by global and nested regional climate models." Monthly Weather Review, Boston, MA 126(12): 3124-3134.
Simulations of the intraseasonal oscillation (ISO) in the Indian summer monsoon by a general circulation model (GCM) and a nested regional climate model (RCM) are described. The ISO is the leading mode of subseasonal variability in both models. It is shown to be associated with circulation and precipitation anomalies that propagate northward from the equatorial Indian Ocean to the foothills of the Himalayas on the 30-50-day timescale. The spatial structure, timescale, and propagation characteristics of the simulated ISO are found to be similar to those of the leading observed intraseasonal mode. In particular, both of the simulated versions and the observed version all involve periodic deepening and filling of the monsoon trough resulting from northward propagation of troughs and ridges from the equatorial region. Some differences do occur, however: the GCM version of the ISO is too zonally symmetric and the ISO is too strong in both models. During the positive phase of the ISO (i.e., when the ISO acts to enhance the monsoon trough), composite low-level circulation anomalies in the monsoon trough region are found to be somewhat weaker in the RCM than in the GCM because the RCM signal is obscured to a greater degree by noise associated with other modes of variability. In the GCM, large precipitation anomalies are found to be associated with the positive and negative phases of the ISO in many areas, particularly at the latitudes of the monsoon trough. However, the use of a fine-resolution nested RCM leads to the identification of important spatial detail not present in the GCM distributions. This is particularly true in mountainous regions, most notably in the foothills of the Himalayas: here the RCM simulates a strong precipitation signal, which appears to represent an orographic component of the response to circulation anomalies associated with the ISO, whereas this precipitation signal is absent in the GCM. The use of a nested RCM also allows the phase relationship between the oscillations in the two models to be studied. The relationship is found to be close in most years, suggesting that the regional ISO in the RCM is modulated by the driving GCM circulation via the lateral boundary forcing on the 30-50-day timescale. Several examples are also found, however, where the GCM and RCM diverge, showing that the northward-propagating mode can occur independently of any global forcing on the same timescale, in agreement with observational evidence.
Bhide, U. V., V. R. Mujamdar, et al. (1997). "A diagnostic study on heat sources and moisture sinks in the monsoon trough area during active-break phases of the Indian summer monsoon, 1979." Tellus Series A, Stockholm, Sweden 49A(4): 455-473.
The diabatic heating over the Indian monsoon trough area, along with its thermal structure are studied for the 1979 summer monsoon based on FGGE level-IIIb upper air data of the European Centre for Medium Range Weather Forecasts. The apparent heat source and the apparent moisture sink over the trough area varied coherently with the rainfall over central India. The spatial and temporal variations of the vertically integrated apparent heat source and moisture sink were found to be coincident. These coincidences suggested that diabatic heating was largely contributed by the latent heat released by cumulus convection. During the active periods, the vertical structure of spatially-averaged heating and drying rates above the monsoon trough area showed higher values, as much as 8 K to 11 K day super(-) super(1) , at the mid-tropospheric level (500 hPa), but much smaller and even negative values during break periods. Analyses of the heating and drying rates at 500 hPa level in a x-t diagram revealed that heat sources and moisture sinks propagated westward across the trough area with a period of 10-15 days (often called monsoon mode). The 30-50 day period of fluctuations showed a close link with the two major active/break phases of monsoon during the season. The monsoon mode became a part of the mid-season fluctuation of monsoon activity between the two major active phases. This paper discusses the spatial distribution of rainfall and heat source and moisture sink over the trough area, and the role of east-west differential heating in the development of weak /break phases of Indian summer monsoon 1979.
Bin, W. and Y. Ding (1992). "An overview of the Madden-Julian Oscillation and its relation to monsoon and mid-latitude circulation." Advances in Atmospheric Sciences, Beijing, China 9(1): 93-111.
In the past decade there has been extensive research into tropical intraseasonal variability, one of the major components of the low frequency variability of the general atmospheric circulation. This paper briefly reviews the state-of-the-art in this research area: the nature of the Madden-Julian Oscillation, its relation to monsoonal and extratropical circulations, and the current theoretical understandings.
Bruce, J. G., J. C. Kindle, et al. (1998). "Recent observations and modeling in the Arabian Sea Laccadive High region." Journal of Geophysical Research, Washington, DC 103(C4): 7593-7600.
Recent observations that include an airborne expendable bathythermograph (AXBT) survey and coincident satellite altimeter measurements together with a high-resolution numerical simulation indicate that the Laccadive High, an anticyclonic circulating feature that forms off the SW coast of India during the NE monsoon, is comprised of multiple eddies. It is hypothesized that in addition to local and remote seasonal forcing, the Laccadive High region is influenced by an intraseasonal signal that originates in the Bay of Bengal.
Cadet, D. (1978). "Importance of moving tropical disturbances over the Indian Ocean during the summer monsoon." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 138-145.
Results are given which help support the idea that the tropical disturbances propagating over the Indian Ocean during the northern summer are an important phenomenon of the broad-scale Indian monsoon. Time-longitude sections of cloudiness over the Indian Ocean during 11/2 mo of the 1975 summer monsoon stress the existence of moving disturbances: eastward-moving ones in the northern Indian Ocean and westward-moving ones in the southern Indian Ocean. These perturbations and their zonal propagation are also evident at ground level. From a comparative analysis with balloon trajectories, it is shown that they can have an influence on the low-level air flow circulation over the Indian Ocean. The intensity of the Mascarene High Pressure, located near 30 degrees S, 50 degrees E, is studied in relation to the occurrence of the westward-moving disturbances. It is shown that these disturbances modulate the anticyclone intensity of the broad-scale monsoon system with a quasi-biweekly period. The relationship found by Findlater (1969) between the low-level, cross-equatorial winds over Kenya and rainfall over the western coast of India is investigated with focus on the tropical disturbances propagating eastward over the northern Indian Ocean. An increase of the intensity of the Somalia jet occurs each time a perturbation originates over the western Arabian Sea and begins moving eastward. The rainfall mechanism over the southern part of the western coast seems to be linked to these perturbations, and rainfall maxima occur a few days before the pressure minima. The relationship, with a 3-4 day lag between wind maxima over Kenya and rainfall maxima over the western coast, can be well explained with the eastward-moving tropical disturbances.
Cadet, D. and H. Ovarlez (1976). "Low-level airflow circulation over the Arabian Sea during the summer monsoon as deduced from satellite-tracked superpressure balloons, Pt. 1, Balloon trajectories." Royal Meteorological Society, Bracknell, Eng., Quarterly Journal 102(434): 805-816.
During the northern summer of 1975, 45 superpressure constant level balloons were released from the Seychelles Islands (4.7 degrees S, 55.5 degrees E) for flight at 955mb (500m), 910mb (100m), and 860mb (1500m) within the tropical boundary layer. The balloons were located by the Random Access Measurement System (RAMS) of the Nimbus-6 satellite. This spacecraft also operated as a data relay system and collected pressure measurements performed by transducers carried by the balloons. The mean lifetime of the balloons was about 4-5 days; some of them lasted more than 8 days. Low-level Lagrangian air trajectories in the southwesterly flow over the Arabian Sea are presented. They show the drastic changes in the airflow direction over the southwestern Arabian Sea associated with high or low activity periods of the monsoon over the western coast of India. During the first period of the experiment, the airflow over the southwestern Arabian Sea joins the low level jet flowing to the east of the Somali coast. This change, caused by intense convective activity near Somalia, can be associated with a break in the monsoon activity. During the second period, the air joins the Bay of Bengal branch of the monsoon over the southwest coast of India. Before the end of July, a complete reversal of the winds over the Seychelles Islands is observed, and some balloons drift toward Madagascar. During the third period, the airflow over the Arabian Sea resumes a southwesterly direction, and at the beginning of August it seems to join again the Bay of Bengal branch of the monsoon. Thus, this experiment shows that low-level ballooning can be used to map air currents over vast unsurveyed areas of the oceans.
Cadet, D. L. (1983). "Mean fields of precipitable water over the Indian Ocean during the 1979 summer monsoon from TIROS-N soundings and FGGE data." Tellus, Series B, Chemical and Physical Meteorology, Stockholm 35(5): 329-345.
An assimilation of precipitable water in three layers (surface-700 mb, 700-500 mb, and 500-300 mb), as inferred by TIROS-N precipitable water data and Level 3-b humidity fields from ECMWF (European Center for Medium Range Weather Forecast) during summer 1979, is presented. The domain of the study is the summer MONEX area (30-120 degrees E, 30 degrees N-30 degrees S), and the period covers May, June, July, Aug. and the first half of Sept. 1979. An objective analysis of the satellite data was made every 12 hr at 0000 and 1200 hr GMT. To overcome the lack of satellite data over cloudy areas, some subjective data were inserted, taking mean TIROS-N temperature profiles in disturbed areas and assuming a certain profile of relative humidity. The fields obtained from the assimilation of TIROS-N data were merged with Level 3-b fields. During the merging process, higher weight was given over land to Level 3-b fields than to TIROS-N fields, whereas the opposite prevailed over the ocean. The accuracy of the final fields has been estimated by using the dropsonde soundings performed from research aircraft during MONEX-79. The relative accuracy is similar to 13%. The fields reveal the variations of precipitable water associated with the establishment of the southwest monsoon. Preonset conditions were characterized by dry air over the Arabian Sea and humid air over the Equator. Two weeks prior to the onset, maxima of PW began to move from equatorial regions northward to the Arabian Sea. Postonset fields show the existence of dry air along the African coast and western Arabian Sea due to the inversion layer, and humid air over the eastern Arabian Sea, where the inversion layer weakens and convection develops. The major break, which occurred around mid-July, affected the fields of precipitable water. The first week of the break in the activity of the monsoon was characterized by a drop of similar to 15% of the amount of precipitable water over the Arabian Sea, compared to an active period. The fields of vertical wind reveal that convection was inhibited over the eastern Arabian Sea during the break. Two-latitude cross sections of weekly mean fields were constructed at different longitudes. They show the time evolution of the fields associated with the onset of the southwest monsoon and with its active/break cycle.
Cadet, D. L. (1986). "Fluctuations of precipitable water over the Indian Ocean during the 1979 summer monsoon." Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm 38(2): 170-177.
Fluctuations of precipitable water (PW) over the Indian Ocean during the 1979 summer season are studied. Time series of space average PW over the Arabian Sea show the existence of a well-marked oscillation of humidity related to the active-break cycle of the monsoon. This cycle around 30-50 days was the major feature of the 1979 monsoon. It also appeared on the pressure and zonal wind fields. This oscillation resulted from perturbations originating over the equatorial eastern Indian Ocean and propagating northward toward India. The arrival over India of a trough ridge characterized by humid-dry conditions gave rise to an active-break monsoon. It is also suggested that this type of oscillation might be related to the date of the onset of the monsoon. Another periodicity around 10-12 days is also found in the PW and meridional wind fields. This fluctuation appears to be associated with a westward propagating mode north of the Equator.
Cadet, D. L. (1990). "Large-scale dynamics of the Indian summer monsoon." Mausam, New Delhi, India 41(2): 155-160.
A global description of the monsoon phenomenon as deduced from the analysis of the comprehensive FGGE/MONEX data sets is presented. The Indian summer monsoon is one of the two components along with the east Asia monsoon of the Asian summer monsoon which influences the large-scale dynamics of the entire tropical belt. The monsoon circulation can be described within the theoretical framework of equatorially trapped waves in response to the localized release of heat. The 30-50 day oscillation is presented because the demonstration of its influence on the activity of the summer monsoon at sub-seasonal scales is one of the major salient results obtained during MONEX.
Cadet, D. L. and P. Daniel (1988). "Long-range forecast of the break and active summer monsoons." Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm 40(2): 133-150.
Ship observations along the major shipping route from Sri Lanka to Sumatra have been processed over a 23-yr period from Jan. 1954 to Dec. 1976. Daily time series of the zonal and meridional components of the wind, pressure, cloudiness, air temperature, and SST have been constructed over the southern Bay of Bengal. The zonal wind component, the pressure, and the cloudiness exhibit a well-defined 30-50-day mode. The fluctuations of these parameters are physically consistent. The period of the low-frequency mode varies over the 23-yr period, but does not show a seasonal or annual cycle. The low-frequency variations of the surface parameters can be associated with the break and active cycle of the Indian summer monsoon. The low-frequency relationship established between the surface meteorological parameters over the southern Bay of Bengal and the activity of the summer monsoon (rainfall at stations in India) is used to forecast the occurrence of the active and inactive periods. The forecast is based upon the fitting of an autoregressive moving average process over a 4-mo period of the time series to extrapolate it over 30, 40, or 50 days. During years when the 30-50-day mode is well defined, high-quality forecasts can be obtained. The forecasts fail during years when the phase of the low-frequency mode varies rapidly during spring and summer. On the average, the dates of the peak of the active/inactive monsoons can be predicted with an accuracy of approximately a week. This study offers new perspectives for the long-range forecast of fluctuations of the activity of the summer monsoon.
Cadet, D. L. and S. Greco (1987). "Water vapor transport over the Indian Ocean during the 1979 summer monsoon, Pt. 2, Water vapor budgets." Monthly Weather Review, Boston 115(10): 2358-2366.
In this second part of the paper, moisture budgets over the Arabian Sea and the Bay of Bengal are investigated for the 1979 summer monsoon season. Over the Arabian Sea, the different terms of the moisture balance equation, except evaporation, strongly fluctuate depending upon the activity of the monsoon. The relative contribution to the monsoon moisture supply by water vapor transport across the Equator and Arabian Sea evaporation varies as the monsoon intensity changes from active through break and back to revival stages. However, it is shown that water vapor from the Southern Hemisphere is the major source of moisture for Indian rainfall. Total evaporation during the active period after the onset of the monsoon is found to be 30-40% of the total eastward flux across the west coast of India. This ratio increases to 40-45% during break conditions, but falls below 20% during a revival phase. These moisture budgets also show that convergence of water vapor flux is limited to the eastern part of the Arabian Sea whereas evaporation exceeds precipitation in the western Arabian Sea. Moisture budgets over the Bay of Bengal depend strongly upon the monsoon intensity and the amount of moisture advected across the western coast of India and into the Bay of Bengal by the monsoon circulation. Moisture supply from the Southern Hemisphere via cross-equatorial flux at the longitude of the Bay of Bengal is very weak. In comparison with weak monsoon periods, a much larger percentage of the water vapor supplied by evaporation and boundary fluxes is transported toward Burma and Malaysia during active monsoon periods, fueling the heavy rainfall there.
Cadet, D. L. and S. Greco (1987). "Water vapor transport over the Indian Ocean during the 1979 summer monsoon, Pt. 1, Water vapor fluxes." Monthly Weather Review, Boston 115(3): 653-663.
Water vapor transport over the Indian Ocean during the 1979 summer monsoon season is studied. The analysis is based upon wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a three-layer precipitable water data set. Fields of zonal and meridional water vapor fluxes show significant variations over the north Indian Ocean in association with the different phases of the 1979 monsoon. Whereas after the onset, the cross-equatorial water vapor flux west of 50 degrees E does not vary much, it undergoes significant fluctuations east of that longitude. The bulk of water vapor crossing the western coast of India comes from the Southern Hemisphere. The latitude band between 10 and 20 degrees S appears as a major source of moisture during the northern summer. The major moisture supply for the western coast of Burma and Thailand is advected over the Bay from the Arabian Sea branch of the monsoon. The water vapor flux across the west coast of India undergoes large-amplitude variations in relation to the active-break cycle of the 1979 monsoon (onset, active, break, and revival periods). During active periods, the moist flow over the Arabian Sea strengthens and deepens. The water vapor flux across the west coast of India is well related to rainfall along the coast. The early retreat of the 1979 monsoon is associated with a decreasing trend in moisture transport over the Arabian Sea. In the Bay of Bengal, the cross-equatorial flux is not affected by the break-active cycle of the monsoon. There are strong surges of northward flux into the bay. Some of them are related to the formation of bay depressions.
Camberlin, P. (1996). "Intraseasonal variations of June-September rainfall and upper-air circulation over Kenya." Theoretical and Applied Climatology, Vienna, Austria 54(3-4): 107-115.
In the Northern Summer, Kenya is located under the influence of the divergent Indian monsoon flow, and therefore is dry except for two separate areas: the coastal strip and the western regions. Analysis of daily rainfall data for June-September 1982 to 1988 has revealed that, although there are many distinct rainfall events between the two regions, an out-of-phase relationship is also evident, rain on the Coast being frequently accompanied by a drop in the precipitation over the Rift Valley area. It is shown that two types of wind forcing accompany these patterns. Alternating westerly and easterly anomalies at the 700 hPa level are associated with persistent wet and dry conditions (respectively) in western Kenya, and the opposite along the Coast. Large speed increases of the cross-equatorial low-level jet over Mombasa are followed by short rain spells in this latter region. These observations are thought to reflect the importance of an influx of moist unstable air from the west, linked to the West African monsoon, to ensure heavy rainfall over the Highlands. Variations in the low-level jet speed, which cannot be easily followed downstream, also have a significant, but less persistent impact on rainfall in the two regions.
Camberlin, P. (1997). "Rainfall anomalies in the source region of the Nile and their connection with the Indian summer monsoon." Journal of Climate, Boston, MA 10(6): 1380-1392.
In light of the droughts and subsequent food crises that have plagued the Ethiopia-Sudan region in the course of its history, and especially during the last 3 decades, the author examines both the interannual and intraseasonal variabilities of the July-September rains and compares them to the Indian summer monsoon. Regional rainfall indexes for the region stretching from Eritrea to Lake Victoria are computed using seasonal totals for the period 1901-88. Daily data for 1982-88 are also considered. Though all these regions are only partly affected by the Indian monsoon cross-equatorial flow and although they are separated from India by an extensive dry belt (Red Sea, Somalia, west Arabian Sea), there exists a close association between summer rainfall variations in India and in the western parts of East Africa. An even stronger relationship is revealed between these latter regions and Bombay surface pressure, with as much as 79% of the variance in common during 1953-88. This relationship has been virtually stable throughout the twentieth century. Although there also exists a statistical connection between East African rainfall and the Southern Oscillation index (SOI), partial correlation coefficients show that the India-East Africa teleconnection is to a large extent independent of SOI. At intraseasonal timescales, this connection with India still holds, although less consistently. Abnormally low pressure along the western coast of India is associated with above normal daily rainfall and westerly wind anomalies over western Kenya. The most significant relationship is found with a lag of 2-6 days after the occurrence of a low pressure anomaly in Bombay. It is suggested that monsoon activity over India is a major trigger for July-September rainfall variability in the East African highlands. Active/strong monsoon conditions correspond to an enhanced west-east pressure gradient near the equator and, therefore, to abnormally strong westerly winds, advecting moisture from the Congo Basin to Ethiopia, Uganda, and western Kenya. This advection is reduced for weak/break monsoon conditions in India. This teleconnection bears some potential for the improvement of intraseasonal and seasonal rainfall forecasting in the Ethiopia /western Kenya area.
Camberlin, P. and N. Philippon (2002). "The East African March-May Rainy Season: Associated Atmospheric Dynamics and Predictability over the 1968-97 Period." Journal of Climate 15(9): 1002-1019.
This paper focuses on the East African March-May 'long rains.' Particularly, it investigates the atmospheric patterns associated to the March-May rainfall anomalies, then proposes a seasonal prediction model. In a preliminary step, in order to define a regional rainfall index, a new form of extended principal component analysis is performed, aimed at capturing both the spatial and intraseasonal rainfall coherence. What emerges is that although the long rains exhibit a low temporal coherence, calling for a separation between the months of March-April and May in teleconnection studies, they show a relatively strong spatial consistency over the Kenya-Uganda inland region. From composite analyses performed using NCEP-NCAR reanalyzed atmospheric data, three major signals appear for that region. Two are during March-April involving ENSO and the latitudinal location of the ITCZ, and ENSO interactions with the northern extratropical dynamics (by way of cool surges toward the Tropics and upper-ridge-trough systems). The third signal is the Asian monsoon in May. As shown using a rainfall index for Ethiopia, the interactions with the midlatitudes get stronger when considering rainfall farther to the north. The predictability study identifies four February indexes, involving several scales and several atmospheric and oceanic parameters, to serve as predictors in linear multiple regression (LMR) and linear discriminant analysis (LDA) models. The predictors, selected by a stepwise procedure, depict both regional (energy gradient and zonal wind) and remote dynamics (ENSO and 500-hPa geopotential height over the Near East): they are consistent with the signals shown in the synchronous composites. The robustness of the LMR and LDA models is demonstrated by the high linear error in probability space (LEPS) scores (44% for continuous variables and 51% for categorical variables) obtained on cross-validated results.
Castro, C. L., T. B. McKee, et al. (2001). "The Relationship of the North American Monsoon to Tropical and North Pacific Sea Surface Temperatures as Revealed by Observational Analyses." Journal of Climate 14(24): 4449-4473.
The North American monsoon is a seasonal shift of upper- and low-level pressure and wind patterns that brings summertime moisture into the southwest United States and ends the late spring wet period in the Great Plains. The interannual variability of the North American monsoon is examined using the NCEP-NCAR reanalysis (1948-98). The diurnal and seasonal evolution of 500-mb geopotential height, integrated moisture flux, and integrated moisture flux convergence are constructed using a 5-day running mean for the months May through September. All of the years are used to calculate an average daily Z score that removes the diurnal, seasonal, and intraseasonal variability. The 30-day average Z score centered about the date is correlated with Pacific sea surface temperature anomaly (SSTA) indices associated with the El Nino-Southern Oscillation (ENSO) and the North Pacific oscillation (NPO). These indices are Nino-3, a North Pacific index, and a Pacific index that combines the previous two. Regional time-evolving precipitation indices for the Southwest and Great Plains, which consider the total number of wet or dry stations in a region, are also correlated with the SSTA indices. The use of nonnormally distributed point source precipitation data is avoided. Teleconnections are computed relative to the climatological evolution of the North American monsoon, rather than to calendar months, thus more accurately accounting for the climatological changes in the large-scale circulation. Tropical and North Pacific SSTs are related to the occurrence of the Pacific Transition and East Pacific teleconnection patterns, respectively, in June and July. A high (low) NPO phase and El Nino (La Nina) conditions favor a weaker (stronger) and southward (northward) displaced monsoon ridge. These teleconnection patterns affect the timing and large-scale distribution of monsoon moisture. In the Great Plains, the spring wet season is lengthened (shortened) and early summer rainfall and integrated moisture flux convergence are above (below) average. In the Southwest, monsoon onset is late (early) and early summer rainfall and integrated moisture flux convergence are below (above) average. Relationships with Pacific SSTA indices decay in the later part of the monsoon coincident with weakening of the jet stream across the Pacific and strengthening of the monsoon ridge over North America.
Cavazos, T., A. C. Comrie, et al. (2002). "Intraseasonal Variability Associated with Wet Monsoons in Southeast Arizona." Journal of Climate 15(17): 2477-2490.
The intraseasonal evolution of the North American monsoon in southeast Arizona during the 1980-93 period is investigated using a neural network-based nonlinear classification technique known as the self-organizing map (SOM). The goal of the SOM algorithm is to discover meaningful low-dimensional structures hidden in the high-dimensional observations. Various daily lagged atmospheric fields (850-hPa meridional winds, 700-hPa specific humidity, 500-hPa geopotential heights, and 850-500-hPa thickness) for the summer season (June-July-August-September) of the 1980-93 period are used in the nonlinear classification of monsoon modes. Special emphasis is given to the wettest monsoon modes. The neural network classification successfully captures the multidimensional interaction of the atmospheric variables during the monsoon evolution, and shows monsoon 'bursts' and 'breaks' in a given year. Spectral analysis of daily summer rainfall in the study area reveals a significant peak in the 12-18-day band; a secondary and significant peak is also found near 40 days. Thus, monsoon bursts and breaks seem to be modulated by low-frequency variability. The SOM nonlinear classification shows that the mature phase of the monsoon is associated with two distinct intraseasonal (>10 days) wet monsoon modes. The signature of the wettest monsoon mode is a zonal three-cell anomalous midtropospheric height pattern over the North Pacific-North American sector, suggesting a large-scale dynamical mechanism, possibly linked to sea surface temperature (SST) anomalies in the North Pacific. This zonal mode, which is most frequent in July and August, is characterized by an enhanced and northeastward-displaced monsoon ridge, large amounts of midtropospheric moisture over the study area, and an out of phase relationship between precipitation in the southwest United States and precipitation in the Great Plains. The zonal mode has been recognized in longer datasets and it is the most typical mode that characterizes the mature phase of the monsoon in the southwest United States. In contrast, the second wettest intraseasonal monsoon mode does not show a monsoon ridge, but a meridional three-cell anomalous midtropospheric height pattern along the west coast of North America, weak height anomalies over the rest of North America, and large amounts of moisture over the study area. Importantly, this meridional mode, which is most frequent in August and September, does not show out of phase links to Great Plains precipitation. The meridional wet mode also shows an anomalous low-level cyclonic circulation off the west coast of central-south Mexico suggesting that convective activity off the southern Mexican coast-possibly associated with the intertropical convergence zone-may cross over the Isthmus of Tehuantepec toward the Gulf of Mexico and the southern United States. This would explain the weak link between precipitation in the Southwest and precipitation in the Great Plains during August and September of the 1980-93 period. At more regional scales, the zonal wet mode is also characterized by a latitudinal gradient of SST anomalies between Baja California and southern Mexico and reversed low-level flow over the Gulf of California. Looking at extreme wet monsoons outside of the study period (e.g., 1955, 1959, 1999) it is shown that the positive SST anomaly pattern along the Pacific coast of Baja California, which characterized wet events during 1980-93, can be completely reversed during other extreme wet events. These contrasting results suggest that interaction between local and remote forcing mechanisms over the study area are complex during extreme events and needs further investigation.
Chakraborty, B. and M. Lal (1994). "Monsoon climate and its change in a doubled CO sub(2) atmosphere as simulated by CSIRO9 model." TAO, Taiwan, PRC 5(4): 515-536.
In this paper, the author presents an assessment of the likely climate changes over the Indian subcontinent and the intraseasonal and interannual variability in summer monsoon rainfall as a consequence of increasing greenhouse gas concentrations in the atmosphere as inferred from CSIRO9 climate model simulations. The data obtained from the control and doubled CO sub(2) experiments with the model, each for 24-equilibrium years, are analyzed in this study. The model demonstrates a reasonable skill in simulating the present-day climate and its interannual variability over the Indian subcontinent. The total seasonal rainfall over India under the influence of southwest monsoon activity as inferred from the control run is in fair agreement with the observed climatology. A rise in area-averaged surface temperature of 2.98 degrees during the monsoon season over India (for land points only) is projected by the model in a doubled CO sub(2) atmosphere. Though no significant change in the monsoon onset date is found, an intensification of monsoonal rainfall is simulated by the model in a warmer atmosphere. The seasonal changes in the other hydrological parameters (evaporation and soil moisture) simulated by the model for the doubled CO sub(2) atmosphere are also discussed.
Chang, C. C. (1981). "Contrasting study of the rainfall anomalies between central Tibet and central India during the summer monsoon season of 1979." American Meteorological Society, Boston, Bulletin 62(1): 20-22.
Based upon a comparison of rainfall anomalies between central India and central Tibet in July and Aug. 1979, a negative correlation between them is found. When an active monsoon prevailed over central India, a break monsoon occurred over central Tibet, and vice versa. The large-scale circulation conditions for an active Indian monsoon are characterized by the presence of a large area of negative height departures over the Indian Peninsula and large areas of positive height departures over central Tibet. Conversely, the circulation conditions responsible for a break monsoon in India are characterized by frequent wave-trough activity over Tibet and the regions to the west of Tibet, and by a dominating high-pressure area over the Indian Peninsula.
Chang, C. P. and K. M. Lau (1982). "Short-term planetary-scale interactions over the Tropics and midlatitudes during northern winter, Pt. 1, Contrasts between active and inactive periods." Monthly Weather Review, Boston 110(8): 933-946.
Objectively analyzed 200-mb winds of four winters are used to study the short-term (several days) teleconnections between planetary-scale circulation components over the monsoon region. The composited structures suggest that, during very active northeasterly monsoon (surge) periods, the midlatitude and tropical circulation components vary in a coherent way. The jet streak and local Hadley circulation over eastern Asia, the divergent flow over the maritime continent, and the equatorial Walker circulations over the Pacific and Indian oceans all strengthen steadily, while the s econdary jet streak over western Asia weakens. During inactive (break) periods the midlatitude circulation components all exhibit reversed changes, while the variations in the Tropics are less coherent, although they still show reversed tendencies. The results basically verify the short-term teleconnection model proposed by Chang and Lau (1980), but more complex processes in the acceleration of the midlatitude jet streaks are indicated. In addition, the contrast between the very active and break monsoon periods suggests the relative importance of tropical versus midlatitudinal forcings in different regions of the monsoon circulation.
Chang, L.-N. and F.-C. Lu (1989). "On temporal variations of low level jets associated with the Asian summer monsoon." Sham, P. and Chang, C. P.
ECMWF FGGE level IIIb data are used to evaluate the temporal variation of the low-level summer monsoon in general and the Somali jet in particular to identify the important features responsible for the maintenance and variation of these circulation systems. Throughout the summer season, the Somali jet remains as quasi-Ekman boundary layer flow so that cross isobaric generation of KE is balanced by frictional dissipation. The kinetic energy of the Somali jet, the pressure gradient and the Rossby wave source in the southwesterly monsoon region all exhibit the same feature of having a low frequency oscillation of a period of 40-45 days, which is in close relation to the onset, active and break periods of the Indian monsoon. Temporal variation of the energetics of the western north Pacific region is also discussed.
Cheang, B.-K. and H.-V. Tan (1988). "Some aspects of the summer monsoon in southeast Asia, May to September 1986." Australian Meteorological Magazine, Canberra 36(4): 227-233.
The rainfall pattern, onset, low-frequency oscillations, and tropical storm activities over the South China Sea, during the 1986 Northern Hemisphere summer monsoon, are documented. On the whole, Southeast Asia experienced below normal rainfall from May to Sept. 1986. The onset date of the monsoon over Southeast Asia was found to fall within the range of onset dates determined by Orgill (1967). There was a distinct 20-40-day oscillation in the surface pressure, upper wind, and rainfall in Southeast Asia during the monsoon. This oscillation was found to be related to the active and break cycle of the monsoon. Tropical storm-typhoon occurrence was below average for all the months except May.
Chen, G. T.-J. (1994). "Large-scale circulations associated with the east Asian summer monsoon and the Mei-Yu over south China and Taiwan." Journal of the Meteorological Society of Japan, Tokyo, Japan 72(6): 959-983.
Grid point data, visible and infrared cloud imageries, and cloud top temperatures during the period of May-June 1981-1986 were used to study the large-scale circulations in Asian monsoon region. Stream function, velocity potential, divergent part of the wind, convection index, cloud top temperature index, and moisture field were analyzed. The distribution of the half-monthly mean of these parameters is presented and discussed to reveal the characteristics of the evolution of large-scale circulations from the pre-Mei-Yu season (1-15 May) to the post-Mei-Yu season (16-30 June) in South China and Taiwan region. Also, convectively active and inactive Mei-Yu seasons and fronts were selected to study the interannual and intraseasonal variations of the large-scale circulation patterns. The results can be summarized as follows: (1) The Mei-Yu over South China and Taiwan occurred concurrently with the onset of the summer southwest monsoon over the South China Sea during the period of May 16-31 (Phase I); (2) The northward advance of the area of deep convection, ITCZ, and subtropical ridge at the post-Mei-Yu season (June 16-30) occurred concurrently with the establishment of the quasi-stationary position of Mei-Yu front over the Yangtze Valley and Japan. At the same time, a quasi-equivalent barotropic monsoon circulation system was well developed with the low-level cyclone under the upper-level anticyclone over northeastern Indian and Burma area; (3) The active Mei-Yu season was characterized by the further southward penetration of the northern (baroclinic) system and moisture flux convergence over the Mei-Yu region. The reversed situations were observed for the inactive Mei-Yu season; (4) The difference for the active and inactive Mei-Yu fronts was mainly controlled by the low-level flows. The active Mei-Yu front was accompanied by the southwest monsoonal flows originating from the Bay of Bengal and the tropical western Pacific, whereas the inactive front the southeasterly or easterly flows of the Pacific high circulation prevailed over the Mei-Yu region. Higher mixing ratio, stronger moisture flux, and flux convergence were observed over the Mei-Yu region for the active front as compared to those for the inactive one; and (5) More frequent occurrence of the active fronts led to an active Mei-Yu season and the reverse was true for an inactive one.
Chen, T.-C. and J.-M. Chen (1993). "The 10-20-day mode of the 1979 Indian monsoon: its relation with the time variation of monsoon rainfall." Monthly Weather Review, Boston, MA 121(9): 2465-2482.
The synoptic structure of the 10-20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1 degrees x 1 degrees rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10-20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15 degrees -20 degrees N and the other at the equator. 2) Both cells of the 10-20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10-20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10-20-day monsoon mode through the modulation of this monsoon mode on the Indian monsoon rainfall. 5) The northern-cell lows are initiated from the redevelopment of the westward-propagating residual lows over the Bay of Bengal when the 30-60-day transient monsoon trough is present there. In addition to these findings, the possible genesis and westward-propagation mechanisms of the 10-20-day monsoon mode are also discussed in this study.
Chen, T.-C. and J.-M. Chen (1995). "An observational study of the South China Sea monsoon during the 1979 summer: onset and life cycle." Monthly Weather Review, Boston, MA 123(8): 2295-2318.
The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30-60- and 12-24-day monsoon modes. The former intraseasonal mode formed the northward migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May-August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30-60-day monsoon trough and the 12-24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30-60-day monsoon ridge lines and the 12-24-day center met near 15 degrees -20 degrees N in the northern South China Sea. The South China Sea monsoon was regulated by the northward-propagating 30-60-day monsoon troughs/ridges that were coupled with the eastward-propagating 30-60-day global divergent circulation through the transient local Hadley circulation.
Chen, T.-C., J.-M. Chen, et al. (1995). "The 12-24-day mode of global precipitation." Monthly Weather Review, Boston, MA 123(1): 140-152.
Global precipitation estimates derived from satellite data at the Goddard Laboratory for Amospheres for 1979-80 were used to explore time variations in global precipitation. Time series of the area-averaged precipitation [P] over the Asian-Australian (AA) monsoon (60 degrees E-120 degrees W), and the extra-AA monsoon (120 degrees W-60 degrees E) hemispheres were used in describing the variations. A distinct seesawlike intraseasonal variation of precipitation between these two hemispheres emerges from the two time series. Two intraseasonal (30-60 and 12-24 day) modes stand out in the spectral analysis of the two [P] time series. The (30-60-day mode is well known, while the 12-24-day mode is identified here for the first time. Using data generated by the Global Data Assimilation System of the National Meteorological Center, an effort was made to investigate the characteristics of the 12-24-day mode of global precipitation via potential functions for the 200-mb wind, water vapor transport, and precipitation. It is found that the 12-24-day mode exhibits a wavenumber 1 structure and propagates eastward. The seesaw intraseasonal variations of precipitation between the AA and extra-AA monsoon hemispheres is caused not only by the 30-60-day mode but also by the 12-24-day mode.
Chen, T.-C. and K. Takahashi (1995). "Diurnal variation of outgoing longwave radiation in the vicinity of the South China Sea: effect of intraseasonal oscillation." Monthly Weather Review, Boston, MA 123(2): 566-577.
It has been observed in numerious studies that (a) tropical cumulus convection undegoes a distinct diurnal variation, and (b) this diurnal variation exhibits a significant contrast between land and sea. In addition, tropical cumulus convection, particularly over the Asian monsoon region and the maritime continent, is greatly modulated by the eastward propagation of the global intraseasonal (30-60-day) oscillation (ISO). Conceivably, the diurnal variation of tropical cumulus convection is affected by this global ISO mode. In order to answer this question, the equivalent blackbody temperature T sub(B) sub(B) observed by the Japanese geostationary meteorological satellite over the South China Sea and its vicinity were analyzed. The major findings in this study are as follows; 1) Centers of the T sub(B) sub(B) diurnal cycles during the northern summers are located over land around the South China Sea region north of the equator, while those in the northern winter exist over land around the southern part of this sea. 2) In the northern summer, the T sub(B) sub(B) diurnal cycle is suppressed over land and enhanced over the open sea during the active ISO phase. This land-sea contrast of the T sub(B) sub(B) diurnal cycle is reversed during the inactive ISO phase. 3) During the northern winter, changes in the T sub(B) sub(B) diurnal cycle between the active and inactive ISO phases become less pronounced because there is less open sea in the southern South China Sea. 4) The eastward propagation of the global ISO mode does not generally induce a significant phase change in the T sub(B) sub(B) diurnal cycle.
Chen, T.-C., R.-Y. Tzeng, et al. (1988). "Development and life cycle of the Indian monsoon: effect of the 30-50-day oscillation." Monthly Weather Review, Boston 116(11): 2183-2199.
The velocity-potential fields generated from the FGGE 3-b horizontal winds of the European Centre for Medium Range Weather Forecasts were subjected to an empirical orthogonal function (EOF) analysis to extract the annual cycle and the 30-50-day mode of the divergent circulations. It is found that the Indian monsoon circulation is portrayed by the annual cycle of the divergent circulation and develops as a classical, giant seabreeze model. Conversely, this monsoon system is modulated by the planetary-scale 30-50-day low-frequency mode to establish an onset-active-break-revival-retreat life cycle. This modulation is accomplished through the following interaction processes. The northeastward propagation of the planetary-scale 30-50-day mode over the Indian monsoon region induces transient local Hadley circulation. Through this type of circulation, the planetary-scale 30-50-day mode couples with and steers northward the low-level, 30-50-day monsoon troughs and ridges that originated around the Equator. The northward migration of these low-level transient troughs and ridges cause, respectively, the deepening and filling of the monsoon trough over central India. The evolution of this monsoon trough results in the intensification and weakening of the Indian monsoon and its life cycle.
Chen, T.-C. and S.-P. Weng (1996). "Some effects of the intraseasonal oscillation on the equatorial waves over the western tropical Pacific-South China Sea region during the northern summer." Monthly Weather Review, Boston, MA 124(4): 751-756.
Previous studies have shown that the life cycle of the Asian summer monsoon is modulated by the 30-60-day oscillation; the arrival of the 30-60-day monsoon trough (ridge) at 15 degrees -20 degrees N intensifies (weakens) the monsoon flow over the South China Sea-western tropical Pacific region and, in turn, extends eastward (westward) the monsoon westerlies (trade easterlies) over this region. Since equatorial waves are important synoptic disturbances to the weather of this region, an effort was made in this study to explore the possible impact of the 30-60-day mode on the occurrence frequency and westward propagation of these waves. The major findings of this study are as follows: 1) the westward intrusion of trade easterlies during the break monsoon facilitates the occurrence and westward propagation of equatorial waves along trade easterlies in the southern flank of the North Pacific anticyclone, and 2) the eastward expansion of monsoon westerlies during the active monsoon varies relative to the activity of equatorial waves in a way opposite to the break monsoon condition. Since this is a pilot study, some future efforts are suggested.
Chen, T.-C. and S.-P. Weng (1999). "Interannual and intraseasonal variations in monsoon depressions and their westward-propagating predecessors." Monthly Weather Review, Boston, MA 127(6, Pt. 1): 1005-1020.
The majority of monsoon depressions develop from the regenesis of westward-propagating residual lows from the east. Most of these residual lows can be traced to weather disturbances in the south China Sea, including tropical cyclones and 12-24-day monsoon lows. Hypothetically, any mechanism causing a variation in the occurrence frequency of these two types of weather disturbances in the western tropical Pacific-south China Sea (WTP-SCS) region may result in a corresponding change in the formation frequency of monsoon depressions over the Bay of Bengal. Two such possible mechanisms are interannual and intraseasonal variations of large-scale summer circulation in the WTP-SCS region induced by 1) the interannual variation of the sea surface temperature (SST) in the eastern tropical Pacific and 2) the northward migration of the 30-60 day monsoon trough/ridge. The National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data and the 6-hourly tropical cyclone track collected by the Japan Meteorological Agency for the period of 1979-94 were analyzed to substantiate the aforementioned hypothesis. The findings are as follows. 1) Interannual variation. Based upon the SST averaged over the National Oceanic and Atmospheric Administration NINO3 region (150 degrees -90 degrees W, 5 degrees S-5 degrees N), the summers of 1982, 1983, 1987, and 1991 and 1981, 1984, 1985, 1988, 1989, and 1994 are defined as warm and cold, respectively. A clear interannual variation can be seen in the frequency of monsoon depressions in the Bay of Bengal: an enhancement (reduction) of monsoon depression activity occurs during cold (warm) summers. This interannual variation of monsoon depression activity is traceable to the corresponding variation of the combined tropical cyclone and 12-24-day monsoon low frequency in the south China Sea. The latter interannual variation results from the development of an anomalous anticyclonic (cyclonic) circulation between 15 degrees and 30 degrees N in the WTP-SCS region in response to the warm (cold) SST anomalies in the eastern tropical Pacific. 2) Intraseasonal variation. There is an intraseasonal variability in the occurrence of tropical cyclones and of 12-24-day monsoon lows over the south China Sea, which is followed by a corresponding variability of monsoon depressions over the Bay of Bengal. The formation frequency of these depressions is dependent on the penetration role of the residual lows of these two types of disturbances across Indochina. These residual lows lead to an intraseasonal change in monsoon depression formation in connection with a deepening/filling of the monsoon trough over northern India and the Bay of Bengal.
Chen, T.-C. and M.-C. Yen (1991). "Interaction between intraseasonal oscillations of the midlatitude flow and tropical convection during 1979 northern summer: the Pacific Ocean." Journal of Climate, Boston, MA 4(7): 653-671.
It has been inferred and illustrated by many studies in terms of outgoing longwave radiation (OLR) and velocity potential ( chi ) that energy is input into the intraseasonal oscillation through tropical cumulus convection, especially over the monsoon region. The existence of this low-frequency oscillation of regional extratropical oscillation has been observed by other studies. Previous findings lead us to speculate upon a hemispheric interaction between tropical cumulus convection and extratropical circulation in terms of an intraseasonal time scale. This study demonstrates that during the 1979 summer a coherent intraseasonal oscillation existed between cumulus convection and divergent circulation over the equatorial western Pacific and mid-Pacific troughs. Moreover, the intraseasonal oscillations of streamfunction ( psi ) associated with these troughs, the tropical OLR, and the tropical psi field exhibit coherent eastward propagations. Based upon these two coherent relations, it is hypothesized that a tropical-midlatitude interaction with the intraseasonal time scale can be established through the chain relation between tropical diabatic heating, global-scale divergent circulation, and extratropical rotational flow depicting the mid-Pacific troughs. A diagnostic scheme employing a chi -maintenance equation and a psi -budget equation is adopted to substantiate the aforementioned hypothesis. The former equation relates the global-scale intraseasonal chi oscillation to tropical diabatic heating, whereas the latter illustrates how the eastward propagation of global-scale intraseasonal chi oscillation induces the intraseasonal psi tendency associated with the mid-Pacific troughs. The hypothesized tropical-midlatitude interaction through the eastward propagation of the intraseasonal chi oscillation emerges clearly from our diagnostic results.
Chen, T.-C. and M.-C. Yen (1991). "Intraseasonal variations of the tropical easterly jet during the 1979 northern summer." Tellus Series A: Dynamic Meteorology and Oceanography, Stockholm, Sweden 43A(3): 213-225.
Numerous studies have investigated the intraseasonal oscillation of various elements of the Indian monsoon, but the tropical easterly jet has been neglected. The 1979 summer data generated by the FGG III-b analysis of the European Center for Medium Range Weather Forecasts were used to examine the intraseasonal oscillation of this jet. It was found that the jet possesses a distinctive intraseasonal oscillation south of its core. Previous studies suggested that the temporal fluctuation of this jet may be related through cumulus convection to that of the low-level Indian monsoon circulation. It was demonstrated by a streamfunction budget analysis that the intraseasonal oscillation of the tropical easterly jet south of its core is primarily induced by the eastward- propagating intraseasonal oscillation of the planetary-scale divergent circulation.
Chen, T.-C., M.-C. Yen, et al. (1993). "The vertical structure of diabatic heating associated with the Madden-Julian oscillation simulated by the Goddard Laboratory for Atmospheres climate model." Journal of Geophysical Research, Washington, DC 98(D5): 8801-8813.
Previous studies have shown that numerical simulations of the Madden-Julian oscillation (MJO) are very sensitive to the vertical distribution of diabatic heating. Since atmospheric diabatic heating is generally difficult to estimate, the vertical diabatic heating profile associated with the MJO is not well known. Judged by its propagation properties and spatial structure, the MJO is reasonably well simulated by the nine-layer Goddard Laboratory for Atmospheres (GLA) general circulation model. Although only a simulation the model MJO may provide an indication of the vertical diabatic heating profile associated with the real oscillation. The diabatic heating structure of the model MJO is illustrated with composite charts made for those times when this low-frequency mode reaches its maximum and minimum amplitudes. These composite charts compare the vertically integrated diabatic heating with potential functions, the vertical distribution of diabatic heating with the east-west mass flux function in the tropics, and the vertical profiles of diabatic heating at the centers of maximum and minimum MJO amplitude. Three interesting features of the model MJO's diabatic heating are revealed: (1) the maximum heating rate of this low-frequency mode is located over the Asian monsoon region and its amplitude is about a half of the maximum value of the seasonal mean heating rate in this region, (2) the vertical diabatic heating rate profile has a maximum at 500 mbar and resembles the seasonal mean total heating profile, and (3) the total diabatic heating is for the most part composed of the latent heat released by cumulus convection.
Chen, T.-C., M.-C. Yen, et al. (2000). "Interaction between the summer monsoons in East Asia and the South China Sea: intraseasonal monsoon modes." Journal of the Atmospheric Sciences, Boston, MA 57(9): 1373-1392.
The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)-Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120 degrees E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30-60-day monsoon trough/ridge from the equator to 20 degrees N and with the westward-propagating 12-24-day monsoon low-high along the latitude of similar to 15 degrees -20 degrees N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north-south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.
Chern, C.-S. and J. Wang (1989). "On the water masses at northern offshore area of Taiwan." Acta Oceanographica Taiwanica, Taipei, March(22): 14-32.
Off the northeast shore of Taiwan, due to the impinging of the Kuroshio toward the shelf break, there is a blocking high pressure region at a depth of 100-200 m. The cool subsurface water is upwelled 70-100 m and outcrops the sea surface most of the time. At the same time, the upwelled bottom water is driven to flow northwestward north of Taiwan. This cool and saline branch of the Kuroshio intrudes eventually into the southern East China Sea and forms a source of the bottom Taiwan Warm Current all year long. The surface water forms a cyclonic circulation pattern above this internal high center, and the low-pressure system is significant to drive waters from the southern East China Sea and Taiwan Strait flowing seaward. On the East China Sea, the water is stratified in summer, and becomes vertically well mixed in winter due to the stirring of strong monsoon winds. However, the water in the eastern portion of the Taiwan Strait is well mixed in summer and becomes salinity stratified in winter. This contrast implies that waters from the strait and southern East China Sea are of different origin. In addition, an eddy in the northern part of the strait is formed during winter, which will prevent the contribution of southerly warm water to the Taiwan Warm Current through the strait at this season.
Chowdhury, A., R. K. Mukhopadhyay, et al. (1990). "Low frequency oscillations in wind and circulation fields over India during northern summer monsoon." Mausam, New Delhi, India 41(4): 603-610.
The study presents results of low frequency oscillations in zonal and meridional wind and divergence and vorticity fields during summer monsoon over India. Harmonic analysis has been adopted for this purpose to detect prominent periodicities. Strength of Hadley circulation has been related to the monsoon rainfall. Inter-annual fluctuations in the oscillations have been examined with the help of Hovmoller diagrams. The contribution of Kelvin and Rossby waves in the excitation of the oscillations has been discussed. Role of easterly and westerly anomalies in the establishment of the monsoon and the active-break cycles has been projected. The vertical structure of the cycles has been investigated and plausible explanation offered, particularly on its tilt and its relation to monsoon activity. Possible mechanism on generation, propagation, intensification and weakening of the oscillations over Indian latitudes has been proposed.
Chowdhury, A., K. C. Sinha Ray, et al. (1988). "Intraseasonal cloud variations over India during summer monsoon season." Mausam, New Delhi 39(4): 359-366.
The authors deal with large-scale variations of cloudiness over India and its vicinity during the southwest monsoon. Daily satellite cloudiness values from June 1 to Sept. 30 for the period 1982-1986 were subjected to harmonic analysis in order to investigate the presence of periodicities of monsoon cloudiness. The results confirmed the presence of 30-60-day periodicities in the monsoon fluctuation as the dominant modes. Associated with these periodicities, poleward propagation of cloudiness is observed in each month of the monsoon season. Periodicity of 120 days appears north of 10 degrees N in years of deficient rainfall, whereas in the equatorial region, the low-frequency mode generally prevails, regardless of the seasonal monsoon rainfall over India.
Cifelli, R. and S. A. Rutledge (1994). "Vertical motion structure in maritime continent mesoscale convective systems: results from a 50-MHz profiler." Journal of the Atmospheric Sciences, Boston, MA 51(18): 2631-2652.
Wind profiler data were used to determine the vertical motion structure in four tropical mesoscale convective systems (MCSs), which occurred during the Down Under Doppler and Electricity Experiment (DUNDEE) near Darwin, Northern Territory, Australia. Three of the MCSs occurred during the monsoon-break convective regime and one occurred during the monsoon regime. In the break regime cases (each with a leading convective and trailing statiform region structure), the wind profiler sampled low-level convective cells on the leading edge of the convective region, trailed by deeper updrafts of comparable magnitude. Surface rainfall measurements from a network of raingauges showed two comparable peaks in rainfall intensity that roughly corresponded to the passage of low-level and deep convective updraft (71%-80% of the system total rainfall was associated with the passage of the convective line). In the stratiform region, the profiler data showed generally weak vertical drafts (<1 m s super(-) super(1) ) and the presence of both mesoscale upward and downward motion (17%-28% of the system total rainfall was associated with the passage of the stratiform region). Deep subsidence in the transition zone located between the convective and stratiform regions was also documented in each of the break regime cases. Composite vertical motion profiles in different regions of the break MCSs were constructed and the salient features of the profiles are described. The composite vertical motion profiles are compared to similar profiles from different graphical regions. The evolution of the monsoon MCS was different from the break regime cases. This system was characterized by a series of convective updrafts embedded in stratiform cloud.
Cifelli, R. and S. A. Rutledge (1998). "Vertical motion, diabatic heating, and rainfall characteristics in north Australia." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 124(548, Pt. B): 1133-1162.
Very-high-frequency wind-profiler data are used to study the vertical draught structure within 13 tropical Mesoscale Convective Systems (MCSs) near Darwin, Australia during the 1989-90 and 1990-91 wet seasons. These studies are supported by single-Doppler radar, soundings, and surface rainfall data to correlate radar reflectivity, thermal buoyancy, and surface rainfall patterns with vertical air-motion structures. Because of Darwin's unique location at the southern tip of the Maritime Continent, vertical draughts in both the monsoon (maritime) and monsoon break (continental) convective regimes can be observed. The break-regime MCSs (six in total) were all squall lines, characterized by a leading line of convection with heavy precipitation and trailing stratiform rainfall containing a characteristic radar bright band. These MCSs exhibited a pronounced life-cycle patterns and were all sampled by the profiler in the mature to dissipating stages. In contrast, the monsoon systems (seven in total) were composed of regions of stratiform cloud with embedded convective bands which moved on-shore in the monsoonal flow. Results from the Darwin rain-gauge network indicated that the majority of the total rainfall in each MCS (break and monsoon) was associated with the convective portion of the system. The break-regime MCSs were all characterized by a low-level (4 km) updraught peak associated with convective cells on the leading edge of each squall line, trailed by deeper convective updraughts in the middle and upper troposphere. For the monsoon cases, the lower-troposphere convective updraughts were typically less than those in the squall lines, yet were stronger in the upper troposphere. The low-level differences in the convective updraughts were consistent with the smaller virtual-temperature excess in the monsoon soundings, as well as the larger vertical radar-reflectivity gradients observed in monsoon convection. Consistent with the differences in vertical air-motion patterns, diabatic heating and moistening profiles for the monsoon MCSs were characterized by a higher-level heating and drying peak compared with the break MCSs. The results have important implications for cumulus parametrizations in numerical models since the large-scale circulation is sensitive to the vertical distribution of diabatic heating in tropical MCSs.
Cleland, S. J. (1998). "The tropical circulation in the Australian/Asian regionAMay to October 1997." Australian Meteorological Magazine, Canberra, Australia 47(1): 71-81.
A summary of the broadscale tropical circulation from 70 degrees E to 180 degrees , for the six months May to October 1997, is presented. Cool-ENSO conditions prevailed during the previous Southern Hemisphere summer period. By the start of this summary period there were indicators (e.g., winds and sea-surface temperatures of the near-equatorial Pacific and the southern oscillation index) of a dramatic swing to warm-ENSO conditions. These intensified in the first two months and persisted throughout the period. The Northern Hemisphere southwest monsoon system became established over much of Indo-China and the Bay of Bengal during an active phase of the 30 to 60-day intraseasonal oscillation (ISO) in May and advanced over India during June. It had retreated from most mainland areas early in October. Outgoing long wave radiation and low-level wind anomalies indicate a near-average monsoon season over India and Indo-China, but areas around Indonesia and Papua New Guinea were much drier than normal, indicative of the strong warm-ENSO event. Two coherent active phases of the ISO were evident early in the period, near the middle of May and the end of June. After this time the ISO signal became difficult to interpret, possibly due to the warm-ENSO event suppressing convection in the central longitudes of the region. There may have been up to six cycles of the ISO during the summary period. A near-average number of tropical cyclones developed during the period. More than 65 per cent of those that affected the northwest Pacific formed east of 145 degrees E, with several also forming in the southwest Pacific, again indicative of the warm-ENSO event.
Cleland, S. J. and P. W. Bate (1996). "The tropical circulation in the Australian/Asian region November 1995 to April 1996." Australian Meteorological Magazine, Canberra, Australia 45(3): 193-202.
A summary of the broadscale tropical circulation from 70 degrees E to 180 degrees , for November 1995 to April 1996, is presented. Weak cool ENSO conditions persisted through the season, with an apparent enhancement of the upward branch of the Walker circulation over the Australasian region, compared to the long-term mean. Mean sea-level pressures were generally less than average over most of the tropical region during the period, and sea-surface temperatures were mostly warmer than climatology. Averaged over the season, the low-level cross-equatorial and westerly monsoon flow was near average, as was the upper-level return flow. There was little evidence of any regular intraseasonal periodicity in the active tropical convection early in the season. However, from the end of January, there appeared to be greater regularity, with a period of about 30 days dominating. Tropical cyclone numbers were close to the long-term mean.
Cleland, S. J., B.-K. Cheang, et al. (1995). "The tropical circulation in the Australian/Asian region: May to October 1992." Australian Meteorological Magazine, Canberra, Australia 44(3): 225-236.
A summary of the broadscale tropical circulation from May to October 1992 in the area of analysis responsibility of the Darwin Regional /Specialised Meteorological Centre--that is from 70 degrees E to the date-line--is presented. Mature El Nino conditions were apparent during the previous austral summer. Early in the summary period, most indicators showed that El Nino began to weaken but did not fully dissipate. Weak warm conditions lingered and were generally re-intensifying at the end of the period. The Northern Hemisphere summer monsoon produced mostly near-normal rainfall totals. Five active phases of the intraseasonal oscillation were noted, with an average periodicity at about 40 days. More than the mean number of tropical cyclones developed during the period.
Dai, Y., F. Xue, et al. (1998). "A land surface model (IAP94) for climate studies. Part II: Implementation and preliminary results of coupled model with IAP GCM." Advances in Atmospheric Sciences, Beijing, China 15(1): 47-62.
The Institute of Atmospheric Physics Land Surface Model (IAP94) has been incorporated into the IAP two-level atmospheric general circulation model (IAP GCM). Global and regional climatology averaged over the last 25 years of 100 year integrations from the IAP GCM with and without IAP94 (``bucket'' scheme) is compared. The simulated results are also compared with the reanalysis data. Major findings are: The IAP GCM simulation without IAP94 has extensive regions of warmer than observed surface air temperatures, while the simulation with IAP94 very much improves the surface air temperature. The IAP GCM simulation with IAP94 gives improvement of the simulated precipitation pattern and intensity, especially the precipitation of East Asian summer monsoon and its intraseasonal migration of the rainbelts. In five selected typical regions, for most of the surface variables such as surface air temperature, precipitation, precipitation minus evaporation, net radiation, latent heat flux and sensible heat flux, the IAP GCM with IAP94 provides better simulations.
Das, S., U. C. Mohanty, et al. (1988). "Study of Kuo-type cumulus parameterizations during different epochs of the Asian summer monsoon." Monthly Weather Review, Boston 116(3): 715-729.
The performances of several versions of the Kuo-type cumulus parameterization schemes have been examined during different phases of the summer monsoon. These phases are the preonset, onset, and a period of break in the monsoon. Special sets of upper air observations that were collected from stationary ships forming polygons over the Arabian Sea and the Bay of Bengal during MONEX-79 were used for this purpose. Cumulus warming, drying, and precipitation rates have been simulated in a semiprognostic way and compared with the observations. The limitations of different schemes for numerical weather prediction are discussed. Among various Kuo-type cumulus parameterization schemes studied in this article, a modified Kuo-scheme is found to provide best results during the summer monsoon. In this scheme, the moistening parameter is determined, on the basis of the relative humidity, and it is tuned for different phases of the monsoon. A comparison of the performance of various schemes during different phases of the monsoon was made. The heating and drying rates were best simulated during a preonset phase, when compared with the other two periods. The largest deviations between observed and simulated values were obtained during a break in the monsoon.
Datta, R. K. and T. K. Mukerji (1975). "Vertical velocity patterns over India and neighborhood during break monsoon and active monsoon periods." Indian Journal of Meteorology, Hydrology, & Geophysics, Delhi 26(3): 399-404.
Vertical velocity patterns at different levels of the atmosphere, over India and neighboring areas, were obtained for a situation each of active and break-monsoon. A five-layer, quasi-geostrophic model was developed for the above computations. These vertical velocity patterns are discussed to bring out their salient features and their differences. A model for the vertical velocity field for active and for break-monsoon was also proposed.
De, U. S., O. Prasad, et al. (1995). "The influence of Southern Hemispheric equatorial trough and rainfall during southwest monsoon." Theoretical and Applied Climatology, Vienna, Austria 52(3-4): 177-181.
Monsoon is a complex dynamical system. Interannual and intraseasonal fluctuations have been studied in the regional and global perspective. Southern Hemispheric Equatorial Trough and its activity during the northern summer appears inversely related to the rainfall activity over the Indian sub-continent in general and central India in particular. During typical active SHET epochs the rainfall activity becomes deficient in the central parts of the country. The activity of the SHET is fluctuating in nature. Its duration is prolonged and intensity is increased during the years of major failure.
De, U. S. and D. V. Vaidya (1987). "Morphology of intraseasonal oscillations in the Indian summer monsoon." Mausam, New Deli 38(4): 395-400.
Intraseasonal variations during the summer monsoon have been documented by various researchers. Recently, low-frequency oscillations in the time scale of 10-15 and 30-50 days have been extensively studied. The morphology of intraseasonal fluctuations on a regional scale (over the Indian subcontinent) has been studied by using mean circulation parameters. The low-frequency oscillations (30-50 days) and their contribution to the fluctuations of rainfall have been discussed. Synoptic-scale (3-5-day) oscillations and the quasi-biweekly oscillations appear more relevant for explaining the major intraseasonal oscillations in rainfall.
Degtyarev, A. I. (1999). "Influence of the Indian summer monsoon on the formation of tropical cyclones over Southeast Asia." Russian Meteorology and Hydrology, New York, NY 7: 38-42.
The ECMWF objective analysis data on the variability of kinetic energy in the tropical atmosphere over the Indian Ocean and Southeast Asia water area in 1995 and 1996 are analyzed. The intraseasonal variability of the Indian summer monsoon is found. Tropical cyclogenesis over the South China Sea and the Philippine Islands is shown to occur more often in the active phase of the Indian monsoon than during its breaks. It turned out that the westerly wind intensification, an active monsoon phase, leads to a deepening of the monsoon trough and creates favorable conditions for the formation of tropical cyclones over the study region.
Degtyarev, A. I. and L. A. Pavlovskaya (1992). "Numerical modeling of the intraseasonal variability of the Indian summer monsoon." Meteorologiya i Gidrologiya, Moscow, Russia 5: 42-50.
A numerical experiment was carried out with the 15-level global spectral model of the general atmospheric circulation, developed at the Hydrometeorological Scientific Research Center of the Russian Federation. One of the goals of the experiment was to model the summer monsoon circulation in the Indian region. The results obtained showed that the model successfully simulates the well-known large-scale features of monsoons. Phases of monsoon circulation (normal, active, and a break) are considered in detail. The experiment confirmed the hypothesis of interlatitudinal interaction, which assumes that the mid-latitude ultralong Rossby waves of a large amplitude penetrate into the Indian region, causing disturbances of the monsoon circulation and forming the break phase.
Desai, D. S. (1986). "Study of thermodynamic parameters in strong and break monsoon." Mausam, New Delhi 37(1): 123-128.
Fluctuations in the intensity and spatial distribution of the southwest monsoon rainfall over India are well known. A spurt in the rainfall activity over India leading to a strong monsoon and a cessation of the rainfall activity leading to a break/weak monsoon are typically associated with different synoptic situations. In these two typical situations, the variations of thermodynamic parameters such as dry-bulb temperature, dew-point temperature, potential temperature, equivalent potential temperature, and saturated moist-static energy are studied, and their possible use in forecasting is highlighted.
Desai, D. S. (1987). "Field of vorticity, divergence, and vertical velocity associated with break and strong monsoon." Mausam, New Delhi 38(4): 419-424.
Contrasting rainfall distribution in a break and strong monsoon is explained by the fields of vorticity, divergence, and vertical motion. The heavy rainfall belt in a monsoon is generally south of the surface position of the monsoon trough. In a strong monsoon, the heavy rainfall belt is confined between the surface and 700-mb (hPa) monsoon trough position. The presence of an E-W synoptic-scale zonal circulation similar to the planetary-scale Walker circulation is observed over the Indian monsoon region. The synoptic-scale E-W zonal circulation is observed over northern and peninsular India during the monsoon season, separately, and/or simultaneously. During strong monsoons, over northern India, there is an ascending motion over northwestern and a descending motion over northeastern India. Over peninsular India, there is an ascending motion along the west coast of peninsular India and a descending motion over the southwestern bay and its vicinity. During break monsoons, this synoptic-scale E-W zonal circulation either weakens or reverses. The strength of a monsoon depends upon the synoptic-scale E-W zonal circulation.
Dhar, O. N. and S. Nandargi (1999). "Role of low pressure areas in the absence of tropical disturbances during monsoon months in India." International Journal of Climatology, Chichester, UK 19(10): 1153-1159.
In this study an effort has been made to find out the contribution of low pressure areas (or lows) towards the rainfall of northern and central India during the monsoon months of June to September in the absence of more intense cyclonic disturbances such as depressions, deep depressions and cyclonic storms. 15 years of data from 1983 to 1997 has been used to study this aspect. This study has shown that the occurrence of moderate to heavy rainfall mainly depends upon their frequency, life span, track followed and origin of these disturbances provided there are no inhibiting meteorological factors like `break' monsoon situations.
Dhar, O. N., M. K. Soman, et al. (1984). "Rainfall over the southern slopes of the Himalayas and the adjoining plains during breaks in the monsoon." Journal of Climatology, Chichester, Eng 4(6): 671-676.
During breaks in the southwest (or summer) monsoon, there is a general cessation of rainfall activity over most of the Indian area, but this activity increases over the southern slopes of the Himalayas and its adjoining plains. In this study, rainfall distribution during the break monsoon situations has been studied, over the southern slopes of the Sikkim, Nepal, and Uttar Pradesh Himalayas (longitude 78-89 degrees E) and their adjoining plains. Rainfall distributions during the major break situations, which occurred during the principal monsoon months of July and Aug. of the period from 1957 to 1969, were studied. This study has shown that, on a break day, positive percentage departures of rainfall as high as 100-300% have occurred at stations located in the outer Himalayas and the adjoining plains of north Bengal, east and central Nepal, Bihar, and east Uttar Pradesh. However, the width of this heavy rain belt is maximum over the hills and plains of southeastern Nepal and the adjoining plains of north Bihar. It was also shown that, on a break day, there is a decrease of rainfall activity over the Himalayas west of 80 degrees E long. Decrease of rainfall also occurs along a narrow belt immediately to the south of the great Himalayan range to the east of 80 degrees E long. outside Sikkim. Over the adjoining Gangetic plains, an increase in rainfall is noticed even to the south of the Ganga River in Bihar and its neighborhood. Thus, because of the increase in rainfall activity during break monsoon situations in the catchments of Himalayan rivers located in Sikkim and Nepal, a peculiar situation arises which is responsible for causing floods in the downstream plains that experience almost drought conditions.
Ding, Y.-H. and E. R. Reiter (1980). "Preliminary study of the variability in the frequency of typhoon formation over the West Pacific Ocean." Colorado. State Univ., Ft. Collins, Environmental Research Paper June(22).
On the basis of 30 yr of typhoon data over the West Pacific Ocean and monthly upper air wind data, the authors studied some climatological aspects of the variability of typhoon formation frequency and its relationship to the tropical general circulation in the Asian monsoon region and in the central and western Pacific. Special emphasis is placed on a contrasting study of the circulation features responsible for months with many typhoons and months with few typhoons. The following major differences in the circulation features between the above two cases were found: 1) the Intertropical Convergence Zone (ITCZ) for months with many typhoons is located near 20 degrees N and extends eastward to 160 degrees E, whereas the ITCZ for months with few typhoons is found near 10 degrees N and terminates about 130 degrees E; 2) for months with many typhoons, conditions characteristic of a break in the monsoon prevail over India. In contrast, during months with few typhoons, the monsoon is very active over India; 3) for months with many typhoons, the tropical upper tropospheric trough (TUTT) is located east of 180 degrees . whereas in months with few typhoons, it can be found west of 150 degrees -160 degrees E; 4) the upper tropospheric Tibetan high shifts eastward to 90 degrees -100 degrees E from its normal position over the Tibetan plateau during months with many typhoons, and this eastward shift of the Tibetan high is closely related to the corresponding eastward extension of the region of typhoon formation during such months; 5) in connection with the eastward shift of the Tibetan high, the upper tropospheric easterly jet contains higher wind speeds over the tropical West Pacific and lower speeds over South Asia during months with many typhoons than during months with few typhoons; and 6) the region enclosed by the ridge line of the Tibetan high, the TUTT trough line, the upper easterly jet, and the coast line of China's mainland is favored by typhoon formation and is characterized by relatively weak vertical wind shears.
Ding, Y.-H. and E. R. Reiter (1983). "Large-scale hemispheric teleconnections with frequency of tropical cyclone formation over the Northwest Pacific and North Atlantic oceans." Archives for Meteorology, Geophysics, and Bioclimatology, Ser A, Meteorology and Geophysics, Vienna 32(4): 311-337.
Large-scale circulation features during months with many and with few tropical cyclones revealed the following conditions. 1) With many typhoons in the Northwest Pacific, a well-defined intertropical convergence zone (ITCZ) extended eastward to 160 degrees E and was displaced to 20 degrees N. At 200 mb, an extensive anomalous anticyclonic circulation prevailed over the western and central Pacific. The circulation at 500 mb over Tibet was often characterized by an upper trough or a low-pressure region. Conditions characteristic of a break in the monsoon prevailed in India. The monsoon trough at 500 mb and at the surface over the Indian peninsula was relatively weak and was accompanied by higher than normal rainfall in the northern part of India and lower than normal rainfall over the peninsula. The polar vortex tended to be weak and moved away from the North Pacific sector. Abnormally warm water was observed over the central and eastern Pacific, and abnormally cold water was seen over the western Pacific in summer as well as in the preceding spring. Sea surface temperatures (SST) in the region of the Kuroshio Current and in the latitude belt 30-40 degrees N were also abnormally warm. 2) Over the North Atlantic, months with many hurricanes were characterized by a well-developed subtropical high-pressure belt, displaced to the north; a deep Icelandic low; a stronger than normal polar vortex with a deep polar trough dipping far southward over the eastern part of the U.S.; a dominant high-pressure ridge over western Canada and the U.S.; and a jet stream displaced northward over these regions. At 200 mb, an anomalous anticyclonic circulation tended to prevail over the subtropical North Atlantic, with stronger than normal upper easterly flow in low latitudes. Positive sea surface temperature anomalies on a seasonal or long-term basis tend to favor the formation of hurricanes.
Drosdowsky, W. (1996). "Variability of the Australian summer monsoon at Darwin: 1957-1992." Journal of Climate, Boston, MA 9(1): 85-96.
The variability of the Australian summer monsoon is reexamined using data covering 35 monsoon seasons. A new, easily applied objective definition of active and break phases of the monsoon, based solely on the zonal wind at Darwin, is proposed. Attempts to define ``wet westerly'' onsets are shown to be misleading, since no clear relationship is found between westerly winds and rainfall, on the timescales associated with the transition between active and break phases. The resulting dates of monsoon onset at Darwin differ from those reported in some recent studies, resulting in significantly different relationships with the Southern Oscillation. In particular, the date of monsoon onset is shown to be significantly related to, and hence predicted from, prior values of the Southern Oscillation index. Also in contrast to a number of recent studies that have highlighted the so-called 40-50-day oscillation in the Australian summer monsoon, no dominant timescales are found in the length of the active periods or in the recurrence time between active phases.
Dunkerton, T. J. (1995). "Evidence of meridional motion in the summer lower stratosphere adjacent to monsoon regions." Journal of Geophysical Research, Washington, DC 100(D8): 16675-16688.
Twenty-one years of rawinsonde data were used together with 8 years of uninitialized European Centre for Medium-Range Weather Forecasts (ECMWF) analyses to describe the climatological structure of large-scale circulations adjacent to monsoon regions in Northern and Southern Hemisphere summers. In the upper troposphere and lower stratosphere, equatorward and poleward motions occur on the eastern and western sides of monsoon regions, respectively. It is shown that significant meridional velocities (>1 ms super(-) super(1) ) penetrate the lower stratosphere up to a maximum height of 50-30 mbar. Largest meridional velocities are observed in connection with the Asian monsoon in northern summer. Although evanescent in height, these motions are relatively important for horizontal transport of constituents in the summer lower stratosphere, when planetary waves are otherwise small. Asian and Mexican monsoons in this season are displaced sufficiently far from the equator, in close proximity to the tropopause break, to have a significant role in stratosphere-troposphere (S/T) exchange. The companion paper by Chen (1995) provides evidence of irreversible S/T exchange in the ``upper middle world'' during northern summer.
Fasullo, J. and P. J. Webster (1999). "Warm pool SST variability in relation to the surface energy balance." Journal of Climate, Boston, MA 12(5, Pt. 1): 1292-1305.
The warm tropical oceans underlie the most convective regions on Earth and are a critical component of the Earth's climate, yet there are differing opinions on the processes that control warm pool SST. The Indo-Pacific warm pool is characterized by large-scale variations in SST approaching 30 degrees C on intraseasonal timescales. In this study, surface heat flux anomalies associated with composite warm episodes over three spatial scales in both the Pacific and Indian Ocean basins are examined. The current study benefits from the recently available National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis dataset that enables the examination of variability in surface evaporation with moderate confidence. Solar flux estimates from the reanalysis are somewhat less reliable than evaporation estimates, however, and two techniques that infer surface shortwave radiation from satellite retrievals of cloud properties are considered. Error in all measurements is quantified. Both shortwave and evaporative flux variability play significant roles in modifying the temperature of the warm pool, though the relative importance of individual flux anomalies depends on SST tendency and geographical location. There also exist differences in the relative heating roles of the flux anomalies among episodes within a fixed location, though in instances the resolved differences are less than likely flux estimation error. Differences also exist between the ocean basins. A more pronounced annual cycle exists in the eastern Indian Ocean, and SST there is less sensitive to surface thermal forcing. Finally, the analysis offers evidence that SST is not regulated by a simple atmospheric thermodynamic response to the surface. Instead, the relationship between warm pool variability and large-scale dynamical features of the Tropics (e.g., intraseasonal oscillation and the seasonal monsoon) is demonstrated. The conclusions are shown to be robust to spatial scale and are consistent with a recent analysis of Tropical Oceans and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment observations.
Ferranti, L., J. M. Slingo, et al. (1997). "Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integrations." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 123(541): 1323-1357.
Monsoon variability on intraseasonal and interannual time-scales is analysed using data from five 10-year European Centre for Medium-Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. The results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. The spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20 degrees N to 5 degrees N, covering most of the land areas, with the other band, of opposite sign, lying between 5 degrees N and 10 degrees S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India. On the intraseasonal time-scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3-component chaotic model. Northward-propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. The timing of the monsoon onset appears to be modulated by the phase of the El Nino/Southern Oscillation during the preceding season, consistent with observational studies. The results suggest that the dominant mode may also represent some components of the observed monsoon variability. The interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad-scale monsoon wind-shear index.
Fieux, M. (1985). "Observational strategy for TOGA in the tropical Indian Ocean." World Meteorological Organization, Geneva, WCRP Publications Series No(65).
As an introduction to the observational strategy for TOGA in the tropical Indian Ocean, the author presents current knowledge of the deep atmospheric convection in the eastern Indian Ocean, its linkage to high sea surface temperatures that affect convergence and the monsoon, the effect of the shift of the convective zone during an El Nino year, the SST variability as a response of the ocean to the variability of the large-scale atmospheric forcing, the highest seasonal variability of the tropical oceans characterizing the western Indian Ocean together with the Somali currents, the highly variable equatorial circulation peculiar to the Indian Ocean, the opposite features of the zonal thermoclines in the Indian Ocean and in the Pacific Ocean, the interannual variability superimposed on the seasonal variability, the association of the interannual variability with El Nino-Southern Oscillation in the Indian Ocean, and the evidence that the most pronounced anomalies of the thermocline are phase locked with the seasonal cycle. The large-scale monitoring network and the observational techniques required to understand the processes controlling SST changes and upper ocean content on intraseasonal, annual, and interannual time scales, and in particular, to determine the relative importance of advection, upwelling, surface fluxes, and heat storage, are described. The observational strategy of large-scale monitoring involves an internationally managed XBT program, a drifting buoys program, a direct sea level measurement program, satellite data collection and analysis, and a voluntary observing ship (VOS) program. Also, process-oriented studies are noted briefly.
Fink, A. (1995). "The physical causes for the variability of intraseasonal tropical convection fluctuations over the Indo-pacific." Koeln, Germany, Universitaet zu Koeln 118.
In the present study the role of variations in tropical sea surface temperature (SST) and atmospheric precipitable water as well as the phase of El Nino-Southern Oscillation (ENSO) for the interannual variability of the convective signal of the Madden-Julian-Oscillation (MJO) is investigated. Anomalous convective activity of MJO is estimated by the difference between the seasonal standard deviation of the 25-70 day filtered OLR and the long-term seasonal mean for the period 1974 through 1994. The respective roles of the forcing mechanisms mentioned above are considered for the year-to-year variability of both the seasonal anomalies of mean convection and the seasonal variations in convective activity associated with MJO-events. Interannual variability in tropical SST and the phase of ENSO are highly correlated with seasonal anomalies of mean convection as well as with seasonal anomalies of MJO-activity over the entire equatorial Pacific from 160 omicron E to the coast of South America. When SST occasionally exceeds 27 omicron C in the eastern equatorial Pacific and /or the climatological low-level divergence disappears in the central equatorial Pacific, the eastward propagating convective MJO-events are observed to penetrate farther into the central and eastern Pacific. This behaviour is typical for, but not exclusively observed during, El Nino seasons. Over the maritime continent deep convection is significantly supressed during El Nino years. A corresponding signal in the MJO-activity index is confined to the Philippine Sea and the Australian monsoon region during boreal winter. It is shown that the MJO-activity within the region of its climatological maximum, the eastern Indian Ocean, was below normal during El Nino winters 1976/77, 1982/83 and 1986/87. However, during the ENSO warm events 1991/1992 and 1993 MJO-activity was close to average over the eastern Indian Ocean. A different stratification of the five El Ninos with respect to anomalies in intraseasonal convection is obtained for the western Pacific warm pool. The atmospheric water vapor content is closly related to seasonal convection anomalies at most locations over the Indian and Pacific Ocean. The intensity of the high-frequency convective perturbations is also significantly correlated to seasonal anomalies in precipitable water. In contrast, convective activity of MJO shows no relation to variations in the amount of precipitable water. Since MJO is composed of a hierarchy of cloud clusters with time scales ranging from 1 to 14 days, which are found to be sensitive to the amount of precipitable water, it is concluded that the interannual variability of MJO over the Indian Ocean and western Pacific ,, warm pool[`] [`] is largely determined by its frequency of occurrence rather than by modulations of its amplitude. This is consistent with rather abrupt changes in periodicity of MJO, which can neither be related to changes in SST in the western Pacific warm pool nor to the phase of ENSO. An indication of a possible mechanism which is linked to the frequency of MJO-events is obtained from the statistically significant correlation between near-surface westerly wind anomalies and increased convective activity of MJO-events which is found in the western Pacific Ocean north of the equator. A possible connection between seasonal westerly wind anomalies and the occurrence of cold surges as a possible extratropical forcing mechanism of MJO is discussed.
Flatau, M., P. Flatau, et al. (1998). "Intraseasonal oscillations and Asian monsoon onset." Conference on the TOGA Coupled Ocean Atmosphere Response Experiment.
The Asian Summer Monsoon exhibits substantial interannual variability (?) which has profound social and economic consequences. This variability is intimately related to the phase of the El Nino /Southern Oscillation (ENSO) but also depends on the regional and intraseasonal behavior of the monsoon. In this paper we present the case of the multiple monsoon onset of 1995, triggered by the passage of the strong MJO episode in early May. The development the initial bogus onset was associated with a delay of the ``real'' onset and severe drought in India at the beginning of June. The delayed onset of monsoon in 1995 (Weller et al., 1998) was preceded by the development of circulation resembling summer monsoon in the middle of May with fairly strong south-westerly flow over Bay of Bengal and Arabian Sea, cross-equatorial flow east of Africa, pressure trough developing over Himalayas and convection in the northern Bay of Bengal. However, by late May, south-westerlies weakened, or even reverse, pressure trough filled and convection over Bay of Bengal disappeared. Monsoon condition started to develop again in early June in the south-eastern part of Bay of Bengal. The monsoon arrived at southern tip of India (the Kerala Coast) on June 8.
Flatau, M. K., P. J. Flatau, et al. (2001). "The Dynamics of Double Monsoon Onsets." Journal of Climate 14(21): 4130-4146.
Double monsoon onset develops when the strong convection in the Bay of Bengal is accompanied by the monsoonlike circulation and appears in the Indian Ocean in early May, which is about 3 weeks earlier than the climatological date of the onset (1 Jun). The initial "bogus onset" is followed by the flow weakening or reversal and clear-sky and dry conditions over the monsoon region. The best example of such a phenomenon is the development of the summer monsoon in 1995, when monsoonlike perturbations that appeared in mid-May disappeared by the end of the month and were followed by a heat wave in India, delaying onset of the monsoon. The climatology of double onsets is analyzed, and it is shown that they are associated with delay of the monsoon rainfall over India. This analysis indicates that the development of bogus onsets depends on the timing of intraseasonal oscillation in the Indian Ocean and the propagation of convective episodes into the western Pacific. There is evidence that an SST evolution in the Bay of Bengal and the western Pacific plays an important role in this phenomenon. It is shown that in the case of the double monsoon onset it is possible to predict hot and dry conditions in India before the real monsoon onset. In the 32 yr of climatological data, six cases of double monsoon onset were identified.
Frederiksen, J. S. and C. S. Frederiksen (1993). "Monsoon disturbances, intraseasonal oscillations, teleconnection patterns, blocking, and storm tracks of the global atmosphere during January 1979: linear theory." Journal of the Atmospheric Sciences, Boston, MA 50(10): 1349-1372.
The results of a study are presented that indicate that a wide variety of atmospheric disturbances, including those associated with storm tracks and blocking in both hemispheres, quasi-stationary global teleconnection patterns, and localized monsoon disturbances, as well as intraseasonal oscillations, may be generated through the instability of the three-dimensional global basic state for January 1979 including a wave-CISK cumulus heating parameterization. The analysis has been conducted with a two-level primitive equation eigenvalue model, and the growing disturbances for various specifications of the strengths of the cumulus heating have been analyzed. Within the parameter range studied, inclusion of explicit moisture in the basic state has little effect on the structures of the storm track and onset-of-blocking modes in both hemispheres, but it increases growth rates as expected. It is also responsible for generating a significant amplitude of the tropical shear streamfunction of low-frequency and quasi-stationary teleconnection pattern modes, particularly in the Australian/South Pacific region where a coupling to the Australian monsoon appears to occur. Remarkable localized quasi-stationary monsoon disturbances of fairly small scale are found, which tend to produce either break or active periods of the Australian monsoon depending on their phase. The CISK heating focuses these modes in the Australian region and increases their growth rates. A group of intraseasonal oscillation modes with quite complex structures and periods between about 20 and 60 days is also found. They have eastward-propagating velocity potentials, peaking in the equatorial regions with a zonal wave number 1, and possibly 2, envelope wave within which are embedded smaller-scale structures. They have equivalent barotropic streamfunctions in extratropical regions with a baroclinic structure, typical of the first internal mode, in the tropical regions. At certain phases they can produce break or active periods of the Australian monsoon. Both a three-dimensional large-scale basic-state flow and a cumulus heating parameterization appear to be necessary for generating, through instability, intraseasonal oscillation modes with realistic structures.
Frederiksen, J. S. and C. S. Frederiksen (1997). "Mechanisms of the formation of intraseasonal oscillations and Australian monsoon disturbances: the roles of convection, barotropic and baroclinic instability." Contributions to Atmospheric Physics [Beitraege zur Physik der Atmosphaere.], Wiesbaden, Germany 70(1): 39-56.
A study is made of the roles of barotropic instability, baroclinic instability and convection in the formation and structures of intraseasonal oscillations and low frequency anomalies. The analysis has been conducted with barotropic and two-level primitive equation models using as basic states the three-dimensional global flow for January 1979, including a Kuo-type cumulus convection parameterization and observed moisture mixing ratio in the baroclinic model. The resulting moist static stability parameter, used in this study, is positive everywhere so that pure wave-CISK instability is not possible. Anomaly correlations have been calculated to provide quantitative measures of the similarity of barotropic modes and dry and moist low frequency baroclinic modes. The differences and similarities in the tropical and extra-tropical structures of the modes have been examined for the different cases. Baroclinic processes appear to be essential for generating small scale localized stationary Australian monsoon disturbances; both the horizontal and vertical shears and moisture appear to be important in determining the structures, growth rates and periods of these modes. Barotropic instability of the zonally varying 300 hPa basic state streamfunction produces a number of modes with qualitatively similar extra-tropical structure to baroclinic model 20-30 day oscillations, 40-60 day intraseasonal oscillations and quasi-stationary teleconnection patterns. These tend to be of slightly larger scale than both corresponding dry and moist baroclinic modes. Dry baroclinic intraseasonal oscillation and quasi-stationary teleconnection pattern modes have relatively poor vertical structures in the tropical regions. A baroclinic three-dimensional basic state and a cumulus convection parameterization are required to capture the crucial first internal mode structure in the tropical regions of intraseasonal oscillations.
Fu, X., B. Wang, et al. (2002). "Impacts of Air-Sea Coupling on the Simulation of Mean Asian Summer Monsoon in the ECHAM4 Model." Monthly Weather Review 130(12): 2889-2904.
Atmosphere-ocean coupling was found to play a critical role in simulating the mean Asian summer monsoon and its climatological intraseasonal oscillation (CISO) in comparisons of the results from a stand-alone ECHAM4 atmospheric general circulation model (AGCM) and a coupled ECHAM4-ocean [Wang-Li-Fu (WLF)] model. The stand-alone simulation considerably overestimates the equatorial Indian Ocean rainfall and underestimates monsoon rainfall near 15 degree N, particularly over the eastern Arabian Sea and the Bay of Bengal. Upon coupling with an ocean model, the simulated monsoon rainfall becomes more realistic with the rainbelt near 15 degree N (near the equator) intensified (reduced). These two rainbelts are connected by the northward-propagating CISOs that are significantly enhanced by the air-sea interactions.Both local and remote air-sea interactions in the tropical Indian and Pacific Oceans contribute to better simulation of the Asian summer monsoon. The local impact is primarily due to negative feedback between SST and convection. The excessive rainfall near the equatorial Indian Ocean reduces (increases) the downward solar radiation (upward latent heat flux). These changes of surface heat fluxes cool the sea surface upon coupling, thus reducing local rainfall. The cooling of the equatorial Indian Ocean drives an anticyclonic Rossby wave response and enhances the meridional land-sea thermal contrast. Both strengthen the westerly monsoon flow and monsoon rainfall around 15 degree N. The local negative feedback also diminishes the excessive CISO variability in the equatorial Indian Ocean that appeared in the stand-alone atmospheric run. The remote impact stems from the reduced rainfall in the western Pacific Ocean. The overestimated rainfall (easterly wind) in the western North (equatorial) Pacific cools the sea surface upon coupling, thus reducing rainfall in the tropical western Pacific. This reduced rainfall further enhances the Indian monsoon rainfall by strengthening the Indian-Pacific Walker circulation. These results suggest that coupling an atmospheric model with an ocean model can better simulate Asian summer monsoon climatology.
Fujinami, H. and T. Yasunari (2001). "The Seasonal and Intraseasonal Variability of Diurnal Cloud Activity over the Tibetan Plateau." Journal of the Meteorological Society of Japan 79(6): 1207-1227.
Seasonal variation of diurnal cloud activity (abbreviated DCA) over the Tibetan Plateau throughout the year is examined using 3-hourly geostationary meteorological satellite (GMS) data for 6-years (1989-1994). The DCA shows two distinct variance maxima in the seasonal cycle. One is in spring (pre-monsoon season), and the other is in the summer monsoon season. The DCA begins in late January, and reaches its maximum from March through April. The active DCA extends over almost the whole of the plateau, especially over the southern part (around 30 degree N, 90 degree E) and the zonally oriented belt between 35 degree N, 80 degree E and 31 degree N, 102 degree E. A short interval between the two active DCAs is found around late May to early June. The DCA starts again over the southeastern region of the plateau (centered around 30 degree N, 101 degree E) in June and moves up to the southern region. From July to August, the DCA is most active over the southern region (around 30 degree N, 86 degree E). After September, the active DCA retreats to the southeastern region. In both spring and summer, the amplitude of the DCA fluctuates on intra-seasonal time scales. In the active period of the DCA in spring, corresponding to the meandering of the upper-level mid-latitude westerly to the south of the plateau with a trough, the cold air mass at the upper-level is clarly seen over the plateau and weak wind speed is observed through the plateau troposphere. This atmospheric structure and a heating of the lower atmosphere during daytime are likely to be responsible for the enhancement of the DCA in spring. In contrast, during summer monsoon season, the increase of humidity and temperature are identified at the lower atmosphere over the plateau, associated with a humid and warm air intrusion from the South Asian monsoon area into the lower atmosphere over the plateau and precipitation due to active convection. This indicates an increase of the instability for moist convection. These features of the atmospheric circulation and the surface heating during daytime induce active moist convection. Corresponding to the enhanced DCA over the southern part of the plateau, the center of the Tibetan high is located over there. A possible mechanism for the intraseasonal variability of the DCA associated with that of the upper-level atmospheric circulation is also discussed.
Gadgil, S. (1990). "Poleward propagations of the ITCZ: observations and theory." Mausam, New Delhi, India 41(2): 285-290.
Many features of the intraseasonal variation of the ITCZ over the Indian longitudes in the summer monsoon including poleward propagations emanating from oceanic regions and traversing both ocean and land to culminate in the seasonal trough have been simulated by a modified version of the Webster model. The mechanism underlying the propagations in the model involves excess convective heating to the poleward side of the convergence zone relative to the equatorward zone and can operate on land or sea. This differential arises in the model from the seasonal gradient of the convective instability and moisture availability and hence the intensity factor of convective heating which is the product of the two factors. If the ratio of the convective heating to the vertical ascent in reality also depends on these two factors, the observed propagations could be generated by the same mechanism.
Gadgil, S. and G. Asha (1992). "Intraseasonal variation of the Indian summer monsoon. I: Observational aspects." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 517-527.
Within the summer monsoon, the circulation and rainfall over the Indian region exhibit large variations over the synoptic scale of 3-7 days and the supersynoptic scales of 10 days and longer. In this paper we discuss some facets of intraseasonal variation on the supersynoptic scale on the basis of existing observational studies and some new analysis. The major variation of the summer monsoon rainfall on this scale is the active-break cycle. The deep convection over the Indian region on a typical day in the active phase is organized over thousands of kilometers in the zonal direction and is associated with a tropical convergence zone (TCZ). The intraseasonal variations on the supersynoptic scale are also coherent on these scales and are related to the space-time variation of the large-scale TCZ. The latitudinal distribution of the occurrence of the TCZ is bimodal with the primary mode over the heated continent and a secondary mode over the ocean. The variation of the continental TCZ is generally out of phase with that of the oceanic TCZ. During the active spells, the TCZ persists over the continent in the monsoon zone. The revival from breaks occurs either by northward propagation of the TCZ over the equatorial Indian Ocean or by genesis of a disturbance in the monsoon zone (often as a result of westward propagations from W. Pacific). The mechanisms governing the fluctuation between active spells and breaks, the interphase transition and the complex interactions of the TCZ over the Indian subcontinent with the TCZ over the equatorial Indian Ocean and the W. Pacific, have yet to be completely understood.
Gagdil, S. (1988). "Recent advances in monsoon research with particular reference to the Indian monsoon." Australian Meteorological Magazine, Canberra 36(3): 193-204.
The major advances in the understanding of the structure of the Indian summer monsoon and its intraseasonal and interannual variations are discussed. The large-scale monsoon rainfall is shown to be associated with a tropical convergence zone (TCZ) having the dynamic characteristics of the ITCZ discussed by Charney (1969). The intraseasonal and interannual variations of the monsoon rainfall arise from the space-time variations of the TCZ. The prominent scales of intraseasonal variations between active spells and breaks are identified as the 15-day scale (associated with westward propagation of synoptic-scale disturbances) and the 40-day scale (associated with northward propagation of the TCZ). Detailed analysis of satellite imagery reveals that the most prominent feature of the intraseasonal variation over the Indian longitudes is the northward propagation of the TCZ from the equatorial Indian ocean onto the heated continent. So far such poleward propagations have not been reported for any other part of the Tropics. Simple models capable of simulating the seasonal transitions as well as the intraseasonal fluctuations of the monsoon have been developed; however, the underlying mechanisms are yet to be fully understood. The structure of the variations on the interannual scale is found to be markedly similar to that on the intraseasonal scale. Thus, analysis of the interrelationship of the continental TCZ with those over the Indian and the Pacific oceans on the intraseasonal scale, should provide insight into the interannual variations as well. On the interannual scale, links between the Asian monsoon and conditions over the Pacific as manifested by the association between the El Nino and Indian droughts have been well established. The relationship between SST and cloudiness over the tropics is shown to be rather complex, with a threshold of 28 degrees C for organized convection. With deeper understanding of the dynamics of the tropical convergence zones, more light is bound to be shed on the intraseasonal and interannual variations of the monsoon in the near future.
Gao, H., J. He, et al. (2001). "Definition of 40-year onset date of South China Sea Summer Monsoon." Journal of Nanjing Institute of Meteorology 24(3): 379-383.
Analysis of 1958 similar to 1997 NCEP/NCAR pentad dataset from April to June indicates that, the time when the average theta sub(se) 850hPa greater than or equal to 335 K and the negative zonal wind changes into positive stably in area (10 similar to 20 degree N, 110 similar to 120 degree E) has a good correspondence with the onset date of South China Sea Summer Monsoon and can be used as a norm to judge the latter. Above condition should last at least 3 pentads and the following discontinuous time be less than 2 pentads, or last 2 pentads then has one-pentad-break then resume at once.
Gopalakrishna, V. V. (1988). "Variability of wind stress and currents at selected locations over the north Indian Ocean during 1977 and 1979 summer monsoon seasons." Mausam, New Delhi 39(2): 159-166.
Intraseasonal variability of wind stress, wind stress curl, and currents at different locations over the northern Indian Ocean during two contrasting monsoon seasons has been investigated by making use of the time series data collected during MONSOON-77 and MONEX-79. In general, the surface wind stress and near-surface current fields are strong during a good monsoon year (1977) as compared to those in a bad monsoon year (1979) under similar synoptic conditions, together with the prevalence of high and low intensities during active-preonset and break phases, respectively. The wind stress curl is found to be more negative and stronger in the Arabian Sea in the active phase, whereas it is closer to zero during break-monsoon conditions in the Bay of Bengal. A better association exists between wind stress and near-surface currents in the Arabian Sea than in the Bay of Bengal.
Goswami, B. N. (1994). "Dynamical predictability of seasonal monsoon rainfall: problems and prospects." Das, P. K.
The conceptual basis and the current status of dynamical prediction of seasonal mean monsoon rainfall are reviewed. The monsoon may be viewed as a combination of a planetary scale wave number one circulation and a regional fluctuating Hadley circulation. The planetary scale component is primarily governed by the sea surface temperature in the Pacific and the general circulation models are successful in simulating it. The models, however, have significant systematic errors in simulating the regional component which seems to be sensitive to a number of physical processes. It is noted that the simulation of mean rainfall over the Indian region is quite sensitive to initial conditions while those over the Pacific or Sahel are not. It is hypothesised that, while over most of the tropics the low frequency variability is governed mainly by slowly varying boundary conditions, the internal dynamics play an important role in determining even the low frequency variability over the Indian region. Evidence that there are strong intraseasonal oscillations over the Indian region during the Northern Hemisphere summer and that these intraseasonal oscillations arise due to internal dynamics are presented. A conceptual picture of the intraseasonal oscillations of the monsoon is presented and it is indicated that several factors of comparable strength may influence the intraseasonal oscillations. Due to these inherent limitations, new techniques such as ensemble forecasting have to be evolved for better prediction.
Goswami, B. N. (1998). "Interannual variations of Indian summer monsoon in a GCM: external conditions versus internal feedbacks." Journal of Climate, Boston, MA 11(4): 501-522.
The potential predictability of the Indian summer monsoon due to slowly varying sea surface temperature (SST) forcing is examined. Factors responsible for limiting the predictability are also investigated. Three multiyear simulations with the R30 version of the Geophysical Fluid Dynamics Laboratory's climate model are carried out for this purpose. The mean monsoon simulated by this model is realistic including the mean summer precipitation over the Indian continent. The interannual variability of the large-scale component of the monsoon such as the ``monsoon shear index'' and its teleconnection with Pacific SST is well simulated by the model in a 15-yr integration with observed SST as boundary condition. On regional scales, the skill in simulating the interannual variability of precipitation over the Indian continent by the model is rather modest and its simultaneous correlation with eastern Pacific SST is negative but poor as observed. The poor predictability of precipitation over the Indian region in the model is related to the fact that contribution to the interannual variability over this region due to slow SST variations [El Nino-Southern Oscillation (ENSO) related] is comparable to those due to regional-scale fluctuations unrelated to ENSO SST. The physical mechanism through which ENSO SST tend to produce reduction in precipitation over the Indian continent is also elucidated. A measure of internal variability of the model summer monsoon is obtained from a 20-yr integration of the same model with fixed annual cycle SST as boundary conditions but with predicted soil moisture and snow cover. A comparison of summer monsoon indexes between this run and the observed SST run shows that the internal oscillations can account for a large fraction of the simulated monsoon variability. The regional-scale oscillations in the observed SST run seems to arise from these internal oscillations. It is discovered that most of the interannual internal variability is due to an internal quasi-biennial oscillation (QBO) of the model atmosphere. Such a QBO is also found in the author's third 18-yr simulation in which fixed annual cycle of SST as well as soil moisture and snow cover are prescribed. This shows that the model QBO is not due to land-surface-atmosphere interaction. It is proposed that the model QBO arises due to an interaction between nonlinear intraseasonal oscillations and the annual cycle. Spatial structure of the QBO and its role in limiting the predictability of the Indian summer monsoon is discussed.
Goswami, B. N. and R. Mohan (2001). "Intraseasonal oscillations and interannual variability of the Indian summer monsoon." Journal of Climate, Boston, MA 14(6): 1180-1198.
How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956-97) daily circulation data from National Centers for Environmental Prediction-National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974-97. Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define ``active'' and ``break'' monsoon conditions. The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique. A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase. A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in the northern position of the tropical convergence zone (TCZ) and decreased (increased) convection and anomalous downward (upward) motion in the southern position of the TCZ. The cycle evolves with a northward propagation of the TCZ and convection from the southern to the northern position of the TCZ. It is shown that the intraseasonal and interannual variations are governed by a common mode of spatial variability. The spatial pattern of standard deviation of intraseasonal and interannual variability of low-level vorticity is shown to be similar. The spatial pattern of the dominant mode of ISO variability of the low-level winds is also shown to be similar to that of the interannual variability of the seasonal mean winds. The similarity between the spatial patterns of the two variabilities indicates that higher frequency of occurrence of active (break) conditions would result in ``stronger'' (``weaker'') than normal seasonal mean. This possibility is tested by calculating the two-dimensional probability density function (PDF) of the ISO activity in the low-level vorticity. The PDF estimates for ``strong'' and ``weak'' monsoon years are shown to be asymmetric in both the cases. It is seen that the strong (weak) monsoon years are associated with higher probability of occurrence of active (break) conditions. This result is further supported by the calculation of PDF of ISO activity from combined vorticity and outgoing longwave radiation. This clear signal indicates that the frequency of intraseasonal pattern determines the seasonal mean. Because the ISOs are essentially chaotic, it raises an important question on predictability of the Indian summer monsoon.
Goswami, B. N. and R. S. A. Mohan (2001). "Intraseasonal Oscillations and Interannual Variability of the Indian Summer Monsoon." Journal of Climate 14(6): 1180-1198.
How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956-97) daily circulation data from National Centers for Environmental Prediction-National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974-97. Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define "active" and "break" monsoon conditions. The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique. A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase. A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in the northern position of the tropical convergence zone (TCZ) and decreased (increased) convection and anomalous downward (upward) motion in the southern position of the TCZ. The cycle evolves with a northward propagation of the TCZ and convection from the southern to the northern position of the TCZ. It is shown that the intraseasonal and interannual variations are governed by a common mode of spatial variability. The spatial pattern of standard deviation of intraseasonal and interannual variability of low-level vorticity is shown to be similar. The spatial pattern of the dominant mode of ISO variability of the low-level winds is also shown to be similar to that of the interannual variability of the seasonal mean winds. The similarity between the spatial patterns of the two variabilities indicates that higher frequency of occurrence of active (break) conditions would result in "stronger" ("weaker") than normal seasonal mean. This possibility is tested by calculating the two-dimensional probability density function (PDF) of the ISO activity in the low-level vorticity. The PDF estimates for "strong" and "weak" monsoon years are shown to be asymmetric in both the cases. It is seen that the strong (weak) monsoon years are associated with higher probability of occurrence of active (break) conditions. This result is further supported by the calculation of PDF of ISO activity from combined vorticity and outgoing longwave radiation. This clear signal indicates that the frequency of intraseasonal pattern determines the seasonal mean. Because the ISOs are essentially chaotic, it raises an important question on predictability of the Indian summer monsoon.
Goswami, B. N., D. Sengupta, et al. (1998). "Intraseasonal oscillations and interannual variability of surface winds over the Indian monsoon region." Proceedings of the Indian Academy of Sciences, Bangalore, India 107(1): 45-64.
The role of intraseasonal oscillations (ISOs) in modulating synoptic and interannual variations of surface winds over the Indian monsoon region is studied using daily averaged National Centers for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalyses for the period 1987-1996. Two dominant ISOs are found in all years, with a period between 30-60 days and 10-20 days respectively. Although the ISOs themselves explain only about 10-25% of the daily variance, the spatial structure of variance of the ISOs is found to be nearly identical to that of high frequency activity (synoptic disturbances), indicating a significant control by the ISOs in determining the synoptic variations. Zonal and meridional propagation characteristics of the two modes and their interannual variability are studied in detail. The synoptic structure of the 30-60 day mode is similar in all years and is shown to be intimately related to the strong (`active') or weak (`break') phases of the Indian summer monsoon circulation. The peak (trough) phase of the mode in the north Bay of Bengal corresponds to the `active' (`break') phase of monsoon strengthening (weakening) the entire large scale monsoon circulation. The ISOs modulate synoptic activity through the intensification or weakening of the large scale monsoon flow (monsoon trough). The peak wind anomalies associated with these ISOs could be as large as 30% of the seasonal mean winds in many regions. The vorticity pattern associated with the 30-60 day mode has a bi-modal meridional structure similar to the one associated with the seasonal mean winds but with a smaller meridional scale. The spatial structure of the 30-60 day mode is consistent with fluctuations of the tropical convergence zone (TCZ) between one continental and an equatorial Indian Ocean position. The 10-20 day mode has maximum amplitude in the north Bay of Bengal, where it is comparable to that of the 30-60 day mode. Elsewhere in the Indian Ocean, this mode is almost always weaker than the 30-60 day mode. In the Bay of Bengal region, the wind curl anomalies associated with the peak phases of the ISOs could be as large as 50% of the seasonal mean wind curl. Hence, ISOs in this region could drive significant ISOs in the ocean and might influence the seasonal mean currents in the Bay. On the interannual time scale, the NCEP/NCAR reanalysed wind stress is compared with the Florida State University monthly mean stress. The seasonal mean stress as well as interannual standard deviation of monthly stress from the two analyses agree well, indicating absence of any serious systematic bias in the NCEP/NCAR reanalysed winds. It is also found that the composite structure of the 30-60 day mode is strikingly similar to the dominant mode of interannual variability of the seasonal mean winds indicating a strong link between the ISOs and the seasonal mean. The ISO influences the seasonal mean and its interannual variability either through increased/decreased residence time of the TCZ in the continental position or through occurrence of stronger/weaker active/break spells. Thus, the ISOs seem to modulate all variability in this region from synoptic to interannual scales.
Goswami, P. and V. Mathew (1994). "A mechanism of scale selection in tropical circulation at observed intraseasonal frequencies." Journal of the Atmospheric Sciences, Boston, MA 51(21): 3155-3166.
It is shown in this work that two prominent intraseasonal oscillations of the tropical atmosphere--namely, the 3-4 day westward propagating wave observed over the equatorial Atlantic and Pacific oceans and the quasi-biweekly (or 10-20 day) oscillation observed over the Indian Ocean summer monsoon region--can be understood as arising from selective excitation of the tropical normal modes in the presence of moist feedbacks and a moisture relaxation timescale. Unlike in earlier studies of a similar nature, the central theme of the present work is that the dynamics of the moisture variable is governed by a moisture relaxation timescale tau . In other words, the large-scale flow is not in exact quasi equilbrium with the precipitational heating. The implications of this hypothesis are investigated by using a shallow-water model of the tropical atmosphere on an equatorial beta plane, with a fixed vertical structure. The two major findings of the present work are: 1) both the 3-4 day wave and the 10-20 day can be understood as intrisic modes of the tropical atmosphere, excited by the same basic mechanism namely, moist feedbacks in the presence of a moisture relaxation timescale, and 2) while the 3-4 day wave is represented by a maximally growing mixed Rossby gravity wave, driven selectively unstable by moist feedbacks, the 10-20-day wave represents a new mode of the tropical atmosphere that is excited by the moist feedbacks in the presence of mean westerlies. For both of these waves, the agreement between observed structure and theoretical predictions is excellent. Thus, the present work presents a mechanism that explains two major low-frequency tropical oscillations as intrinsic modes of the tropical atmosphere.
Grossman, R. L. and O. Garcia (1990). "The distribution of deep convection over ocean and land during the Asian summer monsoon." Journal of Climate, Boston, MA 3(9): 1032-1044.
The highly reflective cloud (HRC) dataset is a daily index of organized deep convection, at one degree resolution, from 17 years of polar-orbiting satellite imagery. These data are used to analyze and discuss the climatological geographical distribution of deep convection observed over the Asian summer monsoon season and its component months (June, July, August and September). Intraseasonal variations of convection for selected regions are examined using normalized pentad time series of regional median HRC values. We also compare HRC data over two regions (western coastal India and western coastal Burma/Thailand) with the results from a two-dimensional numerical model, consisting of a simple differentially heated land-ocean system which predicts that a preponderance of deep convection occurs over the coastal zone. The Burma/Thailand regional comparison supports the model result. Comparison of the model with the western coastal India region is less conclusive, which may be due to the limitations of the model. We conclude that monsoon deep convection, and its attendant sources of latent heat, momentum, and mass sources important to large-scale monsoon dynamics is localized and persistent from year to year. If, as hypothesized by others, tropical cumulonimbus activity is important to stratospheric-tropospheric exchange, this study shows the preferred areas of such exchange during the monsoon. The locations of areas with large HRC amounts are consistent with upstream lifting of low-level, conditionally unstable air by low, coastal mountains. Intraseasonal variability follows variations in sea surface temperature and low-level flow. Upper-level dynamics are also recognized as an important contribution.
Guan, Z. and G. Dong (1996). "Summer monsoon anomaly and diagnosis on upper-tropospheric vorticity budget." Journal of Nanjing Institute of Meteorology, Nanjing, China 19(1): 83-89.
Diagnosis is undertaken of the genesis and maintenance of interannual anomaly in summer monsoon with the aid of the departure vorticity equation in the context of 1980 similar to 1986 ECMWF 200hPa gridded u- and v-winds. Evidence suggests that the features of the interannual anomaly flowfield over the Asian monsoon region exhibit appreciable intraseasonal oscillation; the features, e.g., vigor and phase, as uncovered by the terms of the vorticity equation may differ more or less for different years and areas; striking difference exists in sign and magnitude of monthly means of the terms in this equation over China mainland for the drought and flood years; the climatology of monsoon circulation plays a critical role in the maintenance of interannual anomaly disturbance. Great difference in terms of the equation used for the late spring-early summer Meiyu rainfall may serve as a precursor of anomalous rainfall forecasting.
Gueremy, J.-F. (1989). "Heat and moisture fluxes on the time scale of 20 to 60 days over the Indian monsoon area." Annual Climate Diagnostics Workshop, 14th, La Jolla, CA, Oct.
Low-frequency variability of sensible and latent surface heat flux over the Indian Ocean has been computed by using an atmospheric thermodynamic budget and the similarity theory. The method requires a knowledge of the precipitation field and the outgoing long-wave radiation, and notable, the computation of the large-scale apparent heating and the large-scale apparent moisture sink; and a knowledge of the wind, the temperature, and the specific humidity in the surface layer. The major results are summarized: in the intraseasonal time scale, the variability of temperature and specific humidity is more important over land than over sea, mainly because of soil moisture which provides a low frequency variability to the land areas. The variance percentage of these two variables, for the time scale considered, is of the order of 30% over land, but only 20% over sea. In the case of the influence of different variables on the surface fluxes in the intraseasonal time scale, the wind was found to have a significant influence on surface fluxes over the oceans; but this is not quite evident over the land. Over the ocean, its influence seems to be more important for latent heat than for sensible heat flux. Of two other variables, the temperature appears to exert the greatest influence upon the fluxes, particularly over land. The northward propagation of low-frequency waves related to the surface fluxes and ground variables has been found to conform to the mechanism proposed by Webster, which involves surface hydrological effects to explain northward propagation of low frequency waves over India.
Gupta, G. R. and O. Prasad (1991). "Activity of southern Indian Ocean convergence zone as seen in satellite cloud data during pre-monsoon months." Mausam, New Delhi, India 42(2): 145-150.
The weekly mean cloud cover data for the pre-monsoon months of April and May over the Indian Ocean between 20 degrees S to 20 degrees N latitudes and 40 degrees E to 100 degrees E longitudes have been studied for three good monsoon years (1977, 1983, 1988) and three drought years (1972, 1979, 1987). It is shown that while the characteristics of weekly mean cloud cover data during pre-monsoon months are similar for all the good monsoon years, they varied from one drought year to another. The study reveals some of the interesting features of southwest monsoon. An overall negative relationship between southern Indian Ocean convergence zone (SIOCZ) and monsoon activity is indicated. While at intraseasonal scale this may only be a simultaneous association, the pre-monsoon activity of SIOCZ may possibly have long-range predictive potential to some extent, for Indian monsoon rainfall.
Gupta, H. N. and H. Ali (1987). "Study of twenty years' temperature data of Indian stations at 700, 500, and 300 mb for June, associated with break-monsoon in July." Mausam, New Delhi 38(4): 459-460.
Twenty years (1961-1980) of temperature data collected at Indian stations for June at 700, 500, and 300 mb were analyzed. It is observed that, in June, at 300 mb there is a weak thermal high along 25 degrees N over India; it is distorted in the years having a long break-monsoon in July by intrusion of cold air from the south over central India. A break in monsoon is a synoptic situation where the monsoon trough is located close to the foot of the Himalayas, which leads to a striking decrease of rainfall over most of the country but an increase along the Himalayas and parts of northeastern India and the southern Peninsula.
Gushchin, D., M. A. Petrosyants, et al. (1995). "Atmospheric circulation pattern during the summer monsoon in Northern Australia." Meteozologiya i Gidzologiya [Meteorology and Hydrology.], Moscow, Russia 2: 36-45.
Basing on the winter MONEX and ECMWF wind analysis data as well as on the satellite imageries, the atmospheric circulation in North Australia is studied for two typical stages of monsoon evolution in 1979, i.e. an active phase (February 2 to 10, i) and a break (January 26 to 31, ii). Vertical motions are computed from the continuity equation. Charts of average cloud cover, streamlines and wind speeds are presented. In (i), an equatorial trough is shifted to 20-22 degrees S in Australia; within the intertropical convergence zone, a cyclonic eddy occurs near the equator. As a result, a well-defined band of westerlies is formed at 850 mb level, with the wind speeds comparable with those in the Indian monsoon. Composed profiles of zonal and meridional winds show that steady westerly winds up to 10 m/s occur in the troposphere up to the 200 mb level, above which, easterly winds >10 m/s are observed. Meridional component, 5 to 7 m/s, is directed to the south in the whole troposphere. In general, the circulation pattern of (i) in the North Australia, including distribution of ascending motions, is similar to that in the centre of the Indian Ocean during active stage of the Indian monsoon. For (ii), an anticyclonic pressure field is observed in the region under study, with slight easterly winds and descending motions, while in the equatorial Indian Ocean, cloud formation and precipitation are very active. The water vapour in (i) is transferred to Australia from the Northern Hemisphere; in (ii), the inter-hemispheric exchange has an opposite direction.
Gushchina, D., M. A. Petrosyants, et al. (1995). "Atmospheric circulation pattern of the northern Australian summer monsoon." Russian Meteorology and Hydrology, New York, NY 2: 27-34.
Wnter MONEX observations have been used to construct atmospheric circulation charts during an active phase and a break of monsoon. The vertical velocity fields and water vapor transfers are calculated for the opposite phases. Conditions necessary for the intensification of the monsoon circulation and conditions resulting in the monsoon break are analyzed. Analysis of the maps of vertical velocities has shown that processes in the Australian monsoon are opposite in phase to the processes in the Indian monsoon and in the central Indian Ocean. The results of calculations of the meridional and zonal transfer of water vapor are used to identify the sources of moisture determining the water potential of the monsoon.
Haldar, G. C. and A. M. Sud (1987). "Rainfall over central parts of India during the break monsoon conditions." Mausam, New Delhi 38(1): 113-118.
In the central parts of India, there are occasions when fairly widespread to widespread rains with isolated heavy falls occur, although the lower tropospheric flow pattern exhibits a break in the monsoon. The authors bring out the presence of significant synoptic features at 700- and 500-mb levels which could be associated with such rainfall activity. The thermal patterns existing during such periods have also been studied.
Hamilton, M. G. (1977). "Some aspects of break and active monsoon over southern Asia during summer." Tellus, Stockholm 29(4): 335-344.
At least three synoptic phases of the summer monsoon can be identified north of 10 degrees N. Two of these phases--the break and active monsoon--are extremes. Patterns of cloudiness and rainfall for these two phases differ substantially. These indicate differences in the geographic location and intensity of tropospheric energy sources. Tropospheric circulations over the equatorial Indian Ocean during break and active monsoon also differ. A break is often accompanied by the development of a trough in the lower troposphere northeast of Gan; this appears to enhance rainfall over southeast India. Although cause and effect are difficult to separate with scanty aerological data, synoptic variation over the equatorial Indian Ocean east of 60 degrees -65 degrees E may influence rainfall over western and southern India.
Hamilton, M. G. (1979). "Aspects of tropospheric structure over the Bay of Bengal during active and break monsoon over India in August 1977." Meteorological Magazine, Bracknell, Eng 108(1286): 253-260.
Details are given of tropospheric conditions over the Bay of Bengal which were associated with break and active monsoon spells over eastern India during Aug. 1977.
Hanstrum, B. N., G. Reader, et al. (1999). "The South Pacific and southeast Indian Ocean tropical cyclone season 1996-97." Australian Meteorological Magazine, Canberra, Australia 48(3): 197-210.
Tropical cyclone occurrences were above the long-term averages during the 1996-97 season. The weak La Nina phase which characterised 1996 declined and some early El Nino indicators emerged by the end of the season. These included the appearance of strong westerly equatorial wind anomalies near the date-line, increasing sea-surface temperature in the near-equatorial central and eastern Pacific, a fall of the Southern Oscillation Index to negative values and increased convective activity in the South Pacific convergence zone. The monsoon in the summer hemisphere was of average development or better, with strongest anomalies in the southwest Pacific. Three major cycles of the 30 to 60-day intraseasonal oscillation were diagnosed. The 26 cyclones that formed spanned every month (July 96-June 97) except August and September.
Harr, P. A. and R. L. Elsberry (1991). "Tropical cyclone track characteristics as a function of large-scale circulation anomalies." Monthly Weather Review, Boston, MA 119(6): 1448-1468.
Factors that contribute to intraseasonal variability in western North Pacific tropical cyclone track types are investigated. It is hypothesized that the 700-mb large-scale circulation can affect tropical cyclone track characteristics by enhancing or excluding genesis in certain regions, and concurrently prohibiting or favoring recurving versus straight tracks. A track-type climatology indicates that genesis location alone may explain some of the variability in track type. Although some genesis regions have no preference for straight-moving or recurving tracks, a formation north of 20 degrees N or east of 150 degrees E and north of 10 degrees N favors a recurvature track. These recurving storms are classified as recurving-north, and recurving storms that form in regions with nearly equal probability of straight or recurving tracks are classified as recurving-south. A compositing technique is used to define anomalous 700-mb large-scale circulations that exist during the formation of tropical cyclones that subsequently follow either a straight track or one of the two types of recurving tracks. Anomalous circulations associated with extended periods that do not contain any tropical cyclones are also identified. Physically and statistically different anomalous large-scale circulation patterns exist at the time of genesis for storms following each track type and for inactive periods. The large-scale anomalies describe variations in the positions and intensities of the monsoon trough and subtropical ridge. During genesis of straight-moving and recurving-south storms, anomalous large-scale horizontal cyclonic shear exists throughout the South China Sea and Philippine Sea. During straight-moving storms, cyclonic shear increases because of anomalous easterlies along the southern boundary of an enhanced subtropical ridge. During recurving-south storms, anomalous equatorial westerlies and cross-equatorial flow from the Southern Hemisphere act to increase the cyclonic shear. The track-type climatology is used to predict the subsequent track type based only on genesis location. In a second scheme, the distributions of anomalous 700-mb zonal wind components in 5 degrees latitude bands averaged between 100 degrees and 140 degrees E are used to predict the most likely track type. The large-scale 700-mb anomalies at genesis time determine the subsequent track type in a majority of cases. The skill of this simple scheme exceeds that from the climatological probability of track type.
Hartmann, D. L. and M. L. Michelsen (1989). "Intraseasonal periodicities in Indian rainfall." Journal of the Atmospheric Sciences, Boston 46(18): 2838-2862.
Spectral analysis of a 70-year (1901-70) record of daily precipitation from 3700 stations in the country of India is carried out to search for periodicities on subseasonal time scales during the summer monsoon. Two statistically significant spectral peaks are found. A 40-50 day spectral peak corresponding to the Madden-Julian Oscillation is found over most of the portion of India south of 23 degrees N. The phase of the oscillation is such that the precipitation maximum appears first over the relatively dry southeastern portion of the peninsula. Ten to 12 days later the precipitation peaks simultaneously all along the coast west of the Western Ghats and along a line running across India between 20 degrees and 25 degrees N. The precipitation maximum then spreads slowly northward and loses significance. Cross-spectral analysis shows strong coherence between the precipitation patterns and wind oscillations. The zonal wind oscillations at 850 and 200 mb are about 180 degrees out-of-phase equatorward of about 20 degrees N, but in-phase poleward of 20 degrees N. Compositing of the 40-50 day variance shows the structure of the wind variations associated with the precipitation cycle over India. At the southern tip and along the west coast of India precipitation variations on the 40-50 day time scale seem to be, at least in part, orographically controlled, with upslope winds simultaneous with the precipitation maxima. In central India the precipitation maxima are more closely related to large-scale divergence and convergence patterns, with the largest precipitation clearly associated with cyclonic circulations at low levels which have about the same horizontal structure as the precipitation maximum. In addition to the 40-50 day oscillation, significant spectral peaks and coherent structures in precipitation are found whose characteristics correspond to those of monsoon lows. These are associated with a peak in the precipitation spectrum between about 5 and 7 days, which is strongest on the Bay of Bengal coast near 21 degrees N. The 5-7 day oscillation in precipitation extends west-northwest across the breadth of India along the known track of monsoon lows. The phase of the oscillation indicates that the precipitation anomalies propagate westward across India with a phase speed of about 5 m sec super(-) super(1) . This oscillation explains as much as 20% of the variance of precipitation in some locations. Evidence for a spectral peak near 15 days is sparse, being limited to a few regions in the north of India where the 40-50 day oscillation does not dominate the low-frequency variance.
Hastenrath, S. and A. Rosen (1983). "Patterns of India monsoon rainfall anomalies." Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm 35(4): 324-331.
Pattern characteristics of annual rainfall anomalies over India during 1900-1972 are studied from precipitation records of 31 regional subdivisions and series of annual cyclone frequency and number of break-monsoon days. Methods include spatial correlation, pattern stratification, and principal component and spectral analyses. Pattern stratification with regard to an all-India rainfall index yields largest concordant departures in the central and western portions of the country. Pattern stratification with regard to the largest annual frequency of cyclonic storms in the Bay of Bengal and Arabian Sea show large positive rainfall departures over eastern, southeastern, and western India, and negative departures in the north and a limited area of the southwest; for the collective of years with smallest storm frequency, a partly inverse rainfall anomaly pattern is obtained. Pattern stratification with regard to the years of most numerous break-monsoon days yields deficient rainfall in a large, broadly zonally oriented band across central India and positive departures in most other parts of the country. Stratification with regard to the least numerous break-monsoon days yields a broadly inverse pattern. The first eight principal component patterns account for 74% of the total variance. The first component resembles the pattern obtained by stratification with regard to the all-India rainfall index, with the two time series being highly positively correlated. The third component is approximately inverse to the pattern constructed from stratification with regard to the most numerous break-monsoon days, with the two time series being negatively correlated. The seventh component is similar to the pattern derived from stratification with regard to the largest frequency of cyclonic storms; this is consistent with the positive correlation between the two time series. No physical significance is apparent for the other principal components. The combination of the various analysis techniques thus serves to identify spatial and temporal pattern characteristics of possible physical significance.
He, J. (1988). "The transfer of physical quantities in QDPO and its relation to the interaction between the NH and SH circulations." Advances in Atmospheric Sciences, Beijing, China 5(1): 97-106.
Based on the 1979 FGGE Level III b data, calculation is made of the transfer of sensible and latent heat and momentum due to a quasi-40-day periodic oscillation (QDPO) on a cross-equatorial meridional vertical cross-section, and analysis is done of the characteristics of the transfer at all phases of QDPO, with the following results obtained: 1) During the monsoon's QDPO activation and break phases, a strong transfer of sensible heat to the SH is felt in the upper troposphere over the Asian monsoon region; the conversion of perturbation effective potential into its kinetic energy attains its maximum at 500-300 hPa (15 degrees N), serving as the source of kinetic energy for the quasi-40-day periodic perturbation; an intense transfer of potential energy is found above 200 hPa from the monsoon area to the SH to maintain the QDPO at the tropical latitudes; 2) During the QDPO activation-break (and reverse) transitional phase the conversion of perturbation effective potential into kinetic energy reaches its maximum in the middle and lower troposphere over the SH middle latitudes and an appreciable lower transfer of potential energy occurs towards the SH tropical latitudes and the NH; 3) The upper-troposphere powerful transfer of westerly momentum caused by QDPO is discovered from the SH tropical latitudes to the NH, and the resulting momentum divergence and convergence are unfavorable for the maintenance of the seasonal mean fields of the NH tropical easterly and SH subtropical westerly winds. Finally, possible synoptical processes responsible for QDPO are discussed together with its relation to the interaction between the circulations of both the hemispheres. It is found that QDPO is both the result of and medium for the interaction.
He, J. (1988). "Transfer of physical quantities of QDPO and its relation to the interaction between the NH and SH circulations." Advances in Atmospheric Sciences, Peking 5(1): 97-106.
On the basis of the 1979 FGGE Level 3-b data, calculation is made of the transfer of sensible and latent heat and momentum resulting from a quasi-40-day periodic oscillation (QDPO) on a cross-equatorial meridional vertical cross section, and the characteristics of the transfer at all phases of QDPO are analyzed. During the monsoon's QDPO activation and break phases, a strong transfer of sensible heat to the Southern Hemisphere is felt in the upper troposphere over the Asian monsoon region. The conversion of potential effective perturbation into its kinetic energy attains its maximum at 500-300 hPa (15 degrees N), serving as the source of kinetic energy for the quasi-40-day periodic perturbation. An intense transfer of potential energy above 200 hPa from the monsoon area to the Southern Hemisphere maintains the QDPO at the tropical latitudes. During the QDPO activation-break (and reverse) transitional phase, the conversion potential effective perturbation into kinetic energy reaches its maximum in the middle and lower troposphere over the Southern Hemisphere middle latitudes, and an appreciable lower transfer of potential energy occurs toward the Southern Hemisphere tropical latitudes and the Northern Hemisphere. The upper tropospheric powerful transfer of westerly momentum caused by QDPO is discovered from the Southern Hemisphere tropical latitudes to the Northern Hemisphere, and the resulting momentum divergence and convergence are unfavorable for the maintenance of the seasonal mean fields of the Northern Hemisphere tropical easterly and Southern Hemisphere subtropical westerly winds. Possible synoptic processes responsible for QDPO are discussed together with its relation to the interaction between the circulations of both the hemispheres. It is found that QDPO is both the result of and the medium for the interaction.
He, J. and X. Zhi (1995). "Seasonal interlock of the intraseasonal variations of rainfall in East China." Journal of Tropical Meteorology, Guangzhou, China 11(4): 370-374.
Based on the daily rainfall data of May-to-August from 1954 to 1983, this paper analyzes the climatic characteristics of precipitation in East China and discusses their association with seasonal variations of the East Asian monsoon. It is found from the analysis that both the Jianghuai (the Yangtze and Huaihe river basins) Mei-yu and South China summer monsoon rain have seasonal interlock characteristics; the former shows a single peak distribution while the latter displays a double peak distribution. Meanwhile, the intraseasonal variations of precipitation in both areas show a periodic oscillation of 15-25 days. The peak value of precipitation in the Yangtze and Huaihe river basins and the first peak value in South China are mainly affected by the subtropical monsoon, i. e. they are associated with the seasonal northward advancing and strengthening of the summer monsoon. The second peak value of precipitation in South China is influenced by the South China Sea tropical monsoon and closely related with the tropical cyclone activities in the corresponding period.
Hendon, H. H. and B. Liebmann (1990). "The intraseasonal (30-50 day) oscillation of the Australian summer monsoon." Journal of the Atmospheric Sciences, Boston, MA 47(24): 2909-2923.
The tropical intraseasonal (30-50 day) oscillation manifests itself in the Australian summer monsoon by a pronounced modulation of the monsoonal westerlies. These 30-50 day fluctuations of the monsoonal westerlies are coherent with rainfall and OLR across northern Australia. The OLR fluctuation originates in the Indian Ocean and systematically propagates eastward at 5 m s super(-) super(1) , consistent with previous studies of the intraseasonal oscillation. The detailed evolution of the intraseasonal oscillation of the monsoon is studied via composites of upper air data in and about the Australian tropics. During the summer periods 1957-87, 91 events were identified at Darwin, Australia. The composite oscillation at Darwin has a very deep baroclinic structure with westerlies extending up to 300 mb. The westerly phase lasts about ten days and lags a similar duration rainfall event by about four days. During the westerly phase, the upper troposphere is warm and the extreme lower troposphere is cool. This structure is consistent with midtropospheric latent heating and lower tropospheric cooling due to evaporation of falling rain. The magnitude of the composite oscillation at Darwin is about 5 m s super(-) super(1) in zonal wind, 1 m s super(-) super(1) in meridional wind, 0.5 degrees K in temperature, 5 mm rainfall per day, and 10% in relative humidity. The oscillation at Darwin is readily traced as far west as Cocos Island and as far east as Pago Pago. Above northern Australia, enhanced synoptic scale variability develops during the wet-westerly phase of the oscillation. Analysis of a single station record precludes documentation of the structure of these synoptic fluctuations. In the Northern Hemisphere midlatitudes, a wave train in 500 mb heights appears to emanate from the longitude of the Australian tropics during the wet-westerly phase. The magnitude of this wave train is only about 50 m while the wave train undergoes a systematic evolution as the tropical convective anomaly moves west to east, no sense of dispersion from a localized low-latitude heat source is evident.
Higgins, R. W. and W. Shi (2001). "Intercomparison of the Principal Modes of Interannual and Intraseasonal Variability of the North American Monsoon System." Journal of Climate 14(3): 403-417.
Time series of seasonal-, monthly, and pentad-mean precipitation are subjected to empirical orthogonal function analysis, regression analysis, and compositing techniques to study the principal modes of interannual and intraseasonal variability of the North American Monsoon System (NAMS). The leading principal component (PC) from the summertime seasonal-mean data is associated with El Nino-Southern Oscillation (ENSO) variability while the leading PC from the pentad-mean data is associated with 30-60-day intraseasonal (Madden-Julian) oscillations (MJOs). The leading PC from the monthly mean data is a hybrid of the two above-mentioned modes, capturing aspects of both. The leading PCs are used as reference time series for regressions and composites that reveal the structure of the principal modes and their manifestation in the NAMS. The leading PCs are also used to estimate the fraction of the variance of summer precipitation that is explained by ENSO and by interannual variations of MJO activity. ENSO-related impacts on the NAMS are linked to meridional adjustments of the ITCZ. In its positive polarity, the leading PC of interannual variability is associated with warm (ENSO) episodes and is characterized by an expansion of the ITCZ toward the south, increased precipitation in a zonally oriented band just north of the equator, and decreased precipitation over Mexico and portions of the Caribbean. MJO-related impacts on the NAMS are linked to more regional meridional adjustments in the precipitation pattern over the eastern tropical Pacific. In its positive polarity, the leading PC of intraseasonal variability is associated with an intensification and northward adjustment of the precipitation pattern in the eastern tropical Pacific, with increased precipitation over the warm pool to the west of Mexico and over portions of Mexico and the southwestern United States. Both the interannual and intraseasonal modes have well-defined, but distinct, sea level pressure and surface wind signatures in the eastern tropical Pacific. These features extend to the middle troposphere and are capped by circulation features in the opposite sense in the upper troposphere. The relationship of the MJO to the NAMS is examined in more detail using the leading PC of intraseasonal variability and an objective procedure to identify the phase of MJO events. The leading PC is strongly related to the eastward progression of centers of enhanced (reduced) convection around the global Tropics on intraseasonal timescales. Notably, a strong relationship between the leading mode of intraseasonal variability of the NAMS, the MJO, and the points of origin of tropical cyclones in the Pacific and Atlantic basins is also present.
Higgins, R. W. and W. Shi (2001). "Intercomparison of the principal modes of interannual and intraseasonal variability of the North American Monsoon System." Journal of Climate, Boston, MA 14(3): 403-417.
Time series of seasonal-, monthly, and pentad-mean precipitation are subjected to empirical orthogonal function analysis, regression analysis, and compositing techniques to study the principal modes of interannual and intraseasonal variability of the North American Monsoon System (NAMS). The leading principal component (PC) from the summertime seasonal-mean data is associated with El Nino-Southern Oscillation (ENSO) variability while the leading PC from the pentad-mean data is associated with 30-60-day intraseasonal (Madden-Julian) oscillations (MJOs). The leading PC from the monthly mean data is a hybrid of the two above-mentioned modes, capturing aspects of both. The leading PCs are used as reference time series for regressions and composites that reveal the structure of the principal modes and their manifestation in the NAMS. The leading PCs are also used to estimate the fraction of the variance of summer precipitation that is explained by ENSO and by interannual variations of MJO activity. ENSO-related impacts on the NAMS are linked to meridional adjustments of the ITCZ. In its positive polarity, the leading PC of interannual variability is associated with warm (ENSO) episodes and is characterized by an expansion of the ITCZ toward the south, increased precipitation in a zonally oriented band just north of the equator, and decreased precipitation over Mexico and portions of the Caribbean. MJO-related impacts on the NAMS are linked to more regional meridional adjustments in the precipitation pattern over the eastern tropical Pacific. In its positive polarity, the leading PC of intraseasonal variability is associated with an intensification and northward adjustment of the precipitation pattern in the eastern tropical Pacific, with increased precipitation over the warm pool to the west of Mexico and over portions of Mexico and the southwestern United States. Both the interannual and intraseasonal modes have well-defined, but distinct, sea level pressure and surface wind signatures in the eastern tropical Pacific. These features extend to the middle troposphere and are capped by circulation features in the opposite sense in the upper troposphere. The relationship of the MJO to the NAMS is examined in more detail using the leading PC of intraseasonal variability and an objective procedure to identify the phase of MJO events. The leading PC is strongly related to the eastward progression of centers of enhanced (reduced) convection around the global Tropics on intraseasonal timescales. Notably, a strong relationship between the leading mode of intraseasonal variability of the NAMS, the MJO, and the points of origin of tropical cyclones in the Pacific and Atlantic basins is also present.
Hines, K. M. and D. H. Bromwich (2002). "A pole to pole west Pacific atmospheric teleconnection during August." Journal of Geophysical Research. D. Atmospheres 107(D18).
An observational analysis is presented that reveals an August teleconnection involving both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). The teleconnection includes three primary anomalies in the monthly average surface pressure field: (1) a high southern latitude component including Wilkes Land, Antarctica, the nearby Southern Ocean, and the Ross Sea, (2) a SH midlatitude component near Australia and New Zealand, and (3) a NH subtropical component over the extreme western Pacific Ocean. Surface pressure for the SH high-latitude anomaly is negatively correlated to the other two primary anomalies. The SH midlatitude and NH subtropical component are positively correlated. Furthermore, the teleconnection is correlated to surface pressure near the Aleutian Islands. The teleconnection anomalies result from tropical convection on intraseasonal timescales. Monsoon rainfall over East Asia during August is correlated to the teleconnection. Furthermore, August precipitation at several other locations in the NH and SH is modulated by the teleconnection. Two different sets of numerical experiments with the National Center for Atmospheric Research (NCAR) Community Climate Model version 2 (CCM2) show a similar-appearing teleconnection during late boreal summer. The modeled teleconnection, however, is forced from high southern latitudes.
Hirasawa, N. and T. Yasunari (1990). "Variation in the atmospheric circulation over Asia and the western Pacific associated with the 40-day oscillation of the Indian summer monsoon." Journal of the Meteorological Society of Japan, Tokyo, Japan 68(2): 129-143.
Through use of the outgoing longwave radiation (OLR) and 700 mb height fields for 1979, an investigation was conducted of the variation of the atmospheric circulation over Asia and the western Pacific associated with the 30-60 day variation of the Indian summer monsoon. Results of the analysis of the OLR indicate that convection over the northwestern Pacific along 15 degrees N is active slightly before the active phase of the Indian monsoon over central India, while there is a decrease in convective activity prior to the break phase. This active convection (between 140 degrees E and 150 degrees E) propagates southward with a period of 40 days from 15 degrees N to the equatorial zone. The cloudiness to the north of the Tibetan Plateau (around 55 degrees N, 75 degrees E) and over the subtropical high region (southeast of Japan) reaches a minimum slightly before the active phase of the Indian monsoon. Conversely, cloudiness reaches a maximum prior to the break phase of the Indian monsoon. From the results of the analysis of the 700 mb geopotential heights, it was found that to the north of the Tibetan Plateau the maximum cloudiness corresponds to the stagnation of the trough while the minimum cloudiness is associated with the development of the ridge. The position of the subtropical high shifts northward (along 25-30 degrees N) slightly before the active phase of the Indian monsoon and shifts southward (along 10-20 degrees N) slightly prior to the break phase. The phase of the 30-60 day variation to the north of the Tibetan Plateau precedes that of the subtropical high region. These results suggest a close association among the 30-60 day variation of the Indian monsoon, the convection over the western Pacific, the westerly-wave movements over the Asian continent and the subtropical high in the western Pacific. These large scale interactions affect the weather regime around Japan during the Baiu season. The meridional position of the Baiu front (east of 130 degrees E) around Japan shifts from 40 degrees N, when the subtropical high shifts northward, to 30 degrees N when the subtropical high shifts southward. This is due to the variation of the meridional position of the subtropical high, which is associated with the 40 day variation of the monsoon. The meridional position of the Mei-yu front (west of 130 degrees E), in contrast, does not exhibit a cyclical variation.
Ho, L. and B. Wang (2002). "The Time-Space Structure of the Asian-Pacific Summer Monsoon: A Fast Annual Cycle View." Journal of Climate 15(15): 2001-2019.
Despite the seemingly intricate and multifold time-space structure of the mean Asian-Pacific summer monsoon (APSM), its complexity can be greatly reduced once the significance of fast annual cycles has been recognized and put into perspective. The APSM climatology is characterized by a slowly evolving seasonal transition (slow annual cycle ) superposed by pronounced singularities in the intraseasonal timescale, termed the 'fast annual cycle' in this study. The fast annual cycles show nonrepetitive features from one episode to another, which are often divided by abrupt change events. The APSM fast annual cycles are composed mainly of two monsoon outbreaks, each marking a distinctive dry-wet cycle. The first cycle spans from the middle of May to early July and the second cycle from late July to early September. When the first cycle reaches its peak in mid-June, a slingshot-like convection zone, described as the grand-onset pattern, rules an area from the Arabian Sea to the Indochina Peninsula then bifurcates into a mei-yu branch and a tropical rain belt in the lower western North Pacific. After a brief recess during 20-29 July, the APSM harbors another rain surge in mid-August. This time a giant oceanic cyclone intensifies over the western North Pacific (around 20 'N, 140 'E); thus the rainy regime jumps 10 '-15 ' north of the previous rain belt. This ocean monsoon gyre incubates numerous tropical cyclones. Meanwhile, the convection zone of the Indian monsoon intensifies and extends well into the subcontinent interior. From the first to second cycle the major convection center has shifted from the adjacent seas in the northern Indian Ocean to the open ocean east of the Philippine Islands. The major cloud movement also switches from a northeastward direction in the Indian Ocean to a northwestward direction over the western North Pacific. The two monsoon cycles turn out to be a global phenomenon. This can be shown by the coherent seasonal migration of upper-level subtropical ridgelines in the Northern Hemisphere. During the first cycle all the ridgelines migrate northward rapidly, a sign that the major circulation systems of boreal summer go through a developing stage. After 20-29 July, they reach a quasi steady state, a state in which all ridgelines stand still near their northern rim throughout the entire second cycle. A reconstructed fast annual cycle based on four leading empirical orthogonal function modes is capable of reproducing most fine details of the APSM climatology, suggesting that the subseasonal changes of the mean APSM possess a limited number of degrees of freedom. A monsoon calendar designed on the basis of fast annual cycles (FACs) gives a concise description of the APSM climatology and provides benchmarks for validating climate model simulations.
Holland, G. J. (1986). "Interannual variability of the Australian summer monsoon at Darwin: 1952-82." Monthly Weather Review, Boston 114(3): 594-604.
Fluctuations in the Australian summer monsoon over the period 1952-1982 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure. Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Nino teleconnections.
Huang, R. and F. Sun (1992). "Impacts of the tropical western Pacific on the East Asian summer monsoon." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 243-256.
In this paper, the impacts of the convective activities in the western Pacific warm pool on the interannual and intraseasonal variations of the summer monsoon in East Asia are analyzed by using the observed data for 12 summers from 1978 to 1989. The analyzed results show that both interannual and intraseasonal variabilities of the East Asian summer monsoon are greatly influenced by the convective activities in the warm pool. Generally, the monsoon rainfall is below normal in East Asia and the abrupt change of the monsoon circulation is obvious in the summer of strong convective activities around the Philippines. The impacts of the convective activities in the warm pool on the summer monsoon in East Asia and the East Asia/Pacific teleconnection pattern of summer circulation anomalies due to the convection are discussed by using the theory of planetary wave propagation and the numerical modelling by the IAP-GCM, respectively.
Huang, R., L. Zhou, et al. (2003). "The Progresses of Recent Studies on the Variabilities of the East Asian Monsoon and Their Causes." Advances in Atmospheric Sciences 20(1): 55-69.
The variabilities of the East Asian summer monsoon are an important research issue in China, Japan, and Korea. In this paper, progresses of recent studies on the intraseasonal, interannual, and interdecadal variations of the East Asian monsoon, especially the East Asian summer monsoon, and their causes are reviewed. Particularly, studies on the effects of the ENSO cycle, the western Pacific warm pool, the Tibetan Plateau and land surface processes on the variations of the East Asian summer monsoon are systematically reviewed.
Jiang, N. and H. Luo (1993). "Intraseasonal variations of the atmospheric heat sources and moisture sinks over Asian monsoon region. Part I: Heat sources." Journal of Tropical Meteorology, Guangzhou, China 9(4): 299-307.
Using the ECMWF data of 1980 to 1983, the intraseasonal component of the variation of the atmospheric heat source <Q1> (vertically integrated) over Asian monsoon region is examined in detail. The large intraseasonal variance of <Q1> is mainly confined over the active monsoon region of Asia and surrounding areas, and varies seasonally and interannually. The EOF analysis indicates the relationship of intraseasonal oscillations between regions of India-Burma, the coast of east China and the western Pacific. The spectral analysis of the EOF time coefficients reveals that the dominant period range is around 30-60 day except for the summer of 1980 and the 1982-1983 ENSO period. The propagation of 30-60 day fluctuation in summer varies from region to region by means of lagged correlation analysis.
Jiang, N. and H. Luo (1994). "Intraseasonal variations of the atmospheric heat sources and moisture sinks over Asian monsoon region. Part II: Moisture sinks." Journal of Tropical Meteorology, Guangzhou, China 10(1): 1-8.
Using the ECMWF data of 1980 to 1983, the intraseasonal component of variation of atmospheric moisture sink <Q sub(2) > (vertically integrated) over the Asian monsoon region is examined. Included is the standard deviation, EOF patterns and the propagation of the oscillation. The results show, consistent with <Q sub(1) > especially in summer, that the large intraseasonal variance of <Q sub(2) > is mainly confined to the active monsoon region of Asia and surrounding areas, and that the main EOF patterns indicate the relationship of intraseasonal oscillations between the regions of India-Burma, the coast of east China and the western Pacific while the corresponding EOF time coefficients have their dominant period ranging from 30 to 60 days. On the other hand, between <Q sub(2) > and <Q sub(1) > there are differences in the features of the intraseasonal variation especially in winter. The propagational difference of 30-60 day fluctuations over regions of east Asia and south Asia in summer was found by means of lagged correlation analysis. To some extent, this regional difference indicates the difference of the two Asian monsoon systems.
Jiang, S., X. Yang, et al. (1993). "The characteristics of ``OLR'' for 1991 Mei-yu." Quarterly Journal of Applied Meteorology, Beijing, China 4(3): 301-309.
Using the mean pentad Outgoing Longwave Radiation (OLR) data observed from the NOAA satellites and the conventional weather observations for the 1991 Mei-yu season, a diagnostic study of the characteristics of the OLR is conducted. This paper describes the OLR and its anomaly patterns and the characteristics of propagation of the intraseasonal variation (ISV) of OLR during the 1991 flood Mei-yu periods. It is revealed that the variation of the ITCZ and the subtropical high are related to the evolution of the Mei-yu. The relationship between the anomalies of OLR over Tibet Plateau and the flood Mei-yu is also revealed. The results show that the earlier ITCZ occurrence in the western Pacific and the positive OLR anomalies (snow cover less than normal) over Tibet Plateau in winter are the important indications for the earlier onset date of the 1991 Mei-yu. Finally, by analyzing the features of the OLR pattern for the active and break phases of the 1991 Mei-yu and comparing with that of Indian monsoon, it is found that their onset dates are different from each other and their active phases either same or contrary. It is suggested that the complication of the Mei-yu in China is mainly due to the interaction between the middle latitude cold air and two kinds (east Asia and south Asia) of monsoon events.
Joseph, P. V. (1976). "Climatic change in monsoon and cyclones 1891-1974." Symposium on Tropical Monsoons, Poona, India, Sept.
During 1931-1960, when Indian summer monsoon rainfall was usually good, the severe cyclonic storms of the Bay of Bengal had more westerly tracks. During 1891-1920 and 1965-1974, when the average monsoon rainfall had departures from normal (1931-1960) of 10-20% over large areas of central and northwestern India and, there were many years without monsoon rainfall, causing droughts and famines, severe cyclonic storms had more northerly tracks. Upper wind observations during the past 20 yr show that more northward tracks of cyclones are associated with greater equatorward penetration of the upper tropospheric westerlies over India, a condition which also causes large-scale monsoon failure over India. It can be inferred that during 1891-1974 there was a long-term change in the upper tropospheric westerlies over India, and associated long-term changes in Indian monsoon rainfall and cyclone tracks of the Bay of Bengal. The periods 1891-1920, 1931-1960, and 1965-1974 can be considered as distinct climatic epochs for India. The number of days under break monsoon conditions in July and August in epochs 1891-1920 (A) and 1931-1960 (B) were examined. Epoch A had more break days than epoch B, and longer duration breaks were more frequent in epoch A.
Joseph, P. V. (1978). "Subtropical westerlies in relation to large-scale failure of Indian monsoon." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 412-418.
In the years of large-scale failure of southwest monsoon rainfall over India during the decade 1965-1974, the mean meridional circulation over India during July and August was of the type occurring in association with break monsoon situations, i.e., meridional northward motion in the upper troposphere, north of about 20 degrees N lat. During such years, subtropical westerlies of the upper troposphere seem to protude more southward into the areas immediately west of India during the monsoon season; this feature persists, on many occasions, from the preceding winter, which may explain the preferred northward tracks of the November cyclones of the Bay of Bengal, the occurrence of low, 500-mb altitudes over northwest Indian stations during winter and pre-monsoon seasons, and the observed greater frequency and duration of break monsoon spells associated with the large-scale monsoon failures, as reported earlier by Joseph (1975, 1976).
Jury, M. R., B. Parker, et al. (1994). "Evolution and variability of the ITCZ in the SW Indian Ocean: 1988-90." Theoretical and Applied Climatology, Vienna, Austria 48(4): 187-194.
The structure and variability of the inter-tropical convergence zone (ITCZ) in the SW Indian Ocean in the austral summer is investigated. The ITCZ is identified by satellite microwave (SSMI) precipitable water (PW) values >5 g cm super(-) super(2) , minimum outgoing longwave radiation (OLR) values <220 W m super(-) super(2) and the pattern of convergence in the low level (850 hPa) winds. According to OLR climatology, the ITCZ lies over 15 degrees S latitude to the west of Madagascar (40-50 degrees E), but near 10 degrees S to the east of 60 degrees E. Inter-annual and intra-seasonal variability is induced by the interaction of the convective NW monsoon and subsident easterly trades. Symptoms of the structure and variability are presented using tropical cyclone (TC) tracks, axes of PW exceedences and OLR, 850 hPa wind and PW fields in the period 1988-1990. The shape and intensity of the ITCZ is modulated by the strength of the NW monsoon off east Africa and by standing vortices in the SW Indian Ocean. The topography of Madagascar imparts a distinctive break in convective characteristics, and distinguishes the SE African ITCZ from its maritime counterpart.
Jury, M. R., B. Pathack, et al. (1991). "Transient convective waves in the tropical SW Indian Ocean." Meteorology and Atmospheric Physics, Vienna, Austria 47(1): 27-36.
Wave-organized convective features in the southwest Indian Ocean are described using Hovmoller composites of satellite imagery, OLR anomalies and ECMWF precipitable water departures during the southern summer. Westward movement of large convective elements is noted in the 10-20 degrees S latitude band in about half of the years between 1970 and 1984. A study of 47 convective systems from satellite imagery establishes the climatological features, including zonal propagation speeds for maritime systems in the range -2 to -4 m s super(-) super(1) , wavelengths of 25-35 degrees longitude (3,000 km), lifespans of 10-20 days and convective areas of 7-10 degrees longitude (800 km). Transient convective waves over the tropical SW Indian Ocean are slower and more diverse than their Northern Hemisphere counterparts. Interannual tendencies in the frequency and mode are studied. Wet summers over SE Africa correspond with an increased frequency of westward moving convective systems, whereas in dry summers convective systems tend to be quasi-stationary. INSAT data composites provide additional insight into the convective structure and show that tropical waves penetrated into southern Africa in February 1988. A more quantitative assessment of transient convective waves is provided by Hovmoller composites of OLR anomalies and precipitable water departures. Both display westward moving systems in 1976 and 1984 and highlight the wide variety and mixed mode character of convective waves. A case study is analyzed which illustrates the deepening of a moist, unstable layer coincident with the westward passage of a convective wave.
Kane, R. P. (2000). "ENSO relationship with Indian rainfall in different months." International Journal of Climatology, Chichester, UK 20(7): 783-792.
The rainfall anomalies in All India (AI) and its six subdivisions, North west (NW), North central (NC), west peninsular (WP), North east (NE), east peninsular (EP), and south peninsular (SP), were examined during years of different categories representing ENSO effects, in the period of 1901-1990. Unambiguous ENSOW-U had the best association with droughts in NW, NC, WP, in the monsoon months June, July, August and September, but with large month-to-month variability in some events, indicating the effects of Madden-Julian Oscillations (MJO). In subdivisions NE, EP, SP, effects were mixed, with droughts in monsoon months preceded and/or followed by, or interspersed with, excess rainfall in some months. In La Ninas (C events), effects were generally opposing those of ENSOW-U.
Kang, I. S., K. Jin, et al. (2002). "Intercomparison of the climatological variations of Asian summer monsoon precipitation simulated by 10 GCMs." Climate Dynamics 19(5-6): 383-395.
We assesses the overall performance of state-of-the-art atmospheric GCMs in simulating the climatological variations of summer monsoon rainfall over the Asian-Western Pacific region and the systematic errors that are common to a group of GCMs. The GCM data utilized are obtained from 10 GCM groups participated in the CLIVAR/Monsoon GCM Intercomparison Project. The model composite shows that the overall spatial pattern of summer monsoon rainfall is similar to the observed, although the western Pacific rainfall is relatively weak. For the simulated precipitation over the western Pacific, the models can be classified into two categories. The first category of models simulates the precipitation more confined to the equatorial region and weaker precipitation in the subtropical western Pacific compared to the observed. The second category of models simulates large precipitation in the subtropical western Pacific but the region is shifted to the north by 5-10 degree . None of the models realistically reproduce the observed Mei-yu rain band in the region from the East China Sea to the mid Pacific. Most of the models produce a rain band along the continental side of East Asia. The climatological variations of simulated summer rainfall are examined in terms of their amplitude and their principal EOF modes. All models simulate larger amplitudes of the climatological seasonal variation of Indian summer monsoon than the observed, though most models simulate smaller amplitudes in the western Pacific. The ten model composite produces four leading EOF modes over the Asian-western Pacific region, which are remarkably similar to the observed counterparts. The first and second eigenmodes, respectively, represent the smoothed seasonal march of broad-scale monsoon and the onsets of the Indian and East Asian summer monsoon. The third and fourth modes relate to the climatological intraseasonal oscillation (CISO). In contrast to the model composite, several models fail to reproduce the first principal mode, and most models do not reproduce the observed modes higher than the second. The CISO of precipitation is also examined over the Indian monsoon and the East Asia-western Pacific monsoon regions separately.
Kang, I.-S., C.-H. Ho, et al. (1999). "Principal modes of climatological seasonal and intraseasonal variations of the Asian summer monsoon." Monthly Weather Review, Boston, MA 127(3): 322-340.
Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction produced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985-89. It is shown that the seasonal evolution of the Asian summer monsoon is adequately described by a few leading EOFs. These EOFs capture the variations of regional rainbands over the East Asian and Indian regions. The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cloud bands from northern India, crossing the Korean peninsula to Japan, and dryness over the oceans to the south of cloud bands. This mode is related to the mature phase of Changma rainy season in Korea associated with the northward movement of cloud bands and circulation systems from the subtropical western Pacific. This mode appears as a first principal mode of climatological intraseasonal oscillation (CISO) over the entire Asian monsoon region. The CISO mode has a timescale of about 2 months. The northward moving CISO also appears in the 850- and 200-mb geopotential height fields as a first mode of each dataset. Based on the height variations of the CISO mode, it is suggested that the extratropical CISO during summer is related to a regional index cycle associated with the variation of north-south temperature gradient in East Asia.
Kar, S. C., M. Sugi, et al. (1996). "Simulation of the Indian summer monsoon and its variability using the JMA global model." Papers in Meteorology and Geophysics, Ibaraki, Japan 47(2): 65-101.
Ten-year integrations of the JMA global model at T106 and T42 horizontal resolutions were compared to examine the role of model resolution in simulating the Indian monsoon and its variability. It has been found that both T106 and T42 models simulate mean monsoon climatology reasonably well in terms of the large-scale monsoon flow. The T106 model simulates rainfall distribution in western and northwestern parts of India better than the T42 model. Correlation of interannual variability (IAV) of simulated rainfall with observed IAV is poor. The simulated IAVs of the Indian monsoon rainfall for both models do not have a strong correlation with the SST, either. The T42 model simulated the rainfall variability for 1987/88 better than the T106 model. Monsoon rainfall variability may be largely due to internal dynamics in both models. Internally generated variability may be larger in T106 model than T42 model simulations. Rainfall anomaly patterns obtained from T42 model simulations are better than those of T106 model simulations for the two pairs of good and bad monsoon events (1988/87 and 1983/82). Responses of both the coarser and finer resolution models to the same imposed surface forcing differ over the Indian monsoon region. Both models simulate the synoptic evolution of the monsoon quite well. Monsoon activity in the T106 model is more intense than in the T42 model. Rainfall distribution is better obtained from T106 model simulations than T42 model simulations. This suggests that further work is necessary in relating intraseasonal variations with interannual monsoon variability for understanding the nature of monsoon variability.
Karoly, D. J. and D. G. Vincent (1998). "Meteorology of the Southern Hemisphere." Boston, MA, American Meteorological Society 410.
A comprehensive description of the meteorology of the Southern Hemisphere was originally provided in the monograph of the same title published by the American Meteorological Society in 1972. That monograph was, of necessity, preliminary in nature because the available time series of observational data was short. In the quarter century that has passed since the first monograph, much has happened to warrant an updated edition: new observational techniques based on satellites, anchored and drifting buoys, and more ground-based stations have expanded the observational network to cover the whole hemisphere. The time is right, therefore, for a fresh look at the circulation features of the Southern Hemisphere, which is provided in this monograph, both for the atmosphere and oceans. The present monograph deals in greater detail with regional climates and updates the knowledge of circulation features such as the Australian monsoon; the South Pacific convergence zone; the South Atlantic convergence zone; the subtropical and polar jet streams, cyclones, and storm tracks; and the circulation in the stratosphere. These topics are dealt with mainly in the first four chapters and in chapter 6. Subjects that were not mentioned in the previous monograph are found in the remaining five chapters. They include mesoscale circulations; stratospheric ozone depletion; ocean circulations; air-sea interaction; interannual variability, including the Southern Oscillation and its attending El Nino; intraseasonal variability, including the Madden-Julian oscillation; climate variability and changes; and climate modeling. There is less emphasis on synoptic meteorology in the present edition, as it was covered quite adequately in the first edition. The main topic that is lacking is numerical weather prediction (NWP). This topic was originally envisioned as chapter 11, but it was decided to forego it because NWP developments and improvements generally are not so specific to a geographical region. Because of the increased interest in the Southern Hemisphere that followed FGGE, the American Meteorological Society formed an ad hoc committee (1981) and later a formal committee (1984) on meteorology and oceanography of the Southern Hemisphere, one of the functions of which is to plan and conduct professional scientific meetings. As a result, five international conferences on the meteorology and oceanography of the Southern Hemisphere have been held: in Brazil (1983), New Zealand (1986), Argentina (1989), Australia (1993), and South Africa (1997). The exchange of ideas at these conferences has been an important factor in the initiation of many of the studies that are summarized in this monograph.
Kashino, Y., H. Watanabe, et al. (1998). "Low frequency ocean variability between Mindanao and New Guinea." Journal of the Faculty of Science, Hokkaido University Series VII (Geophysics), Sapporo, Japan 11(2): 411-439.
In order to investigate ocean circulation and its variability, direct current measurement and three hydrographic observations between Mindanao and New Guinea at the Pacific entrance of the Indonesian Throughflow, were performed from 1994 to 1996. Moorings were deployed at 4 degrees 1'N, 127 degrees 31'E and 3 degrees 11'N, 128 degrees 27'E between Talaud Islands and Morotai Island, Indonesia, from February 1994 to June 1995. During boreal winter, north-northwestward flow is dominant between Talaud and Morotai, especially, at Morotai side. Results from hydrographic observations suggest that this flow is a part of the Halmahera Eddy, which shifts southward associated with the seasonal Asian monsoon change from boreal summer to winter. From spring to autumn 1994, westward flow transporting the mixed water between the North Pacific and South Pacific waters was observed at the southern mooring. Clear interseasonal variability with period of 50 days was also revealed during all period of moorings' deployment. This oscillation may be induced by wind variability associated with Madden and Julian Oscillation, otherwise related to eddy activity, such as the Halmahera Eddy, around this region. The Halmahera Eddy appears to be a key regarding these fluctuations which may influence the Indonesian Throughflow.
Kawamura, R., Y. Fukuta, et al. (2002). "A mechanism of the onset of the Australian summer monsoon." Journal of Geophysical Research. D. Atmospheres 107(D14).
An onset mechanism of the Australian summer monsoon that incorporates possible air-sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land-ocean thermal contrast during the premonsoon period results in a well-organized continental-scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600-700 hPa level. Intensified low-level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at similar to 700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large-scale disturbances with ascending motion (such as the Madden-Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics.
Kawamura, R. and T. Murakami (1995). "Interaction between the mean summer monsoon flow and 45-day transient perturbations." Journal of the Meteorological Society of Japan, Tokyo, Japan 73(6): 1087-1114.
The influence of the low-level mean summer monsoon flow on 45-day transient eddies through barotropic interaction is diagnosed by a scalar product between the horizontal shear vector and Eliassen-Palm (E-P) flux, while the dry (moist) baroclinic interaction is measured by a vector product between the vertical wind shear and the eddy sensible (latent) heat transport, which represents the vertical component of E-P fluxes. Activity of 45-day transient eddies was monitored by the evolution of kinetic energy during the 9-year period of 1985-1993. The 45-day wave activity is relatively weak over the SEAM (SouthEast Asian Monsoon) domain. Absence of geographically fixed forcing makes the WNPM (Western North Pacific Monsoon) more violent, with the peak 45-day wave activity occurring in late August, the time of frequent development of vigorous tropical cyclones. Near the WNPM updraft center (15 degrees N, 140 degrees -150 degrees E), the E-P wave energy fluxes are oriented in a direction which causes weakening of the sheared mean zonal and meridional winds, implying barotropic amplification of 45-day waves. The weak mean temperature gradient relegates the dry baroclinic instability to that of secondary importance over the WNPM domain. This is contrasted with the significant moist baroclinic process due to an approximate perpendicular relationship between the eddy moisture transport and the strong vertical shear of the monsoon flow. Curiously, 45-day waves are quite active near Japan even during August's dry, hot climate. Neither barotropic nor dry (moist) baroclinic instability can account for late-summer disturbance activity. Evidence is presented that 45-day convective oscillations occurring over the WNPM domain induce a strong 45-day response near Japan and further eastward, possibly indicating an intraseasonal Rossby wave dispersion along the westerly jet stream. Near the Aleutian Islands, a pronounced poleward eddy sensible heat flux is directed down the mean temperature gradient, resulting in dry baroclinic amplification of mid-latitude 45-day waves via action of the Coriolis torque. An import of moisture from the WNPM source region brings about a moist baroclinic instability of extratropical 45-day waves poleward of the Pacific high.
Keenan, T. D. and L. R. Brody (1988). "Synoptic-scale modulation of convection during the Australian summer monsoon." Monthly Weather Review, Boston 116(1): 71-85.
Time-longitude representations of Japanese Geostationary Meteorology Satellite (GMS) IR imagery indicate the existence of major synoptic-scale banding of convection within the Australian summer monsoon. The bands can be interpreted as active and break phases of major convective activity within the monsoon. This study relates the occurrence of convection and its organization into synoptic-scale bands to observable flow features. GMS Digital Equivalent Blackbody Temperature (T sub(B) sub(B) ) data and wind fields from the Australian Numerical Meteorology Research Center (ANMRC) tropical analysis scheme for the 1978/1979 and 1983/1984 seasons were composited relative to wind field surges and bands of enhanced and suppressed convective activity. Some low-level wind surges in the South China Sea produced a modulation in the convective activity within the preexisting bands but did not seem to be associated with their formation. Surges in the Southern Hemisphere trade wind easterlies and the southerly jet off the west coast of Australia were not associated with any major change in convective activity. The organization of the convection into synoptic-scale bands was associated with the Southern Hemisphere 200-mb flow. Areas of enhanced convective banding were east of upper level tropospheric troughs. The troughs and associated subtropical jet streaks had amplified from the south, interacting and enhancing the monsoonal convection. West of the trough, in the region of subsiding air, the convection was suppressed. Independent studies taken from the 1984/1985 season showed that this type of interaction was discernible for individual cases.
Kemball-Cook, S. and B. Wang (2001). "Equatorial Waves and Air-Sea Interaction in the Boreal Summer Intraseasonal Oscillation." Journal of Climate 14(13): 2923-2942.
A composite study of the life cycle of the boreal summer intraseasonal oscillation (BSISO) was performed using data from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis and National Oceanic and Atmospheric Administration polar-orbiting satellites. Because of pronounced differences in their climatologies, the boreal summer periods May-June (MJ) and August-October (AO) were composited separately. Characteristics of the BSISO life cycle common to MJ and AO were initiation and eastward propagation of the convective anomaly over the Indian Ocean, followed by poleward propagation, with the northward-moving branch having greater amplitude than the southward-moving branch. The transition of convection from the Indian Ocean to the western Pacific occurred next, followed by dissipation of the current cycle and initiation of the subsequent cycle. The MJ and AO life cycles were found to have several significant differences. The MJ shows strong eastward movement of convection along the equator in both the Indian and western Pacific Oceans. Convection in AO has a weaker eastward-propagating signal along the equator and displays a discontinuous jump from the Indian Ocean to the western Pacific. In marked contrast to MJ, AO shows strong northwestward propagation of convection in the western Pacific during the latter half of the BSISO life cycle. The change in the BSISO life cycle from MJ to AO reflects the seasonal shift in the distributions of vertical wind shear and low-level specific humidity from early to late summer. Rossby waves emitted by equatorial convection play a critical role in the BSISO in both the Indian and western Pacific Oceans. These waves are instrumental in the northward propagation of convection in MJ and AO. Both MJ and AO composites suggest that air-sea interactions are present in the BSISO, fostering both eastward and northward propagation of convective anomalies in the Indian Ocean and in the western Pacific. The complexity and pronounced seasonal dependence of the BSISO reflected in the composites suggest that its simulation is a rigorous test for general circulation models.
Kemball-Cook, S., B. Wang, et al. (2002). "Simulation of the Intraseasonal Oscillation in the ECHAM-4 Model: The Impact of Coupling with an Ocean Model." Journal of the Atmospheric Sciences 59(9): 1433-1453.
Three 15-yr integrations were made with the ECHAM-4 atmospheric GCM (AGCM); in the first integration, the model lower boundary conditions were the observed monthly mean sea surface temperatures, and, in the second, the AGCM was coupled to the University of Hawaii 2.5-layer intermediate ocean model. In the third simulation, the SST climatology generated in the coupled run was used to create monthly mean SSTs, which were then used to drive the AGCM in an uncoupled configuration similar to the first run. The simulation of the intraseasonal oscillation (ISO) in these three runs was compared with data from the NCEP reanalysis and outgoing longwave radiation from NOAA polar-orbiting satellites, with particular emphasis on the boreal summer ISO. The overall effect of coupling the AGCM to the ocean model is to improve the intraseasonal variability of the model. Upon coupling, the simulated boreal winter ISO becomes more spatially coherent and has a more realistic phase speed. In the May-June Asian monsoon season, the coupled run shows pronounced northward propagation of convection and circulation anomalies over the Indian Ocean, as in the observations, while northward propagation is absent in the uncoupled run. These improvements in the simulated ISO occur despite the fact that the coupled-run SST climatology has a substantial cold bias in both the Indian Ocean and the western Pacific warm pool. The improvement in the model ISO may be attributed to air-sea interaction whose mechanism is increased low-level convergence into the positive SST anomaly ahead of the convection anomaly. The simulation of the August-October ISO is degraded upon coupling, however. The coupled-run basic state fails to produce the region of easterly vertical shear of the mean zonal wind, which lies on the equator during August-October. This region of easterly shear is critical for the emission of Rossby waves by equatorial convection associated with the ISO. In the absence of easterly shear, the observed northwestward propagation of convection is inhibited in both runs made using the coupled model basic state. The uncoupled AGCM run correctly locates the region of easterly shear and produces an August-October ISO that agrees well with observations.
Khrishnamurti, T. N. and Y. Ramanathan (1982). "Sensitivity of the monsoon onset to differential heating." Journal of the Atmospheric Sciences, Boston 39(6): 1290-1306.
This paper examines some observational aspects of the evolution of energy exchanges and differential heating during the GARP Monsoon Experiment MONEX. The main findings are 1) a large increase in the kinetic energy of the total flow field and the nondivergent component of the flows occurs over the Arabian Sea similar to 1 week prior to the onset of monsoon rains over central India; 2) the field of differential heating moves during the onset period to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy; 3) the release of the eddy available potential energy enhances the kinetic energy of the divergent circulations; and 4) the kinetic energy of the divergent circulation does not increase much with time. This energy is shown to be transferred rapidly to the nondivergent motion via a number of interaction functions. The orientation of the divergent flows is shown to be of prime importance in these transfers during the onset, active, break, and revival periods of the monsoons. On the basis of these observational findings, some numerical prediction experiments are conducted to examine the sensitivity of the monsoon onset to initially imposed fields of differential heating. The results of similar to 96 hr of integrations seem to confirm the large sensitivity in the evolution to the field of heating. The results of these integrations also show that the kinetic energy of the monsoon circulations increases via the rapid increase of interactions among the irrotational and nondivergent modes.
Kishtawal, C. M., P. K. Pal, et al. (1991). "Water vapour periodicities over Indian Ocean during contrasting monsoons from NOAA satellite data." Proceedings, Bangalore, India 100(4): 341-359.
The satellite-derived moisture fields during different phases of two normal and poor monsoon years have been studied. Spectral analysis was performed in different zones of the monsoon region to study the nature and modes of intraseasonal fluctuations of lower layer moisture fields. Seasonal mean fields of water vapour at low and middle layers show a dry anomaly over the Arabian subcontinent and a wet anomaly over the Bay of Bengal during good monsoon years, while the anomalies show an opposite trend during the poor monsoon years. The zonal and meridional propagation of low-frequency oscillations of moisture fields has also been examined. The southward movement of low-frequency oscillations seems to be suppressed in good monsoon years as compared to the poor monsoon years, whereas the northward movement of the same shows no particular difference. Fluctuations in the 30-50 day range are found shifted to the longer time-period side in the poor monsoon years.
Kislov, A. V., E. K. Semenov, et al. (1998). "Estimation of the capability of the INM RAS atmospheric general circulation model to reproduce precipitation and wind in Indian monsoon." Russian Meteorology and Hydrology, New York, NY 8: 11-16.
The capability of the atmospheric general circulation model of the Institute of Numerical Mathematics, Russian Academy of Sciences to reproduce intraseasonal anomalies of the summer Indian monsoon was estimated. The comparison of the results of numerical experiments and observed data has shown that the spatial and temporal variability of meteorological fields in the model atmosphere is formed due to long-term processes. A contribution of the synoptic-scale component proved to be very small; therefore, the model much better detects and reproduces the pattern of monsoon rainfall in a break phase connected with a large-scale reconstruction of the atmospheric circulation and with the displacement of convergence zone to the southernmost Hindustan, as compared to other phases of seasonal monsoon cycle.
Kitoh, A. (1988). "Long-range forecast experiments for the summer of 1984 with the MRI-GCM: sensitivities to the sea surface temperature anomalies and cumulus parameterizations." Journal of the Meteorological Society of Japan, Tokyo 66(6): 913-925.
The impact of the sea surface temperature (SST) anomalies and cumulus parameterizations is investigated for the case of the 1984 northern summer with the use of the MRI general circulation model. Integrations are performed either with the observed SST for 1984 or the climatological SST, or with the original Arakawa-Schubert cumulus parameterization (the A-model) or a modified scheme (the M-model) that imposes an additional constraint between the minimum entrainment rate and the depth of the predicted planetary boundary layer. Intraseasonal oscillations in the low latitudes are better simulated with the M than with the A. However, the forecast skill with the M depends sensitively on the initial conditions. In one case, the observed eastward moving wave in the velocity potential field at 200 mb over the Equator is forecasted well, both in phase and amplitude, to 20 days. By using different initial conditions, the forecast skill for the transient fields was poor. The M-model also has many advantages over the A-model in its climate simulation. It has simulated the Pacific subtropical high in the proper position and succeeded in simulating a Baiu-like rain band. The M-model has a stronger monsoon activity and a stronger sensitivity to the boundary forcing (here the SST anomalies) than the A-model, because of more unstable stratification. The impact of the SST anomaly is better simulated with the M than with the A. Monthly mean forecast skill in the low latitudes varies from month to month, but overall useful skill is found in the 3-mo mean forecast with the M over the Tropics, except at the very beginning of the forecast. The SST anomaly impact is not clearly determined, mainly because of the limited forecast samples and partly because of the modest SST anomalies in 1984.
Kitoh, A. and T. Tokioka (1987). "Simulation of the tropospheric general circulation with the MRI atmospheric general circulation model, Pt. 3, Asian summer monsoon." Journal of the Meteorological Society of Japan, Tokyo 65(2): 167-187.
The Asian summer monsoon simulated by a five-layer tropospheric general circulation model (MRI-GCM-1) is described. Gross features of the July circulations are well simulated. Although the detailed rainfall distribution in the monsoon region (e.g., a maximum over northeastern India) is not well reproduced, the time evolution of the total diabatic heating distribution over the entire monsoon region was reasonably simulated. The 1-mo period from mid-April is a transition period in the model from the northern winter to the summer regime. A reverse transition occurs between Sept. and Oct. It is indicated that both transitions do not occur smoothly, but rapidly, and that circulation changes associated with the monsoon onset are not confined to the monsoon region, but extend globally. This simplified treatment of the land surface processes caused the model to overestimate the latent heat flux over the Tibetan Plateau compared to that observed (Luo and Yanai, 1985). It is found that reduction of the field capacity of the model groundwater over the Tibetan Plateau tends to improve surface energy balances there. Quasi-periodic oscillations are found in the model monsoon system. Their time evolution is similar to that described by Krishnamurti and Bhalme (1976), although the simulated oscillation period is not quasi-biweekly, but 8-9 days. The oscillation period seems to be influenced by the dominant quasi-10-day period of Kelvin waves found in the model tropics. As a result, the timing of the monsoon onset coincides with one of the favorable timings for the intensification of the upper level easterlies over the equatorial Indian Ocean provided by the Kelvin wave. Preceding the monsoon onset, a long-lasting, westward-moving disturbance was formed in the model lower troposphere south of the Equator over the Indian Ocean. Additional studies are needed to confirm the reality of this disturbance and its role concerning the monsoon onset.
Knutson, T. R., K. M. Weickmann, et al. (1986). "Global-scale intraseasonal oscillations of outgoing long-wave radiation and 250-mb zonal wind during Northern Hemisphere summer." Monthly Weather Review, Boston 114(3): 605-623.
Intraseasonal fluctuations of satellite-based observations of outgoing long-wave radiation (OLR) and NMC analyses of 250-mb zonal wind (U250) are described, based on global data from nine Northern Hemisphere summers (May-Oct.). Cross-spectral analysis of the 28-72-day spectral band is used to establish statistically significant relationships for the entire data period. Hovmoller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics. Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the Equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3-6 m sec super(-) super(1) ) along the Equator between 50 and 160 degrees E. The OLR oscillation has a preferred time scale of 30-60 days during May-Oct., based on a sample of > 30 events. The initiation near the Equator of northward-propagating (1-2-m sec super(-) super(1) ) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies. The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the Tropics with the exception of the Northern Hemisphere Tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0-10 degrees S at similar to 6 m sec super(-) super(1) from 40 to 160 degrees E and at similar to 15 m sec super(-) super(1) from 160 degrees E to 0 degrees W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28-72-day U250 anomalies along 30 degrees S(50 degrees S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors. The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250-mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5 degrees N-5 degrees S is not continuous around the globe, areas of significant coherence scattered throughout the Tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.
Kondragunta, C. R. (1990). "On the intraseasonal variations of the Asiatic summer monsoon." Mausam, New Delhi, India 41(1): 11-20.
Intraseasonal fluctuations of the Asiatic summer monsoon are investigated using Outgoing Longwave Radiation (OLR) covering eight northern hemisphere summers (1 May to 30 September 1975-1983, except 1978). The intraseasonal variance is large in the equatorial Indian Ocean and the northwest Pacific. OLR fluctuations in the equatorial Indian Ocean, over India and the northwest Pacific show spectral peaks on time scales 30-60 days, 10-20 days and less than 10 days. This study also reveals intraseasonal variation associated with the interannual variation. Over India below normal summer rainfall years are associated with 30-60 oscillation, near normal summer rainfall years are associated with 30-60 day and 10-20 day oscillations and above normal rainfall years are associated with oscillations less than 10 days. Empirical Orthogonal Function (EOF) analysis has been carried out for the whole Asiatic summer monsoon domain. The first four EOF coefficients explain 15% of the total variance. EOF analysis confirms existence of oscillations at least on three time scales, viz., 30-60 days, 10-20 days and less than 10 days over the Asian summer monsoon region.
Kondragunta, C. R. (2001). "on the intraseasonal variations of the asiatic summer monsoon." Mausam [Mausam] 1: 217-226.
Intraseasonal flucturations of the Asiatic summer monsoon are investigated using Outgoing Longwave Radiation (OLR) covering eight northern hemisphere summers (1 May to 30 September 1975-83, except 1978). The intraseasonal variance is large in the equatorial Indian Ocean and the northwest Pacific. OLR fluctuations in the equatorial Indian Ocean, over India and the northwest Pacific show spectral peaks on time scales 30-60 day, 10-20 day and less than 10 days. This study also reveals intraseasonal variation associated with the interannual variation. Over India below normal summer rainfall years are associated with 30-60 oscillation, near normal summer rainfall years are associated with 30-60 day and 10-20 day oscillations and above normal rainfall years are associated with oscillations less than 10 days. Empirical Orthogonal Function (EOF) analysis has been carried out for the whole Asiatic summer monsoon domain. The first four EOF coefficients explain 15% of the total variance. EOF analysis confirms existence of oscillations at least on three time scales, viz., 30-60 day, 10-20 day and less than 10 days over the Asian summer monsoon region.
Kripalani, R. H., A. Kulkarni, et al. (1997). "Association of the Indian summer monsoon with the Northern Hemisphere mid-latitude circulation." International Journal of Climatology, Chichester, UK 17(10): 1055-1067.
The association between the mid-latitude circulation and rainfall over the Indian region on an intraseasonal time-scale is investigated by considering 11 years (1974-1984) of Northern Hemisphere 500 hPa geopotential heights and rainfall data for the Indian summer monsoon months June through to September. On the basis of extensive correlation analysis between the geopotential heights and rainfall, it is seen that three regions over the mid-latitudes, the Manchurian region, the Algerian region and the Caspian sea region show positive correlation with rainfall over India, with higher values north of 20 degrees N latitude. Lead and lag correlations between the heights at the locations identified above and rainfall over India reveals that some common element of low-frequency variability is influencing the mid-latitude circulation and Indian rainfall. On the interannual scale the connections between the winter-time low-frequency patterns (the Pacific/North Atlantic, the West Pacific Oscillation, the North Atlantic Oscillation and the Eurasian) and Indian summer monsoon rainfall (ISMR) are investigated. Only the West Pacific Oscillation pattern shows a significant relationship with the ISMR. Further, the interannual and the decadal variability is examined by using the Northern Hemisphere zonal index data for the period 1900-1993. Results reveal that the decadal-scale variability of the ISMR and the circulation features of the Northern Hemisphere are connected.
Kripalani, R. H., S. V. Singh, et al. (1995). "Variability of the summer monsoon rainfall over Thailand: comparison with features over India." International Journal of Climatology, Chichester, UK 15(6): 657-672.
In this study, 20 years (1961-1980) of rainfall data for 34 stations over Thailand are used to investigate and understand the intraseasonal and interannual variability of the summer monsoon. Dominant structures of 5-day and seasonal rainfall are determined through empirical orthogonal functions (EOFs). Monthly and seasonal spatial patterns and a map showing the coefficient of variation are also presented. On an intraseasonal time-scale regions with high 30-60-day variances (Madden Julian oscillations--MJOs) and 10-20-day variances (Quasi-biweekly oscillations--QBWOs) are identified using a band-pass Butterworth filter for 5-day rainfall. It is seen that the MJOs are dominant over the Indian region and the QBWOs are dominant over the Thailand region. Extended EOFs (EEOFs) have shown that the most important evolutionary feature over India is the northward propagation associated with the MJOs, whereas over the Thailand region it is westward, associated with the QBWOs. Northward propagation of rainfall anomalies is not observed over the Thailand region. On the interannual scale the area over northwest Thailand is related well with rainfall variation over west-central India. The Darwin pressure tendency (DPT) and the subtropical ridge over India shows significant relation with rainfall over northwest Thailand. However, the Northern Hemisphere surface temperature (NHST), Quasi-biennial Oscillation (QBO), and the West Pacific ridge (WPR) show no significant relation.
Krishna Rao, A. V. R. (1994). "Application of satellite data for monsoon research." Das, P. K.
In this paper, the intraseasonal and diurnal variations of cloudiness during the monsoon season of 1991 are presented using a composite data set of INSAT & GMS satellites. The parameter chosen, to study these aspects, is the cloud top temperature (CTT) derived over every 1 deg lat./long. grid, covering the area 60N-60S and 40E-160W. The composite data set was prepared after deriving a suitable regression relationship between the CTTs obtained through both the satellites, over the common area of view, equidistant from both of them. The maps of horizontal distribution of the CTTs during the months May through September reflect the position of ITCZ very well. Since the mean CTTs indicate the mean cloud height particularly that of convective clouds in each 1 deg square mesh, the variations of CTTs reflect those of convection. The three hourly CTT distribution indicated an east-west fluctuation of convective clouds over the north Bay of Bengal and adjoining India with time. Lowest CTTs are found over north Bay at about noon, and they move westwards with time into the land up to 21Z and swing back to north Bay of Bengal by morning. The diurnal variation of convection over different regions, also, was presented using 3 hourly mean CTT values. A band pass filter of 30-40 days was applied to the CTT anomalies, after removing the seasonal trend from the data set. These anomalies representing maximum cloud zone (MCZ) show, on time-latitude cross-section, a northward movement starting from the equator to about 30N over the Indian longitudes only, with an average phase-speed of 1 deg. lat./day. The average periodicity is found to be 40 days. Elsewhere the mode is not prominent and travels up to 20 deg. N only. The time-longitude cross-section of CTT anomalies show an eastward movement of the anomalies in the equatorial region. Each epoch of the northward movement of the MCZ coincides with a corresponding eastward movement of cloudiness in the equatorial region. The time-longitude cross-section of 3-hourly CTTs in different latitudinal belts indicate a westward movement of the clouds north on 5N, being maximum in July and in the latitudinal belt of 10-15N. These synoptic scale transient systems with associated convective clouds originate near about 160E longitude and move westwards. After entering the Bay of Bengal, most of them weaken west of 80E longitude. They travel with a phase speed of 5 deg. long./day and the average periodicity of their occurrence is found to be 9 days.
Krishna Rao, D. and R. K. Datta (1975). "Circulation during break in the Indian southwest monsoon: a proposed model." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 26(3): 405-409.
A circulation model, to explain the weather conditions over the Indian subcontinent during break-monsoon situations, is proposed. To substantiate the proposed model, the vertical velocity ( omega ) for a few typical break-monsoon cases was computed with a multilayer diagnostic model.
Krishnamurthy, K. and K. S. Ramasastri (1979). "Study of hydrological droughts in Mahanadi River basin up to Hirakud." International Symposium on Hydrological Aspects of Droughts, New Delhi, Dec 1: 558-569.
Hydrological drought is identified by unusually low flows in streams and rivers over extended periods. On the basis of long-term streamflow data of Mahanadi River at Sambalpur-Hirakud, 7 yr of low streamflows in Mahanadi were identified, and the meteorological causes of hydrological drought in these years were examined. From the study of the hydrological droughts in 1962, 1965, 1966, 1969, 1972, 1974, and 1976, three aspects of monsoon were found to be important, in the order 1) break monsoon caused by shifting of the monsoon trough axis to the north, close to the Himalayas, 2) late arrival of monsoon, and 3) subdued cyclonic activity. Also, a marked decreasing trend in monsoon rainfall and monsoon flow was noticed in the catchment of Mahanadi River as far as Hirakud. It was further noticed that there were only 4 yr of low flows during the period 1926-1961, as compared with 8 yr of low flows during the period 1962-1976.
Krishnamurthy, V. and J. Shukla (2000). "Intraseasonal and interannual variability of rainfall over India." Journal of Climate, Boston, MA 13(24): 4366-4377.
A gridded daily rainfall dataset prepared from observations at 3700 stations is used to analyze the intraseasonal and interannual variability of the summer monsoon rainfall over India. It is found that the major drought years are characterized by large-scale negative rainfall anomalies covering nearly all of India and persisting for the entire monsoon season. The intraseasonal variability of rainfall during a monsoon season is characterized by the occurrence of active and break phases. During the active phase, the rainfall is above normal over central India and below normal over northern India (foothills of the Himalaya) and southern India. This pattern is reversed during the break phase. It is found that the nature of the intraseasonal variability is not different during the years of major droughts or major floods. This suggests that a simple conceptual model to explain the interannual variability of the Indian monsoon rainfall should consist of a linear combination of a large-scale persistent seasonal mean component and a statistical average of intraseasonal variations. The large-scale persistent component can be part of low-frequency components of the coupled ocean-land-atmosphere system including influences of sea surface temperature, snow, etc. The mechanisms responsible for the intraseasonal variations are not well understood. This simple conceptual framework suggests that the ability to predict the seasonal mean rainfall over India will depend on the relative contributions of the externally forced component and the intraseasonal component. To the extent that the intraseasonal component is intrinsically unpredictable, success in long-range forecasting will largely depend on accurate quantitative estimates of the externally forced component.
Krishnamurti, T. N. and P. Ardanuy (1980). "10-20-day westward propagating mode and breaks in the monsoons." Tellus, Stockholm 32(1): 15-26.
Daily surface pressure data records for a 40-yr period (1933-1972) were used in this study. The dominant transient modes of the wavenumber frequency spectra of surface pressure along the latitudes of the monsoon trough (i.e., 20 degrees -30 degrees N) are determined from a longitude-time composite of approximately 3 mo of surface pressure data for 10 separate episodes of breaks in the monsoons. This data base is composited relative to a reference (0, 0) that denotes the day of commencement of the break in rainfall over central India and the longitude of central India (i.e., 75 degrees E). These dominant modes, as determined from this composited data, exhibit interesting westward and eastward propagating modes. Some of the salient modes exhibit steady variations of phase from one day to the next. The period of breaks in the monsoon rainfall coincides with a pressure rise associated with the arrival of a ridge of the dominant modes over the reference origin (0, 0). The remaining 30 yr of data are subjected to a test of a hypothesis that the steady propagation of phase of a dominant westward propagating mode can be used to extrapolate and, thus, to predict, the arrival of this ridge. The tests show that a 10-day linear extrapolation of the phase to day 0 exhibits considerable skill in locating the ridge of the monsoon breaks over central India. In over 70% of the cases examined, the arrival of the ridge coincides with a period of the observed breaks in the monsoon. Suggestions for casting this problem in a truly predictive frame are made.
Krishnamurti, T. N., D. Bachiochi, et al. (2000). "Coupled atmosphere-ocean modeling of the El Nino of 1997-98." Journal of Climate, Boston, MA 13(14): 2428-2459.
This study is based on a global coupled atmosphere-ocean model climate prediction that was designed to include 14 layers over the atmosphere and 17 layers within the ocean. In this model an 11-yr data assimilation includes physical initialization of the daily rainfall estimates. No flux corrections are included in the seasonal and annual forecasts of this coupled model. It is first shown that intraseasonal oscillation on the Madden-Julian timescale was an important feature during the onset of the El Nino of 1997. It is shown that this feature is retained in the model's data assimilation and in the forecasts. The forecasts commence on 1 April 1997. The model forecasts showed an El Nino warming of the equatorial Pacific Ocean waters commencing with the excitation of a Kelvin wave. The Nino-3.4 region acquired above-normal sea surface temperature anomalies (SSTAs) by 15 May. The warm SSTs reached a peak by around January 1998. The El Nino made its demise by June 1998. The life cycle of the entire SSTA shows remarkable agreement to the observed anomalies over the Pacific Ocean. The subsurface temperature anomalies exhibit eastward propagating subsurface warm and cold water that are in phase with the El Nino and the La Nina features at the surface. Phenomenologically, this study is quite successful in showing the following. (a) Velocity potential anomalies at the 200-hPa level are good indicators for long-lasting dry spells. In particular the authors have remarkable success in predicting the long-lasting dry spell over Florida (which resulted in major fires over Florida during June 1998, some 14 months into the forecast) and over Indonesia (which resulted in major fires over Indonesia during September and October 1997). This was by far the most promising result of the coupled modeling study. This study also enumerates several areas of the climate of 1997-98 that were not reasonably simulated at the present resolution of the coupled model. The model does not exhibit very high skill in prediction of precipitation anomalies over the Asian-Australian monsoon world, which is most likely due to the resolution and organization of convection issues. (b) A realistic picture is shown of the North American monsoon system (the Mexico-Arizona monsoon) with wet conditions along 110 degrees W, dry conditions along 95 degrees W, and wet conditions along 80 degrees W during the summers of 1997 and 1998. Furthermore, the model successfully shows a stronger North American monsoon system during the post-El Nino year 1998 compared to the El Nino year 1997. This is in accordance with the climatological and observational findings. (c) California rainfall during January and February 1998, arising from the eastward passage of disturbances from the Pacific Ocean, was successfully simulated, although the rainfall amounts at the model resolution were roughly one-third of the observed peak estimates.
Krishnamurti, T. N. and H. N. Bhalme (1976). "Oscillations of a monsoon system, Pt. 1, Observational aspects." Journal of the Atmospheric Sciences, Boston 33(10): 1937-1954.
In this paper, the elements of a monsoon system are defined, and its oscillations are determined from spectral analysis of long observational records. The elements of the monsoon system include pressure of the monsoon trough, pressure of the Mascarene high, cross-equatorial low level jet, Tibetan high, tropical easterly jet, monsoon cloud cover, monsoon rainfall, dry static stability of the lower troposphere, and moist static stability of the lower troposphere. The summer monsoon months over India during normal monsoon rainfall years are considered as guidelines in the selection of data for the period of this study. The salient result of this study is that there seems to exist a quasi-biweekly oscillation in almost all of the elements of the monsoon system. For some of these elements, such as the surface pressure field, monsoon rainfall, low level, cross-equatorial jet, and monsoon cloudiness, the amplitude of this oscillation in quasi-biweekly range is very pronounced. For the spectral representation of the time series, the product of the spectral density times frequency is used as the ordinate and the log of the frequency is used as the abscissa. Dominant modes are also found in the shorter time scales (<6 days). A sequential ordering of elements of the monsoon systems for the quasi-biweekly oscillation is made in terms of their respective phase angle. The principal result is that soon after the maximum dry and moist static instabilities are realized in the stabilizing phase, there occur in sequence an intensification of the monsoon trough, satellite brightness, the Mascarene high, the Tibetan high, and the tropical easterly jet. Soon after that, the rainfall maximum over central India, arising primarily from monsoon depressions, is found to be a maximum. In the second part of this paper, some plausible mechanisms for these quasi-biweekly oscillations are offered.
Krishnamurti, T. N., S.-O. Han, et al. (1995). "Prediction of the dry and wet spell of the Australian monsoon." International Journal of Climatology, Chichester, UK 15(7): 753-771.
This study is a sequel to two recent studies on the monsoonal dry and wet spells. The previous studies addressed the dry and wet spells of the monsoon over India and China on the intraseasonal time-scales. The present study is intended to provide an outlook for the occurrence of dry and wet spells in reference to the Australian monsoon, within roughly a 1-month period from the time of start of the forecast with a global model. In the global model, the definition of a time mean state and an intraseasonal component are retained. The high-frequency component is viewed as one where inclusion in the initial state can contribute to a degradation of month long forecasts and hence is excluded initially. Sea-surface temperature anomalies on intraseasonal time-scales are included in this analysis. The prediction of the wet spells of the monsoon is assessed from the superposition of the predicted intraseasonal cyclonic circulation anomaly on top of the climatological cyclonic anomaly. The dry spell is assessed from an inverse superposition. We consider here a specific period, i.e. March 1992. The early part of this month was characterized by a wet spell over Northern Australia. A dry spell prevailed during the middle of this month and was followed by a wet spell during the latter part of the month. The prediction experiments with a prescribed and a modelled sea-surface temperature anomaly successfully demonstrated the passage of the intraseasonal anomaly, with the superposition of the geometries of the intraseasonal and the climatic components favouring the wet and the dry spell during different periods of the month.
Krishnamurti, T. N. and Y. Ramanathan (1981). "Sensitivity experiments on the monsoon onset to differential heating." Florida. State Univ., Tallahassee, Dept. of Meteorology, Report(81).
The authors examine observational aspects of the evolution of energy exchanges and differential heating during the GARP Monsoon Experiment MONEX. It is found that a large increase in the kinetic energy of the total and the nondivergent component of the flows occurs over the Arabian Sea approximately one week prior to the onset of monsoon rains over central India. The field of differential heating evolves during the onset period and moves to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy. The release of eddy available potential energy goes to enhance the kinetic energy of the divergent circulations. The kinetic energy of the divergent circulation does not increase much with time; this energy is shown to be transferred rapidly to the nondivergent motion through a number of interaction functions. The orientation of the divergent flows is found to be of prime importance in these transfers during the onset, active, break, and revival periods of the monsoons. On the basis of these observational findings, a series of numerical prediction experiments are constructed to examine the sensitivity of the monsoon onset to initially imposed fields of differential heating. The results of 96-hr integrations seem to confirm the large sensitivity of the evolution to the field of heating. The results of these integrations also show that the kinetic energy of the monsoon circulations increases via the rapid increase of interactions among the irrotational and nondivergent modes.
Krishnamurti, T. N. and Y. Ramanathan (1981). "Sensitivity experiments on the monsoon onset to differential heating." Workshop on Tropical Meteorology and its Effects on Medium Range Weather Prediction at Middle Latitudes, Reading, Eng., March.
The observational aspects of the response of zonal flows to differential heating during monsoon onsets is examined, leading to the construction of numerical experiments to study the responses of zonal flow to imposed initial fields of differential heating. The main findings are that 1) large increases in the kinetic energy of the total and the nondivergent components of the flows occur over the Arabian Sea about a week before the onset of monsoon rains over central India; 2) the field of differential exchange evolves during the onset period, moving to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy; 3) the release of eddy potential energy enhances the kinetic energy of divergent circulations; 4) the kinetic energy of the divergent circulation increases little with time and is transformed rapidly to the nondivergent motion through a number of interaction functions; and 5) the orientation of the divergent flows is of fundamental importance in these transfers during the onset, break, and revival periods of monsoons. The numerical experiment testing the sensitivity of monsoon onset to initially imposed fields of differential heating by condensation, radiative processes, and surface heat fluxes involving 96-hr integrations shows a rapid increase of zonal and kinetic energy only when a postonset field of differential heating is used. An increase of the kinetic energy of the monsoon circulation caused by the rapid increase of interactions among the irrotational and nondivergent modes, is demonstrated.
Krishnamurti, T. N., M. C. Sinha, et al. (1992). "Angular momentum, length of day and monsoonal low frequency mode." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 131-166.
In this paper some global aspects of the intraseasonal oscillations on the time scale of 30 to 50 days are explored. Noting that the variability of zonal flow of the monsoon, the atmospheric angular momentum and the length of day are strongly correlated on this time scale, we have made an effort to examine the global variability using the length of day as a point of reference. The scenario of this cycle is presented starting from a super cloud cluster at the near equatorial latitudes. This seems to be accompanied with an acceleration of zonal flows, an increase of the atmospheric angular momentum and an increase in the length of day. The transfer of westerly angular momentum from the earth to the atmosphere occurs over regions of the surface easterlies to the east of the super cloud clusters resulting in an increase in the length of day. During this transition from a mean length of day to a maximum length of day, an active phase of the Indian summer monsoon is noted. The interesting aspect of the length of day transition occurs on its return cycle when the near equatorial cloud cover eases or moves away from the equator with a decrease in the monsoonal zonal flows and a reduction of this component of atmospheric angular momentum. The length of day does not simply go back to an equilibrium value, but the long term data from the laser ranger shows an overshooting beyond that to a minimum value. This transition is characterized in general by monsoon break-like conditions, counter monsoon flows in the low levels and by a transition from high index to low index conditions in the upper troposphere of the middle latitudes. Phenomenologically, some blocking situations have been noted over the higher middle latitudes during this transition. The reduction of the angular momentum is attributed to the transfer of the westerly angular momentum from the atmosphere to the earth via frictional and mountain torques. These torques exhibit a clear relationship to the changes in the atmospheric angular momentum on this time scale. The behavior of the middle latitude low frequency variability is also in part explained by the meridional wave energy flux. That problem is examined in this context with the full nonlinear equations in the frequency domain. It is shown that unlike the linear problems where such fluxes are inhibited beyond the critical latitude, the nonlinear problem permits the temporal oscillations of zonal flows on this time scale. As a consequence, a significant tropical middle latitude coupling is noted by this process. A simple mathematical model of the oscillation is also presented. It is a local theory in which ocean and atmosphere interact. Initially, the atmosphere is stably stratified with weak winds at the sea surface and stronger winds aloft; the ocean has a surface mixed layer of temperature T sub(s) lying over deep cold water. Solar heating gradually increases T sub(s) which leads to atmospheric convection with associated transport of horizontal momentum and increased winds at the sea surface. Increased winds lead to deepening of the mixed layer and a drop in T sub(s) because of mixing of deep cold water with surface waters. Convection ceases, winds decay, and the cycle repeats only after solar heating has once more increased T sub(s) . The period of this oscillation is shown to be on the order of 30 days.
Krishnamurti, T. N., M. C. Sinha, et al. (1998). "Tropical-middle latitude interactions viewed via wave energy flux in the frequency domain." Dynamics of Atmospheres and Oceans, Amsterdam, The Netherlands 27(1-4): 383-412.
Over the upper troposphere of the polar latitudes the zonal flows exhibit a large variance on the time scale of the Madden-Julian oscillation, i.e. roughly 30-50 days. The other prominent regions for these intraseasonal oscillations are the Asian and Australian monsoon belts. These two regions are separated by the so-called critical latitude, to the south of which easterlies generally prevail and westerlies are prevalent to the north. A perplexing issue is that of possible tropical-middle latitude interactions across the critical latitude. The notion of the critical latitude emerged from the linear theories for the wave energy flux which assume a constancy in time for the zonal flows. This same problem, viewed in its full non-linear context, can be cast in a frequency domain. Such a formulation does not assume a constancy of the zonal flows in time but does permit the intraseasonal variations of the zonal flows to be present. The computation of the wave energy flux, from the more complete non-linear system in the frequency domain, requires the handling of linear, quadratic and triple product terms via use of Hayashi's co-spectral method. These results of the present study, based on 6 years of daily global data sets, show that wave energy flux clearly passes from the latitudes of the monsoon to the polar latitudes. A strong convergence of wave energy flux in the polar latitudes suggests the tropical-middle latitude convergence interactions across the so-called critical latitude--when the problem is viewed in the frequency domain.
Krishnan, R. and S. V. Kasture (1996). "Modulation of low frequency intraseasonal oscillations of northern summer monsoon by El Nino and Southern Oscillation (ENSO)." Meteorology and Atmospheric Physics, Vienna, Austria 60(4): 237-257.
In order to improve our understanding of the interannual variability of the 30-50 day oscillations of the northern summer monsoon, we have performed numerical experiments using a 5-level global spectral model (GSM). By intercomparing the GSM simulations of a control summer experiment (E1) and a warm ENSO experiment (E2) we have examined the sensitivity of the low frequency intraseasonal monsoonal modes to changes in the planetary scale component of the monsoon induced by anomalous heating in the equatorial eastern Pacific during a warm ENSO phase. It is found that the anomalous heating in the equatorial eastern Pacific induces circulation changes which correspond to weakening of the time-mean divergent planetary scale circulation in the equatorial western Pacific, weakening of the east-west Walker cell over the western Pacific ocean, weakening of the time-mean Reverse Hadley circulation (RHC) over the summer monsoon region and strengthening of the time-mean divergent circulation and the subtropical jet stream over the eastern Pacific and Atlantic oceans. These changes in the large scale basic flow induced by the anomalous heat source are found to significantly affect the propagation characteristics of the 30-50 day oscillations. It is noticed that the reduction (increase) in the intensity of the time-mean divergent circulation in the equatorial western (eastern) Pacific sectors produces weaker (stronger) low-level convergence as a result of which the amplitude of the eastward propagating 30-50 day divergent wave decreases (increases) in the western (eastern) Pacific sectors in E2. One of the striking aspects is that the eastward propagating equatorial wave arrives over the Indian longitudes more regularly in the warm ENSO experiment (E2). The GSM simulations reveal several small scale east-west cells in the longitudinal belt between 0-130 degrees E in the E1 experiment. On the other hand the intraseasonal oscillations in E2 show fewer east-west cells having longer zonal scales. The stronger suppression of small scale east-west cells in E2 probably accounts for the greater regularity of the 30-50 day oscillations over the Indian longitudes in this case. The interaction between the monsoon RHC and the equatorial 30-50 day waves leads to excitation of northward propagating modes over the Indian subcontinent in both cases. It is found that the zonal wind perturbations migrate northward at a rate of about 0.8 degrees latitude per day in E1 while they have a slightly faster propagation speed of about 1 degrees latitude per day in E2. The low frequency monsoonal modes have smaller amplitude but possess greater regularity in E2 relative to E1. As the wavelet trains of low latitude anomalies progress northward it is found that the giant meridional monsoonal circulation (RHC) undergoes well-defined intraseasonal oscillations. The amplitude of the monsoon RHC oscillations are significantly weaker in E2 as compared to E1. But what is more important is that the RHC is found to oscillate rapidly with a period of 40 days in E1 while it executes slower oscillations of 55 days period in E2. These results support the observational findings of Yasunari (1980) who showed that the cloudiness fluctuations on the 30-60 day time scale over the Indian summer monsoon region are associated with longer periods during El Nino years. The oscillations of the monsoon RHC show an enhancement of the larger scale meridional cells and also a stronger suppression of the smaller scale cells in E2 relative to E1 which seems to account for the slower fluctuations of the monsoon RHC in the warm ENSO experiment. It is also proposed that the periodic arrival of the eastward propagating equatorial wave over the Indian longitudes followed by a stronger inhibition of the smaller meridional scales happen to be the two primary mechanisms that favour steady and regular northward propagation of intraseasonal transients over the Indian subcontinent in the warm ENSO experim
Krishnan, R. and C. Venkatesan (1997). "Mechanisms of low frequency intraseasonal oscillations of the Indian summer monsoon." Meteorology and Atmospheric Physics, Vienna, Austria 62(1-2): 101-128.
This work deals with idealized modelling experiments designed to understand the dynamical evolution of low frequency intraseasonal monsoonal oscillations that result from interactions between the large scale monsoon Reverse Hadley Cell (RHC) and moist convective processes. The monsoon differential heating, which primarily determines the low-level convergence of the large-scale monsoon flow, is found to play a decisive role in affecting the northward progression of the monsoonal modes. A strong north-south differential heating leads to a robust generation and steady maintenance of northward propagating monsoonal oscillations. A weaker land-ocean thermal contrast leads to feeble low frequency monsoonal modes that have relatively longer periods in the 30-50 day band. This increase in the period of the monsoonal oscillations due to weak north-south thermal contrast is in good agreement with the observational findings of Yasunari (1980) and Kasture and Keshavamurty (1987). It is speculated that such an increase in the oscillatory period may be an outcome from an elongation in the meridional scale of the transient Hadley type cells which act as resonating cavities for the monsoonal modes. A Mobile Wave CISK (MWC) form of interaction between the large scale monsoon and the transient circulations associated with the Madden Julian Oscillation (MJO) is projected as a viable physical mechanism for the northward movement of low frequency modes. It is demonstrated that the effective low level convergence, following such an interaction, tends to shift northward relative to the site of interaction. This enables the heating perturbations to be displaced northward which in turn causes the secondary circulations and wind perturbations to follow. The essential criterion for the occurrence of a prolonged northward propagation of the low frequency modes is that the heating perturbations should phase lead the wind perturbations at all times. An examination of the psi - chi interactions on the 30-50 day time scale reveals that the conversion from the transient divergent motions to rotational motions is quite intense (feeble) in the strong (weak) monsoon differential heating experiments. Because of the closer proximity to the monsoon heat source and also due to the latitudinal variation of Earth's rotational effects, the psi - chi interactions tend to be more pronounced to the north of 15 degrees N while they are less robust in the near equatorial latitudes. The regularity of the monsoonal modes is found to depend on the strength of the monsoon differential heating and also on the periodic behaviour of the equatorial intraseasonal oscillations. The monsoonal modes are quite steady and exhibit extreme regularity in the presence of a weak north-south differential heating provided the equatorial forcing due to the MJO varies in a periodic manner. This result supports the findings of Mehta and Krishnamurti (1988) who found greater regularity of the 30-50 day modes during bad monsoon years. The low frequency monsoonal modes are found to be quite sensitive to the moisture availability factor (m) and the vertical profile of heating used in the MWC parameterization. A small increase in the value of (m) is found to significantly intensify the amplitude of the monsoonal oscillations while there is no considerable shift in the spectral frequency within the 30-50 day band as such. The 30-50 day motions show significant enhancement, with a relatively sharp spectral peak around 45 days, when the vertical profile of MWC heating has a maximum in the lower troposphere. However an upward displacement of the heating maximum tends to weaken the low frequency oscillations.
Krishnan, R., C. Zhang, et al. (2000). "Dynamics of breaks in the Indian summer monsoon." Journal of the Atmospheric Sciences, Boston, MA 57(9): 1354-1372.
In this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of ``breaks'' in the Indian summer monsoon (ISM). Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over the Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian Ocean. A major intensification of the convectively stable anomalies over the Bay of Bengal is seen about 2-3 days prior to commencement of a monsoon break. Both observations and modeling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from the Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from the central Bay of Bengal to northwest India in about 2-3 days' time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves the way for the emergence of a convectively unstable anomaly over the equatorial Indian Ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from the central Bay of Bengal toward northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the ISM and also help maintain the mutual competition between convection over the Indian subcontinent and that over the equatorial Indian Ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic flow, which is driven by a steady north-south differential heating, is a primary physical mechanism that controls the so-called monsoon breaks.
Kuhnel, I. (1991). "Cloudiness fluctuations over Eastern Africa." Meteorology and Atmospheric Physics, Vienna, Austria 46(3-4): 185-195.
Based on ESSA-satellite imagery for the period July 1969-June 1970, this study investigates spatial and temporal variations of East African cloudiness. The major results of this work show that the mean annual cloud amounts over East Africa are lower than those in adjacent tropical areas. One of the main reasons for this is the quasi-meridional alignment of the ITCZ over East Africa during the winter months. Within the area itself, the highest mean annual cloud amount values can generally be found in a diagonally oriented zone extending from the eastern Congo Basin to the Ethiopian Highlands. In contrast to the cloudiness north of the equator, which is dominated by oscillation periods in the range of 30-60 days, the cloudiness fluctuations encountered south of the equator show periodicities around 2 days (in the western part) and 20 days (in the eastern part), respectively. The different oscillation patterns, which are roughly separated by the Rift Valley area (longitudinally) and the equator (latitudinally), resemble the signals of the adjacent (African and Asian) monsoon regimes. However, during the winter months oscillation periods around 40 days can be found north of the equator, whereas a quasi-biweekly oscillation appears over the coastal areas in summer. Further details of the seasonal variability of East African cloudiness are discussed.
Kulkarni, P. L. (1986). "Estimates of heat and moisture over the Indian monsoon trough zone during monsoon 1979." Mausam, New Delhi 37(4): 533-536.
In 1979, the southwest monsoon had many unique features. It was largely a deficient monsoon year. Generally, the monsoon was weak in the month of July, and then there was a prolonged break after the middle of Aug. During 1979, the differences in the estimates of mass and heat fluxes in the lower and middle troposphere of the monsoon trough area were studied in comparison with the normal monsoon conditions based upon 20 yr of data from 1951 to 1970.
Kumar, J. R. and S. K. Dash (1999). "Inter-annual and intra-seasonal variation of some characteristics of monsoon disturbances formed over the Bay." Mausam 50(1): 55-62.
The characteristics of monsoon disturbances during drought and flood years for the period 1971-96 are studied to find out their inter-annual variations. Variations of some of the characteristics of monsoon disturbances formed over Bay during 1979-88, with respect to different monsoon conditions such as strong, weak and break monsoons, are also studied. The results show that monsoon disturbance days are higher during flood years than during drought years. Drought years are associated with higher chances of low pressure areas to intensity into depressions, less westward movement, more horizontal extent, intense pressure departure from normal in comparison with flood years. However, more monsoon disturbances tilt significantly during flood years. The rainfall associated with these disturbances is highly variable and does not depend on the intensity, horizontal and vertical extent of the individual system. More number of lows intensify into depressions during storng monsoon conditions compared to those of weak monsoon conditions. Lows and depressions during strong monsoons have more westward movement and longer life period. Generally, very few lows form during break monsoon and none of them intensify into depression. Hence, the presence of mid-tropospheric heating during strong and weak monsoons is essential for the formation of depression. Synoptic systems which abate break monsoon condition and re-establish normal monsoon are also discussed.
Kurbet, A. S. (1989). "Synoptic situations associated with spells of strong and weak monsoon over Madhya Maharashtra." Mausam, New Delhi 40(2): 225-227.
The synoptic situation with the unusual heavy rainfall during Aug. 1969 reported in Madhya Maharashtra is examined. The Aug. 1969 rainfall was among the top two or three rainiest Augusts recorded during the 75 yr of 1901-1975. Rainfall was in excess of normal in nearly three fourths of the region, and by >200% in one fourth of the region. It is shown that, although on most of the occasions of the passage of surface or upper air flows across the South Peninsula during break monsoon conditions, rainfall is scanty in Madhya Maharashtra, on the rare occasions when the upper air cyclonic circulation associated with the system extends to Madhya Maharashtra, the region may get a spell of vigorous monsoon.
Kusuma, G. R., S. Raman, et al. (1991). "Boundary-layer heights over the monsoon trough region during active and break phases." Boundary-Layer Meteorology, Dordrecht, The Netherlands 57(1-2): 129-138.
The thermodynamic structure and the heights of the boundary layer over the monsoon trough region of the Indian southwest monsoon are presented for the active and break phases of the monsoon. Results indicate significant and consistent variation in boundary-layer heights between the active and break phases.
Lal, M., T. Nozawa, et al. (2001). "Future climate change: Implications for Indian summer monsoon and its variability." Current Science 81(9): 1196-1207.
The broad climatological features associated with the Asian monsoon circulation, including its mean state and intraseasonal and interannual variability over the Indian subcontinent, as simulated in the CCSR/NIES coupled A-O GCM in its control experiment are presented in this paper. The model reproduces the seasonal cycle as well as basic observed patterns of key climatic parameters, in spite of some limitations in simulation of the monsoon rainfall. While the seasonality in rainfall over the region is well simulated and the simulated area-averaged monsoon rainfall is only marginally higher than the observed rainfall, the peak rainfall is simulated to be about two-thirds of the observed precipitation intensity over central India. The transient experiments performed with the model following the four SRES 'Marker' emission scenarios, which include revised trends for all the principal anthropogenic forcing agents for the future, suggest an annual mean area-averaged surface warming over the Indian subcontinent to range between 3.5 and 5.5 degree C over the region during 2080s. During winter, India may experience between 5 and 25% decline in rainfall. The decline in wintertime-rainfall over India is likely to be significant and may lead to droughts during the dry summer months. Only a 10 to 15% increase is projected in area-averaged summer monsoon rainfall over the Indian subcontinent. The date of onset of summer monsoon over India could become more variable in future.
Lal, M., K. K. Singh, et al. (1999). "Growth and yield responses of soybean in Madhya Pradesh, India to climate variability and change." Agricultural and Forest Meteorology, Amsterdam, The Netherlands 93(1): 53-70.
This study is aimed at assessing the impact of thermal and moisture stresses associated with observed intraseasonal and interannual variability in key climatic elements on the nature and extent of losses in growth and yield of soybean crop in central India through the use of CROPGRO model. The crops are found to be more sensitive to higher cumulative heat units during cropping season. The yields respond substantially to temporal variations in rainfall (associated with observed swings in the continuity of monsoon). Prolonged dry spells at critical life stages of the soybean crop are found to adversely affect crop development and growth and hence the yields at selected sites. We have also examined the plausible effects of future climate change on soybean yields in the selected region based on simulations carried out for doubled atmospheric CO sub(2) level and with modified weather variables using the available seasonal projections for the future. Our findings on the response of elevated CO sub(2) concentrations in the atmosphere suggest higher yields (50% increase) for soybean crop for a doubling of CO sub(2) . However, a 3 degrees C rise in surface air temperature almost cancels out the positive effects of elevated CO sub(2) on the yield. Soybean crops at selected site are more vulnerable to increases in maximum temperature than in minimum temperature. The combined effect of doubled CO sub(2) and anticipated thermal stress (likely by middle of the next century) on soybean crop is about 36% increase in yield at the selected sites. A decline in daily rainfall amount by 10% restricts this yield gain to about 32%. Deficient rainfall with uneven distribution during the monsoon season could be a critical factor for the soybean productivity even under the positive effects of elevated CO sub(2) in the future.
Lau, K. M. (1989). "Seasonal and intraseasonal variations of the East Asian summer monsoon." Sham, P. and Chang, C. P.
Large scale features associated with the seasonal and intraseasonal variations of the East Asian summer monsoon are reviewed. It is shown that the onset of the Mei-yu in southern and central China is coincident with the arrival of the first wave of organized convection/rainfall associated with the 30-60 day oscillation. The latter propagates eastward along the equator with an average speed of about 3-5 m/s over the Indian Ocean/Western Pacific region. An east-west and a north-south seasaw are found between the Indian Ocean and the Western Pacific, and between the equator and the monsoon region (15-20 degrees N, 60-150 degrees E). These seesaws are associated with the northward and eastward propagation of the rising and sinking branches of the divergent circulation associated with the 30-60 day oscillations. Over the monsoon ITCZ region, westward propagating cyclones with periods of four to five days and wavelengths of 1000-2000 km are predominant. Most interestingly, over the equatorial Western Pacific, westward propagating cloud clusters with periods of 2-3 days and length scales of 500-1000km are found to be embedded in eastward propagating superclusters associated with the 30-60 day oscillations. Some of the clusters are seen to develop into tropical cyclones over the northern South China Sea and the East China Sea and then follow a northeastward path to reach Japan. Others may propagate further westward across Indo China and subsequently develop into monsoon depressions over the Bay of Bengal.
Lau, K. M. (1992). "East Asian summer monsoon rainfall variability and climate teleconnection." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 211-242.
In this paper, recent progress in the study of the East Asian summer monsoon (EAM) and its impact on global climate fluctuations are reviewed. The review is focused on the climatology and variability of the EAM rainfall and its relationship with regional and global scale circulation systems. Climatologically, the EAM rainfall is dominated by convective activities associated with the northward advance of the Mei-yu trough from southern China during April-May to central China during mid-June. After staying in the same position for one to two weeks, the Mei-yu trough disappears abruptly and a new rainfall zone is developed over northern China. This is followed by a quasi-20 days oscillatory rainfall regime which develops over central China. Subsequently, the maximum rainfall zone returns to the coastal region of south and southeast China. Regional features unique to the EAM include the extraordinary length of the extended monsoon season (April to late August), the extent of the northward penetration of the major precipitation, the multiple onset and interspersed propagation and stationary nature of the rainfall. Planetary scale features that directly influence the EAM include the western Pacific Subtropical High, the Tibetan High, the local Hadley and the equatorial Walker circulations. It is stressed that the EAM rainfall is only a small part of the global scale precipitation system which migrates northward from the equatorial Indian Ocean and the Western Pacific to the EAM region and Indian subcontinent during the boreal summer. The EAM possesses a wide range of spatial and temporal scales of variabilities including the seasonal cycle, intraseasonal oscillations, subseasonal scale inter-monsoon interactions, sub-synoptic scale variability and supercluster organization in the western Pacific. These variabilities are in turn linked to interannual variations associated with the biennial oscillation and the El Nino/Southern Oscillation. Also discussed is evidence showing the presence of an atmospheric teleconnection pattern connecting eastern Asia and North America (ANA) via the North Pacific. The ANA has profound impact on the climate of eastern Asia including Japan. Dynamically, it may be associated with a marginally unstable barotropic mode in the mean Northern Hemisphere summertime circulation. This mode is also related to latent heating in the western Pacific near the Philippines as well as the Indian Ocean region. While there are some successes in the general circulation model (GCM) simulation of the planetary scale features of the EAM, most GCMs still have problems obtaining realistic regional rainfall over East Asia and India. The intraseasonal and interannual variability of the EAM are generally not very well-simulated in GCMs. Much work is needed to improve modeling of the variability of the EAM.
Lau, K. M. and C.-P. Chang (1987). "Planetary-scale aspects of the winter monsoon and atmospheric teleconnections." Chang, Chih Pei.
This paper documents the characteristics of the planetary-scale winter monsoon system mainly on the basis of results of several recent observational studies such as the First GARP (Global Atmospheric Research Program) Global Experiment/Monsoon Experiment (FGGE /MONEX). The following topics are discussed: 1) seasonal mean fields; 2) short-term fluctuations of the time scales of a few days, including cold surge genesis, regional energetics, midlatitude tropical interactions (active monsoon periods and winter MONEX), jet stream dynamics, and interhemispheric interactions; 3) intraseasonal variations, including tropical 40-50-day oscillations, atmospheric teleconnections (extratropical pattern, tropical-extratropical pattern), and 30-day oscillations; 4) interannual variability, including teleconnection in rainfall anomalies, winter monsoon during the 1982-1983 ENSO, cold surges and ENSO, and possible interpretation of the interannual variability of the winter monsoon in terms of a bimodal climatic state found over the Tropical Pacific ocean-atmosphere system.
Lau, K. M. and L. Peng (1990). "Origin of low frequency (intraseasonal) oscillations in the tropical atmosphere. Part III: Monsoon dynamics." Journal of the Atmospheric Sciences, Boston, MA 47(12): 1443-1462.
This is the third in a series of papers to study the origin of intraseasonal oscillations. In this paper, we address the issue of the interaction of the monsoon large-scale circulation and intraseasonal oscillations. We show that as a result of the interaction of the large scale monsoon flow with the near-equatorial intraseasonal oscillation, unstable baroclinic disturbances are generated over the monsoon region. These disturbances have spatial scales of approximately 3000-4000 km and periods of 5-6 days with the vertical wave axis tilting eastward with height. The rapid development of these cyclonic disturbances along 15 degrees -20 degrees N and the concomitant weakening of the equatorial disturbances are accompanied by the rapid northward shift of the rising branch of the local Hadley circulation. They may also be identified with the observed sudden jump of the Mei-yu rainband over East Asia and the inverse relationship between the monsoon ITCZ and the equatorial ITCZ over India and East Asia. From a linear stability analysis of quasi-geostrophic motion in a two-level model, it is shown that the westward propagating disturbances generated over the monsoon region are the manifestation of heat-induced unstable Rossby waves. The instability is favored in the region with large vertical wind shear and reduced effective static stability. The monsoon large scale circulation over India and southeast Asia and the plentiful supply of moisture in the region appear to be favorable for the development of these unstable waves. It is argued that the prevailing easterly waves found over the subtropical western Pacific during northern summer may also be due to the above unstable Rossby wave mechanism.
Lau, K. M., P. J. Sheu, et al. (1996). "Evolution of large-scale circulation during TOGA COARE: model intercomparison and basic features." Journal of Climate, Boston, MA 9(5): 986-1003.
An intercomparison study of the evolution of large-scale circulation features during TOGA COARE has been carried out using data from three 4D assimilation systems: the National Meteorological Center (NMC, currently known as the National Center for Environmental Prediction), the Navy Fleet Numerical Oceanography Center, and the NASA Goddard Space Flight Center. Results show that the preliminary assimilation products, though somewhat crude, can provide important information concerning the evolution of the large-scale atmospheric circulation over the tropical western Pacific during TOGA COARE. Large-scale features such as sea level pressure, rotational wind field, and temperature are highly consistent among models. However, the rainfall and wind divergence distributions show poor agreement among models, even though some useful information can still be derived. All three models shows a continuous background rain over the Intensive Flux Area (IFA), even during periods with suppressed convection, in contrast to the radar-estimated rainfall that is more episodic. This may reflect a generic deficiency in the oversimplified representation of large-scale rain in all three models. Based on the comparative model diagnostics, a consistent picture of large-scale evolution and multiscale interaction during TOGA COARE emerges. The propagation of the Madden and Julian Oscillation (MJO) from the equatorial Indian Ocean region into the western Pacific foreshadows the establishment of westerly wind events over the COARE region. The genesis and maintenance of the westerly wind (WW) events during TOGA COARE are related to the establishment of a large-scale east-west pressure dipole between the Maritime Continent and the equatorial central Pacific. This pressure dipole could be identified in part with the ascending (low pressure) and descending (high pressure) branches of the MJO and in part with the fluctuations of the austral summer monsoon. Accompanying the development of WW over the IFA and crucial to its maintenance is a robust meridional circulation, with strong cross-equatorial flow and rising motion near the entrance region of the WW and sinking motion in the extratropical Northern Hemisphere. The presence of a quasi-stationary equatorial heat source near the date line may have provided additional feedback mechanisms for the WWs. Surface pressure and wind surges related to cold air outbreaks off the east Asian continent play an important role in the rapid build up and/or termination of the WWs during TOGA COARE. The establishment of WWs in the near equatorial region may be linked to the modulation of North Pacific storm track activities.
Lau, K. M., G. J. Yang, et al. (1988). "Seasonal and intraseasonal climatology of summer monsoon rainfall over East Asia." Monthly Weather Review, Boston 116(1): 18-37.
This study attempts to bridge a gap in the knowledge and understanding of the East Asian monsoon by presenting a documentation of the seasonal and intraseasonal rainfall climatology of summer monsoon rainfall over East Asia. The rainfall climatology is viewed both in terms of its regional characteristics and in relation to the large-scale circulation with the aim of bringing to light the myriad phenomena associated with the East Asian monsoon and its relevance to current mainstream research in monsoon and climate dynamics. Specifically, climatology of 30- and 10-day rainfall records from East Asian stations are studied and compared with satellite outgoing long-wave radiation and the large-scale circulation field. Results show that two major monsoon rainfall onsets over East Asia are identified climatologically during the period from April to Sept. The first, known as the mei-yu (or plum rain), occurs over central China around the beginning or the middle of June, and the second over northeast China during late July. The multiple onsets occur as the major rainbands make rapid transitions or sudden jumps between three somewhat stationary positions over southern China (premonsoon rain), central China (the Mei-yu front), and northeastern China (the polar front). Also found are the presence of 40- and 20-day oscillations in the rainfall climatology. Both oscillations exhibit structure and propagation consistent with previous studies. Abrupt changes by the major rainbands appear to be related to phase locking between intraseasonal oscillations, such as the 40- and the 20-day mode, and the seasonal variation. A comparison of the rainfall climatology with that over India is also discussed.
Lau, K. M. and S. Yang (1996). "Seasonal variation, abrupt transition, and intraseasonal variability associated with the Asian summer monsoon in the GLA GCM." Journal of Climate, Boston, MA 9(5): 965-985.
In this paper, a description of the global and regional features of the Asian summer monsoon in the Goddard Laboratory Atmospheres (GLA) General Circulation Model (GCM) is presented based on a 10-yr (1979-1988) integration of the model under the Atmospheric Model Intercomparison Project. It is found that the earliest signal of onset of the Asian monsoon is represented by the emergence of convection over the Indo-China ``land bridge'' in early May, followed by the sudden jump of the equatorial ITCZ (Intertropical Convergence Zone) to 10 degrees N over the South China Sea and Indian Ocean in mid-May. This sudden jump is linked to an abrupt northward shift of the ascending branch of the local Hadley circulation, possibly stemming from symmetric instability of the basic flow in May. In agreement with observations, the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic-scale activities identified as easterly waves in the tropical and subtropical western Pacific. These intraseasonal and synoptic activities strongly regulate rainfall variability over east Asia, especially during the premonsoon period. While the model simulates a similar range of variability in the broad-scale structure and evolution of the Asian monsoon as observed, there is considerable scope for improvement in the model's ability to simulate regional features of the monsoon. Compared to observed climatology, the model produces deficient rainfall over northern India but excessive rainfall over the South China Sea and the western Pacific region due to an overactive ITCZ at 10 degrees N. The intraseasonal transition between the equatorial ITCZ and the monsoon (land) convection to the north is weaker than in nature. Moreover, the east Asian monsoon trough and Mei-yu rainband are underdeveloped in the model. These discrepancies are related to the limited northward extension of the model monsoon circulation and the confinement of the upper-level easterly flow near the equator. Overall, the GLA GCM provides a reasonably realistic description of the seasonal and subseasonal climatology of the Asian monsoon and yields important information that sheds new light on the dynamical underpinnings of the multiscale variabilities associated with the Asian summer monsoon.
Lau, K.-M. and P. H. Chan (1986). "Aspects of the 40-50-day oscillation during the northern summer as inferred from outgoing long-wave radiation." Monthly Weather Review, Boston 114(7): 1354-1367.
Intraseasonal variability of tropical convection over the Indian Ocean-western Pacific region during northern summer is studied by using outgoing long-wave radiation (OLR). OLR anomalies are found to propagate eastward along the Equator from the Indian Ocean to the western Pacific and northward toward the Indian subcontinent and southern China. It is found that the dominant mode of tropical convection consists of a dipole with centers located over the Indian Ocean and the western Pacific/South China Sea. This dipole undergoes complex structural changes over a broad period range centered around 40-50 days. During a typical oscillation, an anomaly first develops over the equatorial Indian Ocean. This anomaly then extends eastward to the equatorial western and central Pacific to form an elongated convection zone, while its center is displaced progressively northward from the Indian Ocean into the Indian subcontinent by an anomaly of the opposite sign. The elongated convection zone over the western Pacific then propagates northwestward over the South China Sea into southern China. The simultaneous northward propagation of the dipole centers over India and South China suggests that the two components of the summer monsoon are subject to the same large-scale control. The 40-50-day oscillation is also found to be phase locked to the monsoon onset over India, the mei-yu regime over South China, and in general, the seasonal variation of convection over the equatorial Indian-West Pacific Ocean. However, the eastward propagation along the Equator over the Indian Ocean appears to be present all year round. This suggests that the eastward propagation may be a basic property of equatorially trapped wave modes and that the meridional propagation arises as a result of the interaction between these equatorial modes and the monsoon circulations.
Lau, K.-M. and M.-T. Li (1984). "Monsoon of East Asia and its global associations: a survey." American Meteorological Society, Boston, Bulletin 65(2): 114-125.
Observations concerning the summer and winter monsoons of East Asia and their global associations are reviewed. The seasonal mean structure, transient variation, including intraseasonal to interannual, and synoptic- to planetary-scale fluctuations are discussed separately for the two monsoon components. Similarities and differences between the East Asian monsoon and that of India are surveyed. A description of the current status of monsoon-related observational and theoretical research and important scientific problems are presented. The importance of understanding the long-term anomalies of the monsoon is stressed, and an attempt is made to put the East Asian monsoon in a global perspective, with a view toward identifying with the problems of long-range weather forecasting or short-term climate prediction. Future directions of research are suggested.
Lau, N.-C. and K.-M. Lau (1986). "Structure and propagation of intraseasonal oscillations appearing in a GFDL general circulation model." Journal of the Atmospheric Sciences, Boston 43(19): 2023-2047.
The three-dimensional structure and temporal evolution of quasi-periodic, planetary-scale tropospheric oscillations simulated by a 15-wavenumber GCM are investigated by applying cross-spectral, eigenvector, composite, and temporal correlation techniques to 12 yr of model output. Evident from this diagnostic study is the presence in the model tropics of well-defined, eastward-traveling features with spatial scales of zonal wavenumbers 1 and 2, and with temporal scales of 25-40 and 15-20 days, respectively. The flow pattern associated with the oscillations of both spatial scales is characterized by circulation cells oriented along the equatorial zonal plane, with the zonal wind and geopotential height fluctuations near the sea level being negatively correlated with the corresponding fluctuations in the upper troposphere. The movement of these zonal circulation cells along the equatorial belt is accompanied by systematic migration of the global-scale horizontal divergence field, and by dipolelike precipitation structures within the Indonesian/Pacific sector. The preferred sites for such oscillatory behavior exhibit a notable seasonal dependence, with the most active zonal circulation cells being located in the summer hemisphere. During the northern summer, the 25-40-day oscillations coincide with the occurrence of northward-moving, zonally elongated rainbands over the monsoon region of South Asia. During the northern winter, the 25-40-day phenomena in the Tropics are linked to well-organized extratropical wave trains spanning the Eurasian and Pacific-North American sectors. The principal characteristics of the model-generated phenomena analyzed in this study are compared with corresponding results reported in the observational literature. Although the period of the simulated wavenumber-1 phenomena is somewhat shorter than the corresponding observed values, it is demonstrated that the spatial structure, propagation characteristics, and seasonal dependence of the model features are consistent with observations. The model findings are also interpreted in terms of current theoretical understanding of tropical and extratropical motions.
Lawrence, D. M. and P. J. Webster (2001). "Interannual Variations of the Intraseasonal Oscillation in the South Asian Summer Monsoon Region." Journal of Climate 14(13): 2910-2922.
It is noted that the behavior of the intraseasonal oscillation (ISO) of the south Asian monsoon varies from year to year. An index representing seasonally averaged ISO activity is developed using outgoing longwave radiation data for the period 1975-97. Interannual variations in ISO activity are found to be related to year-to-year changes in the number of discrete events rather than to changes in the characteristic period. Summertime ISO activity exhibits a reasonably strong inverse relationship with Indian monsoon strength but not with total south Asian monsoon strength primarily because of a lack of correlation between ISO activity and the Bay of Bengal component of the south Asian monsoon. Over the 22-yr period examined here, the relationship between Indian monsoon strength and ISO activity is comparable to or even stronger than the well-documented relationship with El Nino-Southern Oscillation (ENSO). However, summertime ISO activity is found to be relatively uncorrelated with ENSO except for a weakly positive correlation at the beginning of the south Asian monsoon season. Therefore, the ISO activity-Indian monsoon relationship is essentially independent of the ENSO-Indian monsoon relationship. ISO activity is uncorrelated with any other contemporaneous or leading sea surface temperature variability.
Lawrence, D. M. and P. J. Webster (2002). "The Boreal Summer Intraseasonal Oscillation: Relationship between Northward and Eastward Movement of Convection." Journal of the Atmospheric Sciences 59(9): 1593-1606.
The summertime intraseasonal oscillation (ISO) is an important component of the south Asian monsoon. Lagged regressions of intraseasonally filtered (25-80 days) outgoing longwave radiation (OLR) reveal that centers of convection move both northward and eastward from the central equatorial Indian Ocean subsequent to the initiation of an ISO. Eastward movement of convection is also seen at Indian subcontinent latitudes (10 '-20 'N). Based on the regression results, the summertime ISO convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 20 'N. Viewed along any meridian, convection appears to propagate northward while equatorial convection propagates to the east. To examine the robustness of the connection between eastward and northward movement, individual ISOs are categorized and composited relative to the strength of the large-scale eastward component of convection in the central equatorial Indian Ocean. It is found that the majority of ISOs that exhibit northward movement onto the Indian subcontinent (42 out of 54 ISOs, or 78%) also exhibit eastward movement into the western Pacific Ocean. It is also found that when convection in the central Indian Ocean is not followed within 10-20 days by convection in the western Pacific Ocean (12 out of 54 ISOs, or 22%), the independent northward movement of convection in the Indian Ocean region is somewhat stunted. The link between the eastward and northward movement of convection is consistent with an interpretation of the summertime ISO in terms of propagating equatorial modes. The northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the Rossby cell that is excited by equatorial ISO convection. A similar convergence pattern is seen for the northern winter ISO, but it does not generate poleward movement due to relatively cool SSTs underlying the surface convergence.
Lee, D.-K. and M.-S. Suh (2000). "Ten-year east Asian summer monsoon simulation using a regional climate model (RegCM2)." Journal of Geophysical Research, Washington, DC 105(D24): 29565-29577.
This paper presents a 10-year (1987-1996) summer climate simulation using a regional climate model. The National Center for Atmospheric Research Regional Climate Model (NCAR RegCM2) was designated over east Asia with a horizontal grid spacing of approximately 50 km. The model was initialized at June 1 and integrated up to the end of August for the 10 years. Initial and boundary conditions were the National Centers for Environmental Prediction/NCAR reanalysis data. The 10-year summer mean biases of the simulated ground temperature and daily precipitation rate over the east Asian land area are about -0.1 degrees C and 1.1 mm/d, respectively. Including the ocean area, the bias of the precipitation rate is significantly reduced to about 0.05 mm/d. More precipitation is simulated in June over the central part of the model domain. The systematic cold biases of the simulated ground temperature take place over the northern part of the domain. The RegCM2 reproduces fairly well the large-scale features associated with the east Asian summer monsoon system, including the subtropical high over the northwestern Pacific Ocean, the planetary-scale trough over Manchuria, the upper level jet and lower-level southerly or southwesterly flow and the precipitation band extended from south China to Japan. The evolution processes of the onset, break phases and abrupt jumps between two adjacent phases of the east Asian summer monsoon are also well simulated. On the other hand, the intensity and magnitude of the large-scale features are not well reproduced. For example, the subtropical high over the ocean is too intensified, in association with the enhancement of the warm air advection in the lower level to the precipitation area. The maximum baroclinic zone and the planetary-scale trough and thus the monsoon rainbelt are shifted northward by 2 degrees -3 degrees compared to reanalysis data. However, some of the systematic errors are hidden in the anomaly analysis over the 10-year RegCM2 climate period.
Li, C. (1996). "A further study on interaction between anomalous winter monsoon in east Asia and El Nino." Acta Meteorologica Sinica, Beijing, China 10(3): 309-320.
The interaction between anomalous winter monsoon in east Asia and El Nino is further studied in this paper. The new results still more proved a previous conclusion: there are clear interactions between El Nino and winter monsoon in east Asia. The continual westerly burst and stronger cumulus convection over the equatorial central-western Pacific caused by stronger winter monsoon in east Asia can respectively excite anomalous oceanic Kelvin wave and stronger atmospheric intraseasonal oscillation in the tropics, then excite the El Nino event through air-sea interaction. In El Nino winter, there are warmer and weaker winter monsoons in east Asia. The El Nino will still reduce the intensity of intraseasonal oscillation and leads it to be barotropic structure.
Li, C., L. Zhenxia, et al. (2001). "Strong/Weak Summer Monsoon Activity over the South China Sea and Atmospheric Intraseasonal Oscillation." Advances in Atmospheric Sciences 18(6): 1146-1160.
The circulation pattern corresponding to the strong/weak summer monsoon in the South China Sea (SCS) region and the associated characteristics of the abnormal rainfall in Eastern China have been studied by using the NECP reanalysis data and precipitation data in China. The results show that the climate variations in China caused by the strong/weak summer monsoon are completely different (even in opposite phase). The analyses of atmospheric intraseasonal oscillation (ISO) activity showed that the atmospheric ISO at 850 hPa near the SCS region is strong (weak) corresponding to the strong (weak) SCS summer monsoon. And the analyses of the circulation pattern of the atmospheric ISO showed that the strong/weak SCS summer monsoon circulation (200 hPa and 850 hPa) result mainly from abnormal atmospheric ISO. This study also reveals that the atmospheric ISO variability in the South China Sea region is usually at opposite phase with one in the Jiang-huai River basin. For example, strong (weak) atmospheric ISO in the SCS region corresponds to the weak (strong) atmospheric ISO in the Jiang-huai River basin. As to the intensity of atmospheric ISO, it is generally exhibits the local exciting characteristics, the longitudinal propagation is weak.
Li, T. and B. Wang (1994). "The influence of sea surface temperature on the tropical intraseasonal oscillation: a numerical study." Monthly Weather Review, Boston, MA 122(10): 2349-2362.
The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial beta plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability. The gradient of the antisymmetric (with respect to the equator) component of SST causes, a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon. During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface east of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin-Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.
Liebmann, B. and D. L. Hartmann (1984). "Observational study of tropical-midlatitude interaction on intraseasonal time scales during winter." Journal of the Atmospheric Sciences, Boston 41(23): 3333-3350.
Eight Northern Hemisphere winters of five- and ten-day average midlatitude 500-mb heights and tropical outgoing IR are used in a correlative study of tropical-midlatitude interaction. The seasonal cycle and interannual variability are removed so that only intraseasonal variability remains. Results indicate that energy predominantly propagates from midlatitudes to the Tropics for both five- and ten-day-averaged data, although the propagation is more apparent in five-day-averaged data. This is because the largest tropical IR patterns are southeastward of the 500-mb point with which the IR field is correlated. The result is interpreted in terms of a quasi-stationary Rossby wave that has an eastward component of group velocity. The SW-NE tilt of the 500-mb height correlation patterns, indicating poleward momentum transport or equatorward wave propagation, also supports the hypothesis that midlatitude flow drives the Tropics. Lead and lag correlations show that, when 500-mb heights lead IR, an upstream development appears in the 500-mb correlation pattern. The field is nearly featureless, however, when 500-mb heights lag IR. Well-defined time evolution is more evident over the eastern Pacific than over the western Pacific. The only indication of possible forcing of the midlatitude flow by the Tropics is from IR anomalies in the region of winter monsoon rainfall over the far western Pacific, which are associated with a pattern of correlations in the 500-mb field of nearly global extent. The pattern may be related to that produced by Simmons et al. with a barotropic model, when steady forcing is used to perturb a zonally varying basic state. Simmons et al. hypothesize that the large global anomalies are the result of the barotropic instability of the basic state. Although the global correlation pattern is statistically significant, it explains only a small fraction of the total variance.
Lin, A. (1998). "The characteristics of low-frequency of oscillation over South China Sea." Journal of Tropical Meteorology, Guangzhou, China 14(2): 113-118.
Using the OLR (outgoing longwave radiation) data during 1975-1993 (except for 1978), the characteristics of interseasonal variation of low-frequency oscillation in South China Sea and its relation to the establishment and activity of summer monsoon, are investigated. It is found: (1) The strength of the oscillation is stronger during the summer monsoon period than that during the winter monsoon period. The establishment of summer monsoon is in the negative phase of the first strong oscillation; (2) Low-frequency oscillation closely relates to monsoon's activeness and interruption (or weakness), the circulation objects of the oscillation during summer monsoon period are the eastward and westward swing of the subtropical high over west Pacific, and the southward and northward vibration of ITCZ; (3) the intraseasonal oscillation accompanied with seasonal variation makes the summer monsoon experience the evolution from establishment in the first oscillation, the strengthening in the second and the third oscillation and the weakening in the fourth oscillation.
Liu, K.-K., G.-C. Gong, et al. (1992). "Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: observations and a numerical simulation." Journal of Geophysical Research, Washington, DC 97(C8): 12511-12526.
A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August-October 1990. The wind direction reversed in mid-September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 mu M) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind change.
Lu, E. and Y. Ding (1997). "Analysis of summer monsoon activity during the 1991 excessively torrential rain over Changjiang-Huaihe River valley." Quarterly Journal of Applied Meteorology, Beijing, China 8(3): 316-324.
The features of monsoon activity and its role (Jianghuai) during the 1991 excessively torrential rain over Changjiang-Huaihe River valley are analyzed. The results show that the first episodes of torrential rain occurs before the onset of summer monsoon and is mainly induced by synoptic systems in subtropical westerlies; while the activity of southwest monsoon becomes most important during the second and third episodes of torrential rain when subtropical high in West Pacific and southeast monsoon stay stable. The maintenance and break of Meiyu are as a result of the activity of southwest monsoon. A significant effect of southwest monsoon is to transport convective air from tropical area (Arabian Sea and the Bay of Bangal) to Jianghuai valley, thus the convective rain formed in the middle and late stages of the Meiyu is much stronger than that in the normal year, especially in the third episode. The distribution and propagation of strong wind core along southwest airflow are also studied.
Lu, E., Y. Ding, et al. (1998). "Nature of precipitation and activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin." Acta Meteorologica Sinica, Beijing, China 12(1): 75-91.
Seasonal variability regarding the nature of precipitation and the activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin (Jianghuai) has been investigated by calculating apparent heat source/apparent moisture sink and analyzing TBB (cloud-top blackbody radiation temperature) data. It is found that three periods of strong ascending motion during the Meiyu season lead to three episodes of heavy rain, and the latent heat due to the precipitation is of the sole heat source of the atmosphere. The nature of precipitation shows distinct seasonal variability, from frontal precipitation of the first episode to the extremely strong convective precipitation of the third episode. TBB field of East Asia may well reflect not only the intensity of convection and rainfall, but also the movement of rain belt and convection belt. In the whole Meiyu season, convection belt mainly stays in Jianghuai, but may shift within the domain of East Asia. Its locating in Jianghuai or not determines the maintenance or break of Meiyu. In the third episode, the narrow convection belt over Jianghuai is mainly caused by southwest monsoon which takes moist and convective atmosphere from tropical ocean.
Lu, J. and Y. Ding (1989). "Medium-range oscillations in the summer tropical easterlies at 200 hPa." Advances in Atmospheric Sciences, Beijing, China 6(3): 301-312.
By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range oscillations in the summer tropical easterlies (10 degrees S-20 degrees N) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves at minimum at 10 degrees N. Three kinds of the medium-range oscillations with about 50-day, 25-day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variation and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3), which all may make important contributions to the 50-day and 25-day oscillations in different years of different latitudes. The quasi-biweekly oscillations are mainly related to the synoptic waves (4-6). In the equatorial region, the 50-day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25-day period with a westward phase propagation in 1980. Both of these are of the mode of zonal wave number 1. Strong westward 50-day oscillation was found in 10 degrees N-20 degrees N in these two years. Regular propagation of the meridional wind 50-day oscillation were also found in the easterlies. The 50-day and 25-day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near the international date line, where the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), are respectively.
Lubis, S. M. and T. Murakami (1984). "Moisture budget during the 1978-1979 Southern Hemisphere summer monsoon." Meteorological Society of Japan, Tokyo, Journal 62(5): 748-760.
FGGE level 3b data is used to analyze the moisture balance over an extensive region (30 degrees S-30 degrees N, 60 degrees E-120 degrees W) during the 1978-1979 Southern Hemisphere summer monsoon. In this study, the onset and break periods are defined by the changes in the moisture budget over the northeastern Australia-western South Pacific region (7.5 degrees -22.5 degrees S, 135 degrees -165 degrees E). The analysis shows that the major moisture source regions for the monsoon rainfall are the Indonesian seas (0 degrees -12 degrees S, 95 degrees -130 degrees E) and the subtropical South Pacific off the east coast of Australia. The contribution of the Northern Hemisphere moisture flux is less than these Southern Hemisphere sources. The monsoon activity shifts eastward and southward from pre- to postonset. During the break period, the monsoon activity shifts still further eastward and poleward. The moisture convergence pattern over Australia illustrates some of the similarities between the break conditions for the Southern Hemisphere summer monsoon and the Indian monsoon. The strong moisture convergence center over northeastern Australia ( similar to 15 degrees S, 140 degrees E) during the active period splits into two centers during the break period. The stronger center is situated at similar to 20 degrees S, 140 degrees E and is similar to 5 degrees poleward of the previous active period position. The other center is more equatorward, at similar to 7.5 degrees S, 140 degrees E. During the break period, there is a strong interhemispheric moisture exchange over the central Pacific between 150 degrees E and the dateline. During the same period, a strong northerly moisture flux around 150 degrees W transports a large amount of moisture into the midlatitudes of the South Pacific.
Madden, R. A. and P. R. Julian (1994). "Observations of the 40-50-day tropical oscillationAa review." Monthly Weather Review, Boston, MA 122(5): 814-837.
Observational aspects of the 40-50 day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.
Mahanti, A. C. (1981). "Stability of monsoon depression." Archives for Meteorology, Geophysics and Bioclimatology, Ser A, Vienna 30(1/2): 39-53.
The object of this paper is to study the stability of a monsoon depression which is a low-level wave disturbance over the Indian region during summer. The stability of a wave disturbance depends intimately upon the phase velocity. The phase velocity of the Rossby gravity wave does not explain the phase velocity of a monsoon depression. Thus, since the convective activity plays an important role in the development of a monsoon depression, the phase velocity of zonally propagating Rossby gravity waves is modified, i.e., the modified dispersion relation considering the effect of convective heating in the equivalent barotropic model, which is derived for the low levels in the Tropics, is obtained. The modified dispersion relation explains the phase velocity of monsoon depression. Beyond the limiting stage, there is no additional depression of the central pressure. Thus, it is essential to study the stability of a monsoon depression at the limiting stage. The stability analysis shows that the monsoon depression is neutral (i.e., it does not break down) as long as there is convective activity, but the monsoon depression becomes unstable (i.e., breaks down) if the convective activity ceases. In the latter case, a possible stream of zonal wind is obtained.
Maloney, E. D. and D. L. Hartmann (1998). "Frictional moisture convergence in a composite life cycle of the Madden-Julian oscillation." Journal of Climate, Boston, MA 11(9): 2387-2403.
A composite life cycle of the Madden-Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20-80 days) 850-mb zonal wind averaged from 5 degrees N to 5 degrees S every 2.5 degrees around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979-95. Water vapor integrated from the surface to 300 mb is composited for the period 1988-92. Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden-Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advection from the west or extratropics associated with Rossby wave circulations. Frictional convergence in front of convection helps to slowly moisten the atmosphere to a state that is again favorable for convection. This moistening may set the timescale for the reinitiation of convection in the Indian and west Pacific Oceans after strong drying and provides a mechanism for slow eastward propagation. A significant correlation exists between surface convergence and column water vapor anomalies in the west Pacific and Indian Oceans. Weaker correlations exist between 850-mb convergence and water vapor anomalies. Over the west Pacific, surface convergence leads positive water vapor anomalies, while 850-mb convergence lags positive water vapor anomalies. Northern Hemisphere summer (May-October) composites show that the phases of the MJO coincide with ``active'' and ``break'' periods of the Indian summer monsoon at intraseasonal timescales. The northward propagation of precipitation across India during the summer monsoon is associated with northward and westward movement of Rossby wave features trailing the main center of equatorial convection associated with the MJO.
Manton, M. J. (1997). "Symposium on climate prediction and predictability: papers presented at the eighth modelling workshop, 12-14 November 1966." Melbourne, Australia, Bureau of Meteorology Research Centre 126.
Numerical modelling forms a substantial component of the activities in all the Bureau of Meteorology Research Centre (BMRC) research groups. The modelling covers spatial scales from the mesoscale to the global, and it includes the ocean as well as the atmosphere. The annual modelling workshop is a forum in which current topics are discussed from both a national and an international perspective. The 1996 workshop (the ``Symposium on Climate Prediction and Predictability'') represented a somewhat different approach from previous years: the sponsorship was broader and the organising committee attempted to provide a sharper focus. The aspects of prediction and predictability that were selected because of their particular relevance were: Simulated climate variability (e.g., AMIP and coupled model experiments) including intraseasonal and interannual variability; Climate predictability studies; Ocean modelling and initialisation of climate predictions; and Coupled model climate prediction. The papers presented here describe recent work in the field of climate prediction and predictability, an area of immense significance and importance to Australia. The presentations and associated discussion helped sharpen the focus of the Australian scientific community in considering our role in the World Climate Research Programme's CLIVAR (Climate Variability and Predictability) initiative. Table of contents include: Seasonal prediction and predictability studies at ECMWF, David Anderson; The globalization of high performance computing and numerical simulation, David Blaskovich; Prediction and predictability studies at the CCCma, G. J. Boer; Climate sensitivity in a GCM--how non-linear is it?, R. A. Colman, S. B. Power and B. J. McAvaney; Cross-validation of statistical seasonal forecasts, Wasyl Drosdowsky; Simulations of Australian climate variability, C. S. Frederiksen; Eddy parameterizations and systematic kinetic enery errors in atmospheric circulation models, Jorgen S. Fredericsen; Effect of oceanic eddy physics on simulation of global warming, Anthony C. Hirst; Multi-seasonal hindcasts for 1972-1992, B. G. Hunt; Simulating the climate of the 20th century, David Karoly, David Sexton, Chris Folland and David Rowell; Limitations to the predictability of ENSO, Richard Kleeman and Andrew M. Moore; Lessons for climate prediction from rainfall variations over the past century, Ants Leetmaa; A theory for the Southern Hemisphere monsoon: implications for predictability, John McBride and Jun-Ichi Yao; Large scale variability of the southern tropical Indian Ocean, Gary Meyers and Yukio Masumoto; Skill assessment for ENSO using ensemble prediction, Andrew M. Moore and Richard Kleeman; Impact on the Australian-Pacific region of the ENSO signal in a global coupled GCM, S. P. O'Farrell; A coupled general circulation model for seasonal prediction and climate change research, S. Power, F. Tseitkin, R. Colman, B. McAvaney, J. Fraser, R. Kleeman and A. Sulaiman; Modelling studies of the Australian and Indian monsoons, K. Puri and R. Colman; The annual cycle, climate drift and ENSO forecasting, A. Rosati; A numerical simulation of water pathways between the subtropical and tropical Pacific Ocean: implications for ENSO prediction, Lewis M. Rothstein; Assessing atmospheric seasonal predictability using an ensemble of multi-decadal AGCM simulations, David P. Rowell, John R. Davies, Alison C. Renshaw and Chris K. Folland; The Australian climate variability ocean model, A. Schiller, P. McIntosh, G. Meyers and J. S. Godfrey; Prediction and diagnosis of Chinese monsoonal rainfall from conditions in the Philippines warm pool, Ian Simmonds and Daohua Bi; Prediction and predictability: some implications for observing system design, Neville R. Smith; Dynamical climate predictions at the Japan Meteorological Agency, Kiyoharu Takano; Predictions of global warming using a simple energy balance model, Ian G. Watterson; The climate response to the instantaneous removal o
Manton, M. J. and J. L. McBride (1992). "Recent research on the Australian monsoon." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 275-285.
Owing to the data flowing from a number of observational programs, there has been over the last few years a sustained research effort on improving our understanding of the Australian monsoon. This paper discusses the findings of that research, including results on the large-scale structure of the monsoon, interannual variability, onset, intraseasonal variability, and mesoscale structure. Although there has been significant progress, much work remains to be done on relating regional aspects of the monsoon to the general tropical circulation.
Mantua, N. J. and D. S. Battisti (1995). "Aperiodic variability in the Zebiak-Cane coupled ocean-atmosphere model: air-sea interactions in the western equatorial Pacific." Journal of Climate, Boston, MA 8(12): 2897-2927.
The behavior of two coupled ocean-atmosphere models of intermediate complexity, the Zebiak-Cane (ZC hereafter) and Battisti version B88 hereafter) of the ZC coupled ocean-atmosphere model, are reviewed and compared to the observed climate record from the tropical Pacific region. A major difference between each system lies in the modes of variability that each supports. In the observations, variability at timescales shorter than that associated with El Nino and the Southern Oscillation (ENSO) is ubiquitous and difficult to characterize in terms of clearly coupled ocean-atmosphere interactions. The B88 model supports only a single unstable mode: the ENSO mode. In the ZC model two distinct modes are present: the interannual ENSO mode, and the mobile mode: a near-annual, westward propagating instability. It is demonstrated that differences in the basic-state climatology and thermodynamic parameters are keys to understanding the differences between the ZC and B88 coupled model behaviors. It is found that interactions between the ENSO and the mobile mode is the cause for irregular variability in standard ZC model simulations. The mobile mode instability is inherently dependent on shallow-water equatorial wave dynamics: anomalies at the air-sea interface are found to phase lock with the gravest symmetric oceanic Rossby mode. The westward propagating instability generates large disturbances in the dynamic ocean fields even though the surface anomalies are relatively weak. The timescale of the recurrence of the mobile instability is that of the free-equatorial-wave basin mode, which is about 9 months. Interactions between the ENSO and mobile modes play an important role in the behavior of the standard ZC model. The phase (cold versus warm) of the model ENSO cycle determines the strength of the mobile mode instability, while ocean disturbances generated by the mobile mode interfere with the model ENSO regularity. It is demonstrated that the coexistence of these distinct, unstable, coupled ocean-atmosphere instabilities is the key element in producing aperiodic behavior in the standard ZC coupled model. The cause for aperiodic variability in the perpetual month simulations is linked to air-sea interactions in the far western Pacific. By explicitly suppressing the air-sea interactions in the far western Pacific, the ZC model ENSO cycle becomes much more periodic than that in the standard model. There is no convincing evidence for robust coupled ocean-atmosphere interactions in the observed western Pacific region. However, uncoupled atmospheric variability associated with intraseasonal oscillations and the south Asian monsoon are very energetic features in this region. The results imply that wind stress variability in the western equatorial Pacific may act as a stochastic forcing that interrupts what might otherwise be a pure ENSO cycle.
Martin, G. M. (1999). "The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK Meteorological Office Unified Model." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 125(557, Pt. A): 1499-1525.
The quality of the simulation of the Asian summer monsoon in the climate version of the UK Meteorological Office's United Model, and the impact upon this of increased horizontal resolution, is investigated using two atmosphere-only model runs forced with observed sea surface temperatures (SSTs) and sea ice extents. The runs each cover the period from 1979 to 1988, but have different horizontal resolutions, with one at climate resolution (2.5 degrees latitude by 3.75 degrees longitude, about 300 km at midlatitudes) and the other at global forecast model resolution (0.833 degrees latitude by 1.25 degrees longitude, approximately 100 km at midlatitudes). The characteristic monsoon circulation and the spatial distribution of precipitation are in good agreement with observations. However, the model has a tendency to overestimate the strength of the monsoon, and also exhibits an early monsoon onset. The large-scale interannual variations in circulation appear to be simulated reasonably well (as far as can be determined using this short dataset), although the magnitude of the interannual variability of precipitation is overestimated. However, the regional circulation and precipitation changes between El Nino and La Nina years show some significant differences between the model and the observations. The dominant mode of intraseasonal variability seen in both model simulations is, in agreement with observations, associated with the active/break cycle of the monsoon (although this only explains about 10% of the total variance in both simulations). There is some evidence that the SST changes associated with El Nino may produce a coherent forcing of the secondary (east-west) mode of intraseasonal variability during the onset phase of the monsoon in the model. However, comparison with observations suggests that this may not be representative of what occurs in the real atmosphere. There is no evidence that the SST variations are causing the system to prefer either the active or the break monsoon phase, as was suggested by Palmer. With increased model horizontal resolution, extra detail is provided in the precipitation distribution, but the mean monsoon simulation is scarcely altered and the systematic errors remain. The interannual variations in circulation and precipitation appear not to be greatly altered, and the overall pattern of intraseasonal variability is also unaffected. This study suggests that the systematic errors in the monsoon simulation are not a result of poor horizontal resolution, but may be due to problems with the model physics.
Matsumoto, J. (1989). "Synoptic features of heavy monsoon rainfall in 1987 related to the severe flood in Bangladesh." Japanese Progress in Climatology, Tokyo, Japan, December.
The seasonal rainfall distribution, intraseasonal variation of monsoon activity, and daily synoptic features associated with the occurrence of heavy rainfall and flood disasters in Bangladesh in 1987 were investigated. Heavy rainfall along the monsoon trough in northern Bangladesh and adjacent India during the break monsoon phases and the transitional periods from the break to the active phase caused floods in early July and from late July to early August. The floods from mid- to late August and in late September occurred during the active monsoon phases. In these cases the inflow from the upper tributaries of the Ganges River as well as heavy rainfall caused by the monsoon depressions were the main factors causing floods in Bangladesh.
McBride, J. (1998). "Indonesia, Papua New Guinea and tropical Australia: the Southern Hemisphere monsoon." Karoly, David J. and Vincent, Dayton G.
The meteorology of the region is described briefly in the context of traditional monsoon meteorology. Various studies are summarized concerning the morphology of monsoon onset and retreat, as well as active and break patterns. The major emphasis of this chapter is on the role of the Indonesian-Australian region in global dynamics. As noted originally by Ramage (1968), during the southern summer the major global tropical heat source is located over this region; for this reason, the region is referred to as the maritime continent. The associated latent heat release can be considered, from energetic considerations, to be the forcing mechanism for the observed large-scale circulation of the global Tropics. The role of the region is also described in the global meridional circulation of mass between the troposphere and the stratosphere. The water vapor budget of the stratosphere is such that it must be dried through mechanisms involving overshoot of deep tropical cumulonimbus cells. Analysis of data from field experiments directed at this problem suggest that, once again, the major location for dehydration of the global stratosphere may be the maritime continent during the Southern Hemisphere summer. This chapter also briefly summarizes some aspects of the Indonesian throughflow phenomenon. This is a transport of water from the Pacific Ocean to the Indian Ocean through the seas and channels in the maritime continent region. The throughflow is believed to play a major role in global climate, as the associated net heat transport is a substantial fraction of the total heat absorbed by the Pacific Ocean. Consequently, seasonal and interannual variations in throughflow can have a major influence on large-scale climate.
McBride, J. L., N. E. Davidson, et al. (1995). "The flow during TOGA COARE as diagnosed by the BMRC tropical analysis and prediction system." Monthly Weather Review, Boston, MA 123(3): 717-736.
The evolution of the large-scale flow through the four-month intensive period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena: 1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured; 2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon; 3) the existence of a major westerly-easterly-westerly cycle of the Madden-Julian low-frequency wave occurring during the latter half of the experimental period; and 4) the development and subsequent movement of tropical cyclones in both (Northern and Southern) hemispheres. The Madden-Julian event consisted of two eastward progressions across the domain of satellite-observed cloud, south of the equator. The horizontal scale of the cloud regions is approximately 10 degrees latitude x 40 degrees longitude and the eastward phase speed is approximately 3.7 m s super(-) super(1) . Linear correlation studies substantiate the eastward movement of both cloud and zonal wind across the domain. The correlation analysis reveals a strong relationship between cloud and low-level zonal wind, with the cloud variations leading those in wind by approximately five days. Time-longitude sections of relative vorticity show that the synoptic activity also progressed eastward with the cloud, and its structure is suggestive that the controlling dynamics (for the synoptic activity) may be the energy dispersion mechanism of Davidson and Hendon. The development of each westerly event was accompanied by a major change in the Southern Hemisphere deep-layer mean flow from easterly to westerly. Examination of flow fields and satellite imagery for individual days shows that the peak of the first westerly event is associated with the flow patterns surrounding two Southern Hemisphere tropical cyclones. The subsequent rapid evolution to an easterly state occurs as the cyclones move eastward and southward, and the monsoon flow collapses in their wake. There is an accompanying ridging at low levels in the subtropics and the establishment of the Southern Hemisphere subtropical jet. The subsequent reestablishment of the monsoon (the second westerly event) occurs from west to east with the eastward moving cloud bands. There is also a suggestion that an equatorward extension of a Southern Hemisphere upper-level trough may have played a role. Major active and break periods are identified over four tropical subdomains over the TOGA COARE region. These are most easily defined in the Southern Hemisphere subdomains. They are characterized by a slowly varying signal in the satellite-observed average cloud-top temperature. Superimposed on this is a rapid transition between the active and break states.
McBride, J. L. and W. M. Frank (1999). "Relationships between stability and monsoon convection." Journal of the Atmospheric Sciences, Boston, MA 56(1): 24-36.
Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon. Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to mid-tropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occured in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection. Convective activity is weakly but inversely related to convective available potential energy variations. Day-to-day variations in CAPE are dominated by variations in equivalent potential temperature of the source level (boundary layer) air. The physical effect is one of changing the moist adiabat along which the air parcel rises. In temperature-log pressure space, moist adiabats diverge in the upper half of the troposphere. Since CAPE variations are dominated by changes in the moist adiabat of the rising parcel, the day-to-day CAPE changes occur almost totally in positive area variations above the 600-hPa level. The above results are discussed in the context of other studies in the literature. It is proposed that stabilization of the atmosphere in response to deep convection occurs almost entirely through the modification of CAPE through decreasing theta sub(e) of the source air in the boundary layer. This occurs over relatively small spatial scales, whereas variations in lapse rate through the deep troposphere are hypothesized to occur over the relatively large scales associated with monsoon active and break events.
McBridge, J. L. (1987). "Australian summer monsoon." Chang, Chih Pei.
The current state of knowledge of the summer monsoon of the Southern Hemisphere is reviewed with particular reference to northern Australia. The introduction describes the general characteristics of the flow as can be determined from climatological charts and from the research conducted in the early 1960s by Troup and Berson. This is followed by a treatment of the relationship between the monsoon and the Southern Oscillation. Subsequent sections summarize and discuss new findings on the monsoon which have resulted from research using data from the 1978-1979 winter MONEX. The subjects covered include large-scale structure of the monsoon circulation, monsoon onset, regions of enhanced and suppressed convection, tropical midlatitude interactions, active and break monsoon, 40-50-day oscillations, tropical cyclones, and monsoon depressions.
McHugh, M. J. and J. C. Rogers (2001). "North Atlantic Oscillation Influence on Precipitation Variability around the Southeast African Convergence Zone." Journal of Climate 14(17): 3631-3642.
The relationship between the North Atlantic oscillation (NAO) and austral summer (December-February) rainfall variability over southeastern Africa is described. Thirty-one stations in 0 degree -16 degree S and 25 degree -40 degree E have statistically significant correlations to the NAO index over varying periods of record starting since 1895 and form a regional normalized rainfall index of southeast African rainfall (SEAR) correlated to the NAO index (NAOI) at r = -0.48 over 1894/95-1989/90, although the relationship is r = -0.70 since 1958. The spectrum of the SEAR index has significant amplitude at 7.6 yr, a periodicity commonly associated with the NAO, and the NAOI/SEAR cospectrum has its largest power at this periodicity. NCEP-NCAR reanalysis data, extending from 1958/59 to 1995/96 are used to evaluate moisture and circulation field variations associated with both NAO and SEAR indices. Precipitable water over southeastern Africa varies significantly such that anomalously high (low) convective rainfall occurs over southeast Africa when the NAO is weak (strong). Unusually wet summers are associated with anomalous equatorial westerly flow originating in the subtropical Atlantic and traversing the continent. Relatively dry summers are associated with increased southeasterly monsoon flow originating over the subtropical Indian Ocean. The NAO linkage to southeastern African rainfall is especially pronounced in 300-hPa zonal winds where five highly significant elongated bands of alternating zonal wind anomalies extend from the Atlantic Arctic to equatorial Africa. The latter 300-hPa equatorial band exhibits westerly (easterly) flow during wet (dry) austral summers and undergoes regional divergence (convergence) over southeastern Africa. The westerly flow, along with orographic uplift, has an element of instability due to the vertical component of the Coriolis parameter that assists rain production during wet summer. Potential interactions between the NAO and ENSO in producing regional latitudinal ITCZ shifts are discussed.
Meehl, G. A., M. Wheeler, et al. (1994). "Low-frequency variability and CO sub(2) transient climate change. Part 3: Intermonthly and interannual variability." Climate Dynamics, Berlin, Germany 10(6-7): 277-303.
Components of interannual, intermonthly, and total monthly variability of lower troposphere temperature are calculated from a global coupled ocean-atmosphere general circulation model (GCM) (referred to as the coupled model), from the same atmospheric model coupled to a nondynamic mixed-layer ocean (referred to as the mixed-layer model), and from microwave sounding unit (MSU) satellite data. The coupled model produces most features of intermonthly and interannual variability compared to the MSU data, but with somewhat reduced amplitude in the extratropics and increased variability in the tropical western Pacific and tropical Atlantic. The relatively short 14-year period of record of the MSU data precludes definitive conclusions about variability in the observed system at longer time scales (e.g., decadal or longer). Different 14-year periods from the coupled model show variability on those longer time scales that were noted in Part 1 of this series. The relative contributions of intermonthly and interannual variability that make up the total monthly variability are similar between the coupled model and the MSU data, suggesting that similar mechanisms are at work in both the model and observed system. These include El Nino-Southern Oscillation (ENSO)-type interannual variability in the tropics, Madden-Julian Oscillation (MJO)-type intermonthly variability in the tropics, and blocking-type intermonthly variability in the extratropics. Manifestations of all of these features have been noted in various versions of the model. Significant changes of variability noted in the coupled model with doubled carbon dioxide differ from those in our mixed-layer model and earlier studies with mixed-layer models. In particular, in our mixed-layer model intermonthly and interannual variability changes are similar with a mixture of regional increases and decreases, but with mainly decreases in the zonal mean from about 20 degrees S to 60 degrees N and near 60 degrees S. In the coupled model, intermonthly and interannual changes of variability with doubled CO sub(2) show mostly increases of tropical interannual variability and decreases of intermonthly variability near 60 degrees N. These changes in the tropics are related to changes in ENSO, the south Asian monsoon, and other regional hydrological regimes, while the alterations near 60 degrees N are likely associated with changes in blocking activity. These results point to the important contribution from ENSO seen in the coupled model and the MSU data that are not present in the mixed-layer model.
Mehta, A. V. and T. N. Krishnamurti (1988). "Interannual variability of the 30- to 50-day wave motions." Journal of the Meteorological Society of Japan, Tokyo 66(4): 535-548.
In this observational study, the 30-50-day motion fields, at 850 and 200 mb, for 1980-1984 are analyzed by using the synoptic approach. This analysis examines their interannual variability. A similar analysis of the 30-50-day motion fields during the FGGE (1979) year showed eastward propagating planetary-scale divergent waves around the globe throughout the FGGE year at 200 mb, and a family of northward propagating trough and ridge lines over the MONEX region (30-150 degrees E, 30 degrees S-40 degrees N) during the summer of 1979 at 850 mb. The sensitivity of the calculations of low-frequency waves to the width of the time filter was analyzed. Results show that the phenomenon is robust and is not very sensitive to the width of the filters. The 850-mb streamline-isotach analysis of the 30-50-day time scale waves over the MONEX region for the years 1980-1984 shows the northward propagating trough and ridge systems during May, June, and July 1982 and 1983. The speed of northward propagation is similar to 0.7 degrees lat./day during the summers of 1979, 1982, and 1983. During the summer seasons of 1980, 1981, and 1984, the trough and ridge lines do not exhibit a steady northward propagation. The direction and the speed of propagation are found to be variable from month to month during these years. The trough and ridge systems during 1980 and 1981 are either stationary or show some northward propagation. During the summer of 1984, the propagation of the trough and ridge system is northward in May and June and southward in July and Aug. Interannually, the onset and break periods of the Indian summer monsoon exhibit a close relationship to the passage of the 30-50-day trough and ridge systems over India. A global analysis of velocity potential at 200 mb shows eastward propagating waves during 1979 and between Oct. 1982 and Oct. 1983. During the remaining period, the direction of propagation is less well defined. The amplitude of the divergent wave is found to be variable from one year to the next. The 30-50-day wave motions exhibit a similar behavior during 1979, 1982, and 1983, both at the 200- and the 850-mb levels. 1982-1983 was an El Nino year, and 1979 was characterized by weak warm sea surface temperature anomalies of the order of 1 degrees C over the eastern equatorial Pacific Ocean. Both of these years were characterized by deficient rainfall over India, with somewhat above normal rainfall over the region south of India.
Mehta, A. V. and E. A. Smith (1997). "Variability of radiative cooling during the Asian summer monsoon and its influence on intraseasonal waves." Journal of the Atmospheric Sciences, Boston, MA 54(8): 941-966.
Infrared radiative cooling rates are calculated over the Asian summer monsoon between 5 degrees S-20 degrees N and 40 degrees -135 degrees E at a spatial resolution of 5 degrees x 5 degrees for the summer seasons of 1984 and 1987. A medium spectral resolution infrared radiative transfer model with specified temperature, moisture, clouds, and trace gas distributions is used to obtain the cooling rate profiles. Cloud distributions for the two summers are obtained from Indian National Satellite measurements. Seasonal mean and intraseasonal variations of clouds and radiative cooling rates over a 21-76-day range of periods are examined. The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15 degrees N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseasonal variability of clouds is also large over these centers (% variances >25%). Consistently, seasonal mean cooling rates are at a maximum (3 degrees -5 degrees C day super(-) super(1) ) in the upper troposphere between 300 and 400 mb, related to cloud-top cooling. The cooling rates below 400 mb are between 1 degrees and 3 degrees C day super(-) super(1) . The cooling rates exhibit intraseasonal amplitudes of 1.0 degrees -1.5 degrees C day super(-) super(1) . The largest amplitudes are found between 300 and 500 mb, indicating that cooling rate variability is directly related to intraseasonal variability of convective clouds. Spatial distributions of clouds and cooling rates remain similar during the 1984 and 1987 summer seasons. However, during 1987, intraseasonal amplitudes of deep convective cloud amount and cooling rate over the Indian Ocean are 10%-15% larger than in 1984. It is shown that intraseasonal variability of cooling rates over the Indian Ocean can perturb convective heating by 10%-30% in the upper and lower troposphere. Based on a one-dimensional radiative-convective equilibrium model, it is estimated that the radiative damping timescale over the Indian Ocean region is similar to 3 days. Based on this damping timescale and in conjunction with a model of equatorial Kelvin waves with first baroclinic mode, it is hypothesized that the variable cloud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasonal oscillations is so broad.
Miller, M. J., A. C. M. Beljaars, et al. (1992). "The sensitivity of the ECMWF model to the parameterization of evaporation from the tropical oceans." Journal of Climate, Boston, MA 5(5): 418-434.
Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropical oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin-Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s super(-) super(1) . The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convection. It is shown that these revisions to the transfer coefficients at very low wind speeds (<5 m s super(-) super(1) ) have a dramatic positive impact on almost all aspects of the model's simulation of the tropics. These include much improved seasonal rainfall distributions (with the virtual elimination of a tendency to generate double ITCZ in both winter and summer), a much improved Indian monsoon circulation, and substantially reduced tropical systematic errors. The model previously had an easterly bias in the zonal-mean upper tropical tropospheric flow with a corresponding cold bias in the deep tropics; it is shown that the flux revision substantially reduces this. Furthermore, the revision to the fluxes greatly enhances the model's ability to represent interannual and intraseasonal variability (see also the companion paper by Palmer et al.).
Mo, K. C. (2000). "Intraseasonal modulation of summer precipitation over North America." Monthly Weather Review, Boston, MA 128(5): 1490-1505.
The intraseasonal rainfall variability over North America is examined using singular spectrum analysis (SSA) and composites of outgoing longwave radiation anomalies (OLRAs), 200-hPa divergence and a gridded rainfall dataset over the United States. The evolution of the Arizona and New Mexico (AZNM) monsoon based on composites indicates that rainfall anomalies propagate eastward from the North Pacific through AZNM, the Great Plains, to the eastern United States. During summer, the wet and dry periods of the AZNM monsoon are modulated by an oscillatory mode with a period of 22-25 days (22-day mode). This is also the dominant mode associated with rainfall events over the Great Plains. The influence of the Madden-Julian Oscillation (MJO) on the AZNM monsoon is secondary. The strongest impact of the MJO is on precipitation over Mexico. SSA performed on the 200-hPa divergence and OLRAs averaged over Mexico show only one oscillatory mode with a period of about 36-40 days. The 22-25-day mode also exists in the vertically integrated moisture fluxes over the Great Plains. During the wet periods of the AZNM monsoon, more moisture is transported from both the Gulf of Mexico and the Gulf of California to AZNM. The situation reverses when the oscillation reaches the other phase. The 22-day mode is linked to tropical convection. When rainfall associated with the 22-day mode travels eastward from AZNM to the Great Plains, the OLRA composites show westward propagating waves just north of the equator. When enhanced convection reaches the western Pacific, rainfall diminishes over AZNM. When convection in the western Pacific is suppressed and enhanced convection is located in the central Pacific, rainfall intensifies over AZNM.
Mo, K. C. and V. E. Kousky (1993). "Further analysis of the relationship between circulation anomaly patterns and tropical convection." Journal of Geophysical Research, Washington, DC 98(D3): 5103-5113.
Tropical-extratropical connections are investigated for 250-mbar stream function fluctuations in both intraseasonal and interannual bands. The principal modes of anomalous 250-mbar stream function are computed and related to anomalous outgoing longwave radiation (OLR), which is used as a proxy for deep tropical convection. The leading mode for both boreal winter and summer displays a large zonally symmetric component, which appears in both the intraseasonal (IS) and interannual (IA) bands. In the IS band, the dominant mode is an oscillation with a period of 48 days. Correlations between the principal component of this mode and OLR anomalies indicate that this pattern is directly related to convection in the tropical Pacific. The second mode for the boreal winter consists of a wave train from the Pacific Ocean east-northeastward to western North America and then east-southeastward to the subtropical Atlantic Ocean. This mode also exists in both IA and IS bands. Simultaneous and lagged correlations between the amplitude time series of this mode and OLR anomalies indicate that extreme amplitudes of this wave train are preceded by extremes in the OLR anomalies (cloud band activity) in the subtropical North Pacific. Enhanced cloud band activity in the vicinity of Hawaii, associated with an anomalous cyclonic 250-mbar circulation in the same region precedes the maximum development of a 250-mbar ridge south of the Aleutian Islands, trough over western North America and another ridge along the east coast of the United States. The second mode for boreal summer (June-August) is associated with variations of the Asian monsoon.
Mohanty, U. C., S. K. Dube, et al. (1983). "Study of heat and moisture budget over the Arabian Sea and their role in the onset and maintenance of summer monsoon." Meteorological Society of Japan, Tokyo, Journal 61(2): 208-221.
The influence of the heat, moisture, and moist static energy (MSE) budget, over the Arabian Sea and adjoining area (0-30 degrees N and 30-75 degrees E), on the onset and activities of the Asian summer monsoon has been studied. The data base for this study consists of twice daily FGGE Level-3b analysis for the period May 16-July 15, 1979. The pentad mean variation, the vertical distribution, and period averages of the various terms in the energy budget equations are closely examined to find their influence on the monsoon activities. The study indicates the significant increase in the net enthalpy, latent heat energy (LHE), MSE, and a number of budget parameters well in advance of the onset of the monsoon over the Kerala coast. A decreasing trend is observed in most of these parameters approximately five days before the break-monsoon condition that began over India on July 16, 1979. The vertical distributions of the budget parameters reveal that, during the active monsoon period, secondary maxima of horizontal heat and MSE flux divergences are observed in the upper troposphere which are replaced by minima during the weak monsoon circulation. The broad features of the budget studies over the Arabian Sea are in good ageement with the large-scale energetics (Mohanty et al. 1982, 1982). Some of the significant departures in the results of the two studies are discussed.
Mohanty, U. C., S. K. Dube, et al. (1982). "On the role of large-scale energetics in the onset and maintenance of summer monsoon, Pt. 2, Moisture budget." Mausam, New Delhi 33(3): 285-294.
The effect of large-scale moisture and moist static energy (MSE) budget on the onset and activities of the summer monsoon is investigated. The data base for this study consists of twice daily First GARP Global Experiment (FGGE) level-3b analysis of temperature, relative humidity, geopotential, and wind fields for a tropical belt from 20 degrees S to 40 degrees N and 0 to 150 degrees E, during May-July 1979. The daily variation, vertical distribution, and period averages of the various terms in latent heat energy (LHE) and MSE budget equation are closely examined to determine their influence on the activities of monsoon. This study indicates a significant increase in the net LHE, horizontal convergence of moisture, and diabatic moisture sink (indicator of excess condensation than evaporation) about 2 weeks before the onset of the monsoon over the Kerala coast. Further, a decreasing trend is observed in the horizontal convergence of moisture and moisture sink about one week before the break monsoon condition which began over India on July 16, 1979. However, the various terms of the MSE budget equation fail to depict any significant trend with the advance of the monsoon. The vertical distribution, period averages, and the boundary fluxes also confirm these findings.
Mohanty, U. C., S. K. Dube, et al. (1982). "On the role of large-scale energetics in the onset and maintenance of summer monsoon, Pt. 1, Heat budget." Mausam, New Delhi 33(2): 139-152.
The effect of large-scale heat budget on the onset and activities of the Southwest Asian monsoon is studied for the tropical belt (20 degrees S-40 degrees N and 0 degrees -150 degrees E). For this study, the level-3b analysis of the First GARP Global Experiment (FGGE) during May-July 1979 is used. The daily variation, vertical distribution, and period averages of the various terms in the heat budget equation are closely examined to ascertain their influence on monsoon activities. It is found that there is significant increase in the net enthalpy, horizontal convergence of heat, and diabatic heating approximately two weeks before the onset of the monsoon over the Kerala coast. A decreasing trend in these values is observed approximately one week before the break monsoon condition, which began over India on July 16, 1979. The vertical distribution, period averages, and the boundary fluxes also confirm these findings.
Mohanty, U. C., S. K. Dube, et al. (2001). "On the role of large scale energetics in the onset and maintenance of summer monsoon -- I: Heat budget." Mausam [Mausam] 1: 165-178.
The effect of large scale heat budget on the onset and activities of southwest Asian monsoon is studied for the tropical belt (20 deg. S-40 deg. N and 0 deg. H-150 deg. E). For this study the level-IIIb analysis of the First GARP Global Experiment (FGGE) during May-July 1979 is utilised. The daily variation, vertical distribution and period averages of the various terms in heat budget equation are closely examined to find out their influence on the activities of monsoon. It is found that there is significant increase in the net enthalpy, horizontal convergence of heat and diabatic heating about two weeks before the onset of monsoon over Kerala coast. It is further found that a decreasing trend in these values is observed about one week before the break monsoon condition, which started over India on 16 July 1979. The vertical distribution, period averages and the boundary fluxes also confirm the above findings.
Mohanty, U. C., S. K. Dube, et al. (2001). "On the role of large scale energetics in the onset and maintenance of summer monsoon -- II: Moisture budget." Mausam [Mausam] 1: 179-188.
The effect of large scale moisture and moist static energy (MSE) budget on the onset and activities of summer monsoon is investigated. The data base for this study consists of twice daily First GARP Global Experiment (FGGE) level IIIb analysis of temperature, relative humidity, geopotential and wind fields for a tropical belt from 20 deg. S to 40 deg. N and 0 deg. E to 150 deg. E, during May-July 1979. The daily variation, vertical distribution and period averages of the various terms in latent heat energy (LHE) and MSE budget equation are closely examined to find out their influence on the activities of monsoon. The study indicates significant increase in the net LHE, horizontal convergence of moisture and diabatic moisture sink (indicator of excess condensation than evaporation) about two week before the onset of monsoon over Kerala coast. Further, a decreasing trend is observed in horizontal convergence of moisture and moisture sink about one week before the break monsoon condition, which started over India on 16 July 1979. However, the various terms of the MSE budget equation do not depict any significant trend with the advance of the monsoon. The vertical distribution, period averages and the boundary fluxes also confirm the above findings.
Mohanty, U. C., R. P. Pearce, et al. (1984). "Numerical experiments on the simulation of the 1979 Asian summer monsoon." European Centre for Medium Range Weather Forecasts, Reading, Eng., Technical Report Oct(44).
The authors describe the results of three experimental integrations of the ECMWF operational prediction model conducted to examine its ability to predict the onset and subsequent development of the 1979 Asian summer monsoon. Each integration began on 12 Z on June 11, the date of onset of monsoon rains over southern India. The first extended over 50 days and included a monsoon break. The other two integrations, each of ten days, were conducted to compare the simulation of the onset by the model by using different convection schemes proposed by Kuo and Arakawa-Schubert (A-S). Moisture, enthalpy, and wind analyses of the predictions are compared with FGGE analyses, with emphasis on the Arabian Sea region. These show that the rapid intensification of the low-level wind, associated with the monsoon onset, was not reproduced in any of the predictions, although the A-S scheme was more successful. Both schemes tended to establish the main regions of convection too far to the East, off Sri Lanka. The main sources of error are discussed, including initialization procedures. It is concluded that the main source of error in simulating the summer monsoon onset is the failure of the convection schemes to provide latent heat release in the correct locations.
Molinari, R. L., D. Olson, et al. (1990). "Surface current distributions in the tropical Indian Ocean derived from compilations of surface buoy trajectories." Journal of Geophysical Research, Washington, DC 95(C5): 7217-7238.
Three different satellite-tracked drifting buoy data sets are compiled and used to generate a monthly climatology of surface currents in the tropical Indian Ocean. Buoys were deployed between 1975 and 1987. The data density is maximum on and near the equator and decreases poleward. Drift characteristics of the different buoy configurations are compared using a structure function analysis. The differences in windage effects are consistent with the buoy designs and small compared with the signals studied. The currents in the tropical Indian Ocean during boreal winter and spring can be characterized as two counterrotating gyres. A southern clockwise rotating gyre bounded on the south by the South Equatorial Current (SEC) and on the north during winter by the Equatorial Countercurrent (ECC) and during spring by the Equatorial Jet (EJ). A northern counterclockwise rotating gyre is bounded on the south by the ECC and EJ, depending on season, and on the north by the North Equatorial Current (NEC). The two gyre systems break down during boreal summer. During this season, the SEC is located closer to the equator, and the NEC is replaced by the eastward flowing Indian Monsoon Current (IMC). The western boundary circulation becomes more complicated from late spring through early autumn with the observation of two intense smaller scale gyres. The large-scale southern gyre reappears during boreal autumn with the reappearance of the EJ. The northern gyre begins to reappear in December, with the reversal of the IMC and the reappearance of the NEC. The monthly buoy speeds are compared with a monthly climatology generated from ship drift reports. Differences between the two climatologies are in general small except in regions of few trajectories. The annual cycles in amplitudes and phases of the major currents in the region are thus comparable.
Moray, P. E. (1976). "Southwest monsoon over Gujarat region: typical situations of droughts and exceptionally heavy rain." Symposium on Tropical Monsoons, Poona, India, Sept.
In this paper, synoptic situations over Gujarat region (including Saurashtra and Kutch) are studied. After reviewing the general meteorological conditions over the area, a survey is made of the synoptic systems affecting the Gujarat region. Rainfall data for the last 30 yr from the selected stations in the region are also analyzed in relation to the southwest monsoon, a main source of rainfall over the region. Typical synoptic situations giving exceptionally heavy rain over some areas and synoptic conditions of break monsoon leading to drought occurrences are also discussed.
Mpeta, E. J. and M. R. Jury (2001). "Intra-seasonal convective structure and evolution over tropical East Africa." Climate Research 17(1): 83-92.
Intra-seasonal convection oscillations over southwestern Tanzania during the December to February season are examined using 15 yr (1979 to 1994) of pentad Outgoing Longwave Radiation (OLR) data and kinematic/thermodynamic parameters from ECMWF data. Most significant spectral energy for an area-averaged OLR index is concentrated in periods of 16 to 33 d. Time-longitude diagrams of filtered OLR, and zonal wind anomalies at 850 hPa averaged over the 7.5 to 10 degree S latitude band for the period November to May reveal eastward (62%), westward (11%) and quasi-stationary convective (27%) features. Phase speeds for transient cases are in the range of 2 to 8 m s super(-1). In the eastward propagating case study, 1 to 15 January 1993, map sequences reveal that deep convection shifts northeastward as a southern mid-latitude trough couples with the ITCZ. Convective events over tropical east Africa are associated with an influx of northeasterly Indian monsoon flow followed by increased westerlies from the Guinea/Congo region. Eastward propagating OLR anomalies couple with zonal circulations, suggesting that a transient Walker cell emerges periodically from East Africa.
Mukherjee, A. K., G. Gurunadham, et al. (1978). "Clouding over the Arabian Sea and the synoptic situation over India during monsoon." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 118-124.
Satellite cloud pictures for the period 1971-1975 were analyzed to examine the coverage and type of clouding over the Arabian Sea in relation to two main synoptic features of the monsoon season, i.e., 1) break in monsoon and 2) movement of monsoon depressions. It is found that whenever breaks appear and the monsoon trough goes to the foot of the Himalayas, the Arabian Sea north of 15 degrees N lat. is covered with stratocumulus cloud lines. With the revival of the monsoon, these cloud lines decrease. This situation continues when the monsoon depressions form and move westward until they come to 80 degrees E long. When the monsoon depressions move further, either to the west of 80 degrees E long. or to the north of 25 degrees N lat., the stratocumulus cloud lines increase over the Arabian Sea.
Murakami, M. (1984). "Analysis of the deep convective activity over the western Pacific and Southeast Asia, Pt. 2, Seasonal and intraseasonal variations during northern summer." Meteorological Society of Japan, Tokyo, Journal 62(1): 88-108.
The seasonal and the intraseasonal behaviors of the deep convective clouds are investigated by using the activity index devised in Pt. 1 of this paper. The monthly mean distributions for the period May-Aug. 1979 reveal many seasonal characteristics that have been inferred in the past from the cloudiness or from the outgoing long-wave radiation (OLR). The latitudinal change of the distribution clearly shows the northward advance of the convective area over Southeast Asia, the formation of a highly convective area that brings the bai-u (mei-yu, in Chinese) rain, and the northward shift of the convective belt associated with the ITCZ over the western Pacific. The latitude-time section of this activity reveals the existence of the major intraseasonal variation with a 30-40-day periodicity. Over the Indo-China peninsula and the Bay of Bengal, it appears in the form of the repetitive progressions from the equatorial southern Indian Ocean. The more detailed analysis using the bandpass filter reveals that a southward progressing component also exists which comes down from the Tibetan Plateau. A similar situation occurs over the western Pacific. The composite analysis performed on the wind variation at the 850-mb level verifies that the above 30-40-day variation is associated with the active-break cycle of the Indian monsoon. When the monsoon westerly is most intensified over India, the enhanced deep convection appears over the region ranging from northeastern India to the Philippines. In contrast, the suppressed condition appears over the equatorial Indian Ocean and the Tibetan Plateau. Furthermore, the convective activity associated with the ITCZ is also intensified at this stage over the tropical western Pacific.
Murakami, T. (1980). "Empirical orthogonal function analysis of satellite-observed, outgoing long-wave radiation during summer." Monthly Weather Review, Boston 108(2): 205-222.
Some of the characteristic features of the Asiatic summer monsoon were investigated by using outgoing long-wave radiation data obtained from NOAA polar orbiting satellites during the summer months in 1974-1977. Monthly mean, outgoing, long-wave radiation patterns clearly reflect pronounced monsoon activity over the India-Southeast Asian and Philippines-western North Pacific regions. Between these two regions, values of outgoing long-wave radiation are relatively high and indicate less pronounced monsoon activity over the South China Sea along similar to 110 degrees E. Empirical orthogonal function analysis was applied to identify the major modes of intraseasonal oscillations present in outgoing long-wave radiation data during the summer. Spectral analyses of the amplitude functions for the four largest eigenvectors exhibit marked peaks in a period range of similar to 20-30 days. The first four eigenvector patterns were examined to investigate the zonal and meridional phase propagation of 20-30-day perturbations. Zonal phase propagation is westward (eastward) at similar to 10-20 degrees N (Equator). It appears that rainfall fluctuations between active and break monsoon periods over the Indian region are strongly associated with these zonally propagating, 20-30-day perturbations in outgoing long-wave radiation fields. Distinct northward phase propagation was found over the South China Sea along similar to 110 degrees E; therefore, this region is characterized by eastward propagating modes near the Equator, westward moving perturbations around 10-20 degrees N, and northward penetration into China.
Murakami, T. (1981). "Orographic influence of the Tibetan Plateau on the Asiatic winter monsoon circulation, Pt. 4, Long-period oscillations." Meteorological Society of Japan, Tokyo, Journal 59(2): 201-219.
An empirical orthogonal function analysis was applied to 12-20-day-filtered wind, temperature, and geopotential height data for the 1978-1979 winter (Dec. 1, 1978-Feb. 28, 1979) at seven levels (the surface, 850, 700, 500, 300, 200, and 100 mb) over the Tibetan Plateau region (20 degrees -50 degrees N and 57.5 degrees -117.5 degrees E). The resulting eigenvector series were truncated at eigenmode 6 to remove unwanted, small-scale noise that is of questionable accuracy. This was followed by the compositing of each meteorological variable at every level and grid point, with reference to changes in the first eigenvector coefficients for 12-20-day-filtered meridional wind data at 200 mb. Composite charts of 12-20-day-filtered vorticity perturbations at 200 mb exhibit prominent standing oscillations anchored in the vicinity of northeastern India and Burma. Nonlinear interaction between the winter mean absolute vorticity field and 12-20-day disturbances is primarily responsible for the development of 200-mb vorticity perturbations over this region. These standing vorticity perturbations are superimposed upon weak, eastward propagating modes that move slowly ( similar to 4 m [small solid circle] sec super(-) super(1) ) in the subtropical westerlies ( similar to 45 m [small solid circle] sec super(-) super(1) ) south of the Tibetan Plateau. Velocity potential fields at 200 mb are greatly enhanced by local topographic effects and tend to reduce the eastward advection effect of the strong westerly flow. Over Indochina and the northern South China Sea along similar to 100 degrees -110 degrees E, north-south-oriented, divergent vertical overturning (departure from winter mean) is thermally indirect (direct) when an upper level trough (ridge) system intensifies over the northeastern India-Burma region. Substantial temperature changes ( plus or minus 1.5 degrees C) in the lower troposphere below similar to 700 mb over southern China are associated with these intraseasonal variations in local vertical overturnings.
Murakami, T. (1984). "Global aspects of monsoons." National Workshop on the Global Weather Experiment: Current Achievements and Future Directions, 1st, Woods Hole, MA., July 2: 517-539.
Recent developments are surveyed in three areas of monsoon research: 1) global aspects of the monsoon onset, including symmetric and asymmetric heating fields, transition from the Southern to Northern Hemisphere summer monsoon, and monsoon onset over central India; 2) orographic influence of the Tibetan Plateau upon the Northern Hemisphere summer monsoon, including simplified model experiments with the eight-layer, two-dimensional numerical model of the summer monsoon at 80 degrees E (Murakami et al., 1970), an experiment without mountains (Webster and Cho, 1980), a primitive equation model with five atmospheric layers and one oceanic layer over the 0-180 degrees E, 25 degrees S-55 degrees N region (Kuo and Quian, 1982), and global GCM numerical experiments; 3) synoptic-scale disturbances over and around the Tibetan Plateau, involving Tibetan vortices and edge cyclones; and 4) tropical 40-50-day oscillations, which include the relationship between 40-50-day oscillations and active and break monsoons during the Northern Hemisphere summer, the behavior of the 40-50-day oscillations during the transition from the southern to northern summer monsoon, and stratospheric 40-50-day oscillations during the 1979 northern summer.
Murakami, T. (1987). "Intraseasonal atmospheric teleconnection patterns during the Northern Hemisphere summer." Monthly Weather Review, Boston 115(9): 2133-2154.
At times, the 30-60-day filtered, outgoing long-wave radiation (OLR) perturbations exhibited a systematic eastward propagation across the equatorial Indian Ocean-western Pacific during the five summers (May 1-Sept. 30) of 1979-1983. Such occasions are defined as E phase, whereas periods of irregular movement are designated as NE phase. Global-scale behavior of the 30-60-day filtered streamfunction and velocity potential fields differs significantly from E to NE phase. During E phase at 200 mb, a series of time-clustered, space-overlapping disturbances develop over the northern and southern subtropics. Although individual disturbances are nearly stationary, a wave packet clearly propagates eastward with an approximate phase speed of 8 degrees long./per day and a space scale of wavenumber 1. Sandwiched between the Southern and Northern Hemisphere wave packets are relatively weak equatorial zonal wind perturbations that also move eastward. At 850 mb, E phase behavior is characterized by strong 30-60-day southerly surges originating over the cold midlatitude Indian Ocean. These southerly surges coincide with widespread convection over the monsoon region (0-20 degrees N, 40-160 degrees E), where a strong continent-ocean heat contrast exists. The 30-60-day southerly surges are less pronounced over relatively orographic free regions of the western hemisphere Tropics. During NE phase, no systematic eastward propagation occurs in the streamfunction and velocity potential fields. Compared with the E phase, the streamfunction patterns are less organized, with dominant wavenumbers >2. At 850 mb, southerly surges are limited to the immediate vicinity of the African east coast.
Murakami, T. and T. Nakazawa (1985). "Tropical 45-day oscillations during the 1979 Northern Hemisphere summer." Journal of the Atmospheric Sciences, Boston 42(11): 1107-1122.
Based on FGGE level-3b data, the structural features of 45-day perturbations over a tropical belt (15 degrees N-15 degrees S) during the 1979 summer are detailed. At the Equator, 45-day perturbations, which are primarily associated with the zonal wind components of wavenumber 1, propagate eastward (8 degrees of longitude/day) and upward (0.7 km/day), probably indicating downward energy flux. In the Southern Hemisphere Tropics (0-15 degrees S), the 45-day zonal mean wind perturbations propagate downward with an approximate phase speed of 0.8 km/day. In the Northern Hemisphere Tropics, they are largely of standing character, with the maximum amplitude (3 m sec super(-) super(1) ) near 200 mb at 15 degrees N. There is a strong association between monsoon activity over South Asia and changes in the intensity of the equatorial Walker circulation. When active monsoons occur over South Asia, the Walker circulation becomes stronger than usual with prominent 850-mb easterlies (200-mb westerlies) over the eastern Pacific east of the date line and above normal 850-mb westerlies (200-mb easterlies) over the Indian Ocean and the western Pacific west of the date line. Equatorial convective activity appears to be above normal near the date line, as evident by abnormally strong ascending motions. During the break monsoon phase, the equatorial Walker circulation is depressed below normal. There are two bridges (one in the Eastern Hemisphere from similar to 50 to 150 degrees E and another in the Western Hemisphere between 170 and 70 degrees W, through which the 45-day perturbations of the Southern Hemisphere Tropics at 200 mb interact with those in the Northern Hemisphere Tropics. In comparison, the central Pacific, between similar to 150 and 120 degrees W, is the only favorable channel for interhemispheric interaction because of transient disturbances with time scales shorter than similar to 30 days. This interhemispheric interaction, caused by short-period transient disturbances at 200 mb, is abnormally enhanced when the Asiatic monsoon is active.
Murakami, T., T. Nakazawa, et al. (1984). "On the 40-50-day oscillations during the 1979 Northern Hemisphere summer, Pt. 1, Phase propagation." Meteorological Society of Japan, Tokyo, Journal 62(3): 440-468.
Based on FGGE level 3b data, the structural features of 40-50-day oscillations over an extensive region (30 degrees S-30 degrees N, 30 degrees E-150 degrees W) during the 1979 summer are detailed. The analysis confirms earlier investigations that these low-frequency modes are associated primarily with the zonal wind oscillations. These 40-50-day perturbations propagate northward and eastward, which is most clearly defined over the monsoon region north of the Equator, from 60 to 150 degrees E. The monsoon region is characterized by prominent spectral peaks in the 850-mb meridional winds with periods shorter than 10 days, probably reflecting the activities of monsoon disturbances. However, the local Hadley circulation, as defined by averaging the meridional component of the wind between 60 and 150 degrees E, exhibits a distinct spectral peak in the period range of 40-50 days. Similarly, the square of the meridional winds, which is a measure of synoptic-scale disturbance activity, also changes with a period of 40-50 days. These features, which are similar to the group velocity phenomena, are pronounced only over the central monsoon region (10-20 degrees N, 60-150 degrees E). The low-frequency modes propagate northward and become most intensified near 10-20 degrees N through mutual interaction between synoptic-scale disturbances, the local Hadley circulation, and the zonal mean flows over the monsoon region. At the Equator, the 40-50-day zonal wind perturbations propagate systematically eastward (500 km/day) and upward (0.7 km/day). In the equatorial region, the low-frequency oscillations owe their existence to a lateral geopotential wave-energy flux from the monsoon region, which represents the major energy source for 40-50-day perturbations via the conversion from potential to kinetic energy. Compared to that at the Equator, the phase propagation of zonal wind perturbations along 15 degrees N, although moving eastward, is not as systematic. At this latitude, zonal wind perturbations are pronounced in the lower troposphere over the monsoon region and in the upper troposphere over the Western Pacific. An integral part of E-W interaction between these two regimes is downward progression of westerly (or easterly) perturbations over to the Arabian Sea region. The downward phase of westerly (easterly) modes corresponds to the commencement of active (break) monsoons over South and Southeast Asia.
Murakami, T., T. Nakazawa, et al. (1984). "On the 40-50-day oscillations during the 1979 Northern Hemisphere summer, Pt. 2, Heat and moisture budget." Meteorological Society of Japan, Tokyo, Journal 62(3): 469-484.
In July 1979, large-scale apparent heat sources exceeded 2 degrees C /day over the eastern Arabian Sea, the northern Bay of Bengal, the central South China Sea, and the Equatorial Pacific near the dateline. These heating centers are embedded in the monsoon trough at 850 mb and are in good agreement with regions of strong upward motions, apparent moisture sinks, and small outgoing long-wave radiation values. In May, the moisture supply for the rainfall near the west coast of Burma and Malaysia comes primarily from the Bay of Bengal, not from the Southern Hemisphere. In midsummer (June-Aug.), the cross-equatorial moisture flux off the east coast of Kenya is not large enough to maintain the rainfall over South and Southeast Asia. Thus, evaporation over the Arabian Sea constitutes the key contribution to the moisture supply for monsoon rains. The northward and eastward passage of 40-50-day perturbations is related to the phase changes between active and break monsoons over central South and Southeast Asia. When active monsoons begin, the large-scale apparent heat sources, Q sub(1) , and moisture sinks, Q sub(2) , become above normal over the Arabian Sea. About 5-7 days later, both Q sub(1) and Q sub(2) reach their maxima over the Bay of Bengal. This is followed by the intensification of Q sub(1) and Q sub(2) over the South China Sea region similar to 5 days later. Regions of above normal Q sub(1) and Q sub(2) also propagate northward across the monsoon region. Similarly, regions of break monsoons with below normal Q sub(1) and Q sub(2) propagate eastward and northward.
Murakami, T. and A. Sumi (1981). "Large-scale aspects of the 1978-79 winter circulation over the greater WMONEX region, Pt. 2, Long-period perturbations." Meteorological Society of Japan, Tokyo, Journal 59(5): 646-671.
Some of the characteristic features of large-scale disturbances over the greater WMONEX region were investigated by using 12-30-day filtered wind, vorticity, and divergence data at 850 and 200 mb for the 1978-1979 winter (Dec. 1, 1978-Feb. 28, 1979). Composites of these meteorological variables at each grid point and level were made with regard to changes in the first eigenvector (empirical orthogonal function) coefficients for 12-30-day filtered vorticity data at 200 mb. Over South and Southeast Asia, the 200-mb circulation undergoes massive changes in association with the eastward passage of upper tropospheric troughs and ridges. The mechanisms that are responsible for the development, slow eastward propagation ( similar to 4 m sec super(-) super(1) ), and dissipation of these disturbances are identical to those described by Murakami (1981). Pronounced long-period ( similar to 20-days) disturbances at 200 mb can also be traced from Japan, across the central North Pacific, to the equatorial Pacific. These upper tropospheric disturbances are maintained primarily through barotropic nonlinear interactions with the winter mean flow and disturbances of all period ranges. Over the western South Pacific, 200-mb long-period disturbances are most intense off the east coast of Australia. In this area, vertical vorticity transports caused by convective motions appear to be significant. At 850 mb, northerly (anomaly) surges are predominant over the Afghanistan-Pakistan, Japan-Philippines, and central North Pacific regions. Along the western periphery of the Tibetan Plateau, low-level southwesterlies ahead of an eastward moving trough flow up the extremely steep topographic gradient. This induces strong low-level convergence, which results in increased westerly divergent winds near the trough and contributes to the enhancement of the local northerlies. Orographic enhancement is also responsible for the acceleration of the northerlies over the Japan-Philippines region. Conversely, over the central North Pacific along about 170 degrees W, intensification of the northerlies is associated largely with transient disturbance activity poleward of 30 degrees N. Along the Equator, zonal wind and divergence fields at 850 mb exhibit marked intraseasonal ( similar to 20-day) changes. Here, anomalous westerlies (easterlies) and convergence (divergence) over the central Pacific coincide with easterlies (westerlies) and divergence (convergence) over the Indian Ocean. The E-W pressure gradient appears to be the primary factor for initiating changes in the equatorial 850-mb zonal winds.
Murakami, T. and A. Sumi (1982). "Southern Hemisphere summer monsoon circulation during the 1978-79 WMONEX, Pt. 2, Onset, active and break monsoons." Meteorological Society of Japan, Tokyo, Journal 60(2): 649-671.
The onset of the 1978-1979 Southern Hemisphere summer monsoon at around Dec. 23-27 was initiated by a series of events that occurred away from the Indonesia-Australia region. Low-level monsoon westerlies were first established over the western South Pacific (160 degrees E-170 degrees W, 0-10 degrees S) in response to the intensification of the trade winds over the North Pacific and cross-equatorial northerlies near 170 degrees E. The monsoon became fully established with the westward expansion of this low-level westerly regime into the Indonesian-Arafura Sea region. The western South Pacific was also a key area for the initiation of the break monsoon that prevailed from around Feb. 8 to 17. The initial phase shift from a westerly (active) to a predominantly easterly (break) regime occurred over this region as a result of vorticity advection caused by northerly divergent winds (low-level Hadley circulation) and planetary boundary layer friction effects.
Nagar, S. G. and S. V. Singh (1991). "Variations of surface pressure over India during the southwest monsoon." Theoretical and Applied Climatology, New York, NY 44(2): 95-112.
Intraseasonal and interannual variations of surface pressure and surface pressure anomalies are studied using 19 years of daily surface pressure data at 60 stations in India during the southwest monsoon season. The interannual variability of surface pressure is highest over northwest India and a ridge runs eastward near 23 degrees N. The spatial pattern of total daily intra-seasonal variations is similar to that of interannual variability. The seasonal cycle accounts for about 50% of the intraseasonal variance over northwest India. When the seasonal cycle is removed the high standard deviations are observed over the area adjoining the head of the Bay of Bengal where the frequency of occurrence of the intense cyclonic disturbances is highest. The area of maximum intraseasonal variability keeps shifting westward as one goes from June through to September. A harmonic analysis is performed to study the contribution to the variances by the 30-60 days and 10-20 days period ranges during excess and deficient monsoon years. EOF analysis of the 5-day mean pressure anomaly fields is conducted to study the most dominant modes of variation. The evolutionary features of surface pressure fields are studied with the help of Hovmoller diagrams and EEOF analysis. The relationships between westward and northward progression of anomalies and their influence on monsoon conditions is also studied.
Nagar, S. G., S. V. Singh, et al. (1992). "Relationships between circulation and rainfall over India during the southwest monsoon season. Part I: Surface pressure." Theoretical and Applied Climatology, New York, NY 45(4): 265-275.
Rainfall over India during the southwest monsoon season exhibits large intraseasonal fluctuations. The surface pressure fields illustrate the important circulation changes and the general conditions of active and break monsoon situations. We have studied the relationship between these two successive fields at daily to monthly time scales using monthly data, from July through September for an 11 year period (1966-1976). Lag relationships were also investigated to ascertain the nature of evolutionary patterns through which pressure affects rainfall and so assess the potential for predicting rainfall with the use of pressure fields. Finally, the relationship between pressure and rainfall (linear or non-linear) was examined with the use of quartile plots.
Nakazawa, T. (1992). "Seasonal phase lock of intraseasonal variation during the Asian summer monsoon." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 597-611.
By using the climatological mean daily value from the 10-year objective analysis data set and the 12-year OLR data, the enhancement of the Asian summer monsoon activity, which occurs intermittently with a period of 30-60 days, is examined. In particular, this work describes the seasonal phase lock of the intraseasonal variation (ISV). In the Northern Hemisphere summer, the enhancement of the convective activity and the low-level monsoon westerlies is evident along 10 degrees N around early June and middle July over the Indian Ocean and around middle June and late July to early August and early-middle September over the western Pacific. The enhancement is also found around late May and middle July over the equatorial Indian Ocean. The first enhancement in late May and early June over the Indian Ocean corresponds to the climatological monsoon onset, dated June 1 in South India and in southern Japan. The second enhancement in late July over the western Pacific is accompanied by a rather abrupt change. The monsoon westerlies in the lower latitude suddenly extend to the western Pacific up to 150 degrees E. The penetration of the westerlies to the subtropical western Pacific is associated with the eastward propagation of the ISV from the Arabian Sea and the equatorial Indian Ocean in middle July. The westerlies over the subtropical western Pacific result in cyclonic circulation over the western Pacific and anti-cyclonic circulation over Japan, which brings the withdrawal of the Baiu and the summer season in Japan.
Nanjundiah, R. S. (2000). "Seasonal simulation of the monsoon with the NCMRWF model." Current Science, Bangalore, India 78(7): 869-875.
The development of a climate version of the National Centre for Medium Range Weather Forecasting (NCMRWF) model and its implementation on a message passing platform are discussed. The ability of the model to simulate the Indian Summer Monsoon is studied. It is found that the model realistically simulates most of the major features of the Indian Summer Monsoon. The intraseasonal variations of the Indian monsoon are also studied.
Nanjundiah, R. S., J. Srinivasan, et al. (1992). "Intraseasonal variation of the Indian summer monsoon. II: Theoretical aspects." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 529-550.
A class of simple zonally symmetric models of increasing complexity viz. Webster andChou (1980a), Gadgil and Srinivasan (1990) and Srinivasan et al. (1991) have yielded increasingly realistic simulation of the intraseasonal variation of the tropical convergence zone (TCZ) over the Indian region in the summer. A new model in this class, which incorporates the space-time variation of surface pressure is discussed in this paper. As in the earlier models, the prominent feature of intraseasonal variation in the Indian region viz. poleward propagations of the TCZ is simulated. The rate of propagations in this model is more realistic and the period between propagations is also more realistic being about thirty five days. A new feature simulated by this model is the active phase of the monsoon with the TCZ persisting over the continental trough for about 20 days. The underlying mechanisms, unravelled by analysis of this and earlier models of this class are discussed in this paper.
Nikaidou, Y. (1989). "PJ-like north-south oscillations found in 4-month integrations of the global spectral model T42." Journal of the Meteorological Society of Japan, Tokyo 67(4): 587-604.
The relationship between precipitation over the tropical western Pacific and sea level pressure of the subtropical high to the south of Japan was investigated by making use of the global spectral model. Two experiments in 4-month numerical integration starting from the initial value on May 1, 1984 with and without an SST warm anomaly of 1.5 degrees C over the tropical western Pacific were examined. In both integrations, the subtropical high with a barotropic structure was formed around Japan in midsummer. Clear positive correlation was found between sea level pressure of the subtropical high and precipitation over the tropical western Pacific in the case of the integration with the SST anomaly. The relationship between the subtropical high and the tropical western Pacific could also be ascertained by EOF analyses of the precipitation. The EOF analyses indicate a close relationship of precipitation among the middle latitudes around Japan, the subtropics and the tropics over the western Pacific more clearly in the integration with the SST anomaly than without the SST anomaly. This relationship of precipitation has a resemblance in many respects to the intraseasonal PJ (Pacific-Japan) oscillation found by Nitta (1987), which is the north-south oscillation of cloud amount over the West Pacific and the Far East. It was also found in the integration with the SST anomaly that Rossby waves propagate along a great circle from the south of the Japan Islands to the west coast of North America, accompanied by an increase in precipitation over the tropical western Pacific. The wave propagation is similar to the phenomena reported by Nitta (1987). It took about 5 days for Rossby waves to propagate from Japan to North America. The increase in precipitation over the tropical western Pacific in the experiment caused not only the northeastward propagation of Rossby waves but also the westward propagation of fluctuations in precipitation along the south Asian monsoon region.
Nikitin, A. E. (1991). "Character of the Northern Hemisphere atmospheric general circulation during the different phases of the Indian monsoon." Izvestiya Fizika Atmosfery i Okeana, Moscow, Russia 27(10): 1161-1171.
In order to investigate the possible causes of variations in the intensity of the Indian monsoon, the distribution of meteorological elements and energetic characteristics in the Northern Hemispheric atmosphere during the active phase and the break of monsoon in June-July of 1979 is studied. The variations in the monsoon evolution can be connected with the change of the circulation regime in the atmosphere on the whole. More low-frequency regime corresponds to the break phase. The advection of vorticity and fluctuations in the rates of energy exchange between the zonal flow and the eddies in quasistationary troughs connected with the blocking systems on the North of Eurasia probably can be the immediate cause of the break. However, the changes in the tropical zone linked with the break can lead to redistribution of sources and sinks of energy in the eastern part of Eurasia as a consequence of the changes of flow over Tibet, changes in the intensity and position of Pacific storm-track and as an outcome, to the substantial changes of circulation conditions in middle latitudes.
Ohsawa, T., T. Hayashi, et al. (2000). "Intraseasonal variation of monsoon activities associated with the rainfall over Bangladesh during the 1995 summer monsoon season." Journal of Geophysical Research, Washington, DC 105(D24): 29445-29459.
The rainfall over Bangladesh during the 1995 summer monsoon season has been investigated in terms of the intraseasonal variation of monsoon activities. The rainfall over Bangladesh is basically dominated by the north-south oscillation of the monsoon trough. The rainfall increases when the monsoon trough is located at the foot of the Himalayas, because synoptic-scale convective activity is much more vigorous to the south of the monsoon trough axis than to the north of it. In addition, the strong southwesterly wind to the south of the monsoon trough intensifies local convective activity owing to the effects of the orography to the north and east of the country. It is also found that the monsoon rainfall over Bangladesh in 1995 varies with a periodicity of similar to 20 days, and this rainfall variation is closely associated with synoptic-scale monsoon activities spreading over South and Southeast Asia. The active /break cycle of the rainfall variation during the 1995 summer monsoon season can be mostly explained by the northward propagation of what is called the 10-20 day variation of monsoon activities.
Palmer, T. N. (1994). "Chaos and predictability in forecasting the monsoons." Das, P. K.
Day-to-day evolution of weather is only predictable up to ten days or so in advance. This can be understood in terms of the chaotic nature of atmospheric dynamics. On the other hand, skillful longer range predictions of seasonally averaged rainfall have been successfully made, as exemplified by the monsoon forecasts from the India Meteorological Department. This dichotomy is explored using the prototype Lorenz 3-component chaotic model. The model is applied to the Indian monsoon problem, with break and active monsoon periods corresponding to the Lorenz-model regimes. The dichotomy is resolved by taking into account the impact of lower boundary forcing anomalies on the regime probability density function. By studying the impact of such anomalies in a chaotic model, it can be seen that seasonal prediction is necessarily probabilistic, and requires ensemble integration techniques when tackled using dynamically-based forecast models.
Pant, G. B., K. Kumar, et al. (1988). "Long-term variability of the Indian summer monsoon and related parameters." Advances in Atmospheric Sciences, Beijing, China 5(4): 469-481.
The long-term variability of the Indian summer monsoon rainfall and related regional and global parameters are studied. The cubic spline is used as a digital filter to smooth the high frequency signals in the time series of the various parameters. The length of the data series varies from 95 to 115 years during the period 1871-1985. The parameters studied within the monsoon system are: (a) monsoon rainfall of the country as a whole; (b) number of break-monsoon days during July and August; (c) number of storms /depressions in Bay of Bengal and Arabian Sea during summer monsoon season; and (d) dates of onset of summer monsoon over South Kerala Coast. The parameters studied outside the monsoon system are: (a) the Wright's Southern Oscillation Index (June-July-August); (b) the January mean Northern Hemispheric surface air temperature anomaly; and (c) the east equatorial Pacific sea surface temperature anomaly. In order to examine the variability under various degrees of the smoothing, the series are filtered with splines of 50% variance reduction frequency of one cycle per 10, 20 and 30 years. It is observed that the smoothed time series of the parameters within the monsoon system comprise a common slowly varying component in an episodic manner distinctly showing the excess and deficient rainfall epochs. The change of intercorrelations between the time series with increasing degree of smoothing throws some light on the time scales of the dominant interactions. The relation between Southern Oscillation and east equatorial Pacific sea surface temperature and the Indian summer monsoon seems to be dominant on the interannual scale. The low frequency variations are found to have significantly contributed to the instability of the correlations of monsoon rainfall with parameters outside the monsoon system.
Pant, P. S. (1980). "Phases of the summer monsoon and oscillations of the equatorial trough." Mausam, New Delhi 31(2): 215-221.
In continuation of earlier studies by Pant (1978), the oscillations of the equatorial trough and the associated phase of the summer monsoon over the Indian subcontinent were studied. For this purpose, the data relating to MONEX-79 were considered. A weak or break monsoon phase is associated with the in situ appearance of the planetary oceanic equatorial trough in the lower troposphere around 10 degrees N. The revival of the monsoon activity is associated with the northward progress of the southern trough and its reestablishment around 20 degrees N. The studies of midtropospheric temperature changes over Tibet and Sinkiang show clearly that the setting in of the weak or break monsoon conditions is concurrent with the occurrence of very low temperatures in the middle troposphere, particularly over Sinkiang. The physical picture that emerges from this study is that, under low zonal index conditions, advection of cold air behind large amplitude, mid-latitude troughs results in cooling of the Tibetan Plateau area in the middle levels. This results in the destruction of the reverse Hadley cell and the reestablishment of the direct cell with its upward limb near 10-15 degrees N. This physically consistent picture explains many of the observed characteristics of the weak and active phases of the monsoon described by several authors. The study results indicate possibilities of prediction of the monsoon phases over periods of a week to 10 days, based upon the temperature changes over the Asian continent to the north of India.
Pant, P. S. (1983). "Physical basis for changes in the phases of the summer monsoon over India." Monthly Weather Review, Boston 111(3): 487-495.
The summer monsoon over India is described as ``active'' when large parts of India, particularly the central parts and the west coast, receive normal or above-normal rainfall. It is described as having a break when the rainfall is below normal over most parts of India, except in the hills in the north and in the southeastern parts of the country. It is shown in this paper that, when a break sets in, there is an in situ formation of a zonally oriented trough at 10-15 degrees N at 700 mb over south India and the adjoining seas. Simultaneously, there is a sharp fall in the 500-mb temperature over the Tibetan Plateau to much below normal values. As the plateau warms and attains near-normal temperature, the trough in the south shifts northward and occupies the normal active monsoon trough position ( similar to 20 degrees N) over north India. Considering the importance of meridional circulations in the Tropics, it is reasonable to expect that the changes in the 700-mb flow with the phase change are associated with corresponding changes in the meridional circulation. Computations of vertical motion along 75 degrees E with the help of a quasi-geostrophic model show that, under break monsoon conditions, the Hadley cell with its upward limb in the trough at 10-15 degrees N over south India and descent further north, predominates; whereas, in the active monsoon phase, the monsoon cell, with a rising limb at similar to 20 degrees N and descent in the south, is prominent. The physical process through which the sudden cooling over the Tibetan Plateau brings about a change in the meridional circulation, and, therefore, a change in the phase of the monsoon from an active to a break situation, is explored through a quasi-geostrophic streamfunction for the meridional circulation. It is seen that eddy fluxes, particularly of heat, are one to two orders of magnitude larger at the northern end of the monsoon cell at the time of disruption of the monsoon. The vertical motions induced by these eddy fluxes are seen to be in the right direction to bring about a switchover from the monsoon cell to the Hadley cell. The reestablishment of the monsoon cell and with it the active phase appears to take place slowly through radiational heating over the Tibetan Plateau.
Parasnis, S. S. (1991). "The convective boundary layer during active and break conditions of summer monsoon." Journal of the Atmospheric Sciences, Boston, MA 48(7): 999-1002.
A case study of the convective boundary layer (CBL) during active and break conditions of Indian summer monsoon has been carried out. Conserved variable analysis of the observations showed differences in CBL thermodynamic structures during active and break monsoon periods. The monsoon boundary layer structures during the summer monsoon are comparable with the FGGE CBL structures over tropical oceanic regions of Pacific.
Parasnis, S. S. and S. S. Goyal (1990). "Thermodynamic features of the atmospheric boundary layer during the summer monsoon." Atmospheric Environment Part A: General Topics, Oxford, England 24A(4): 743-752.
Characteristic variations in the thermodynamic parameters of the atmospheric boundary layer at Pune (18 degrees 32'N, 73 degrees 51'E, 559 m a.s.l.) have been studied using the aerological observations collected during the summer monsoon seasons of 1980 and 1981, and temperature observations from aircraft during the summer monsoons of 1976, 1979-1981. This study showed suppression of the mixed layer, absence of inversion/stable layers and decreased convective instability in the lower layers during the period of active monsoon conditions. The reverse was observed during the periods of weak/break monsoon conditions. Temperature stratification of sub-cloud layer has been classified into four different categories depending on the extent of the mixed layer and the gradient of potential temperature in the overlying stable layer. It was observed that these categories cover all types of weather conditions which prevailed during the monsoon season. The results are discussed with the possible association of the weather conditions prevailed during the active and break monsoon periods.
Parthasarathy, B. (1991). "Indian monsoon variability." WMO Tropical Meteorology Research Programme 183.
Atmospheric circulation including monsoon circulation shows fluctuations on various time scales. Two main aspects of the monsoon variability will be discussed here, namely (i) intraseasonal and (ii) interannual scale. The complex and pulsatory character of the southwest (summer) monsoon is well known. It is brought out by many scientists that oscillations of type (a) 5-7 days, (b) 10-15 days and (c) 30-60 days are prominent among these. The details of these oscillations and their spatial temporal characteristics will be discussed. The interannual and decadal variability of the Indian summer monsoon is an important aspect to be studied and understood. The behaviour of the Indian summer monsoon during the last 120 years (1871-1990) will be presented and discussed. These variations of monsoon may be linked with the conditions over Asian/Indian Ocean and Pacific regions.
Parthasarathy, B. and G. B. Pant (1986). "Summer monsoon rainfall over different regions of India and circulation features during 1981-84." World Meteorological Organization, Geneva, Programme on Long-Range Forecasting, Research Report Series 1986 1(6): 235-246.
Indian summer monsoon (June-Sept.) rainfall exhibits considerable year-to-year variations in its occurrence over different regions, producing such abnormalities as droughts and floods. The behavior of monsoon rainfall in different meteorological subdivisions of India and corresponding regional circulation features for the period 1981-1984 are presented. In 1982, almost all regions of India received below-average monsoon rainfall; the regions that were affected the most were Bihar State and the Vidarbha and Tamil Nadu subdivisions. Many subdivisions received good monsoon rainfall in 1983, mainly peninsular India and the Gujarat subdivisions. Many subdivisions received more or less normal monsoon rainfall in 1981 and 1984. The regional circulation features, namely, storms /depressions, onset/withdrawal, break-monsoon conditions, and west coast troughs, suggest that the years of above-normal, normal and below-normal monsoon rainfall amounts are observed to have noticeable differences in the regional circulation features.
Pfister, L., K. R. Chan, et al. (1993). "Gravity waves generated by a tropical cyclone during the STEP tropical field program: a case study." Journal of Geophysical Research, Washington, DC 98(D5): 8611-8638.
Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere-Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110-km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100- to 150-km-diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three-dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110-km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400-600 m, a lifetime of 4.5-6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi-biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above-cloud diabatic cooling.
Ploshay, J. J., W. F. Stern, et al. (1992). "FGGE reanalysis at GFDL." Monthly Weather Review, Boston, MA 120(9): 2083-2108.
The reanalysis of FGGE [First GARP (Global Atmospheric Research Program) Global Experiment] data for 128 days during two special observing periods has been performed, using an improved data-assimilation system and the revised FGGE level II dataset. The data-assimilation scheme features forward continuous (in time) data injection in both the original and the new systems. However, the major revisions in the new system include a better first guess and a more efficient dynamical balancing for the assimilation of observed data. The results of the implementation of this system are assessed by intercomparisons among the new FGGE analysis of other institutions such as ECMWF (European Centre for Medium-Range Weather Forecasts) and NMC (National Meteorological Center, Washington, D.C.), and also the original GFDL (Geophysical Fluid Dynamics Laboratory) analysis. The quality of the new GFDL analysis in now comparable to those of the other two institutions. However, the moisture analysis appears to be appreciably different, suggesting that the cumulus convection parameterizations and the boundary-layer moisture fluxes in the models are responsible for this discrepancy. A detailed investigation of the results has been carried out by comparing the analyses with radiosonde observations. This verification reveals that temperature and wind differences have been reduced considerably from the original to the new GFDL analysis; they are now competitive with those of ECMWF and NMC, while with regard to the geopotential height, differences of the GFDL reanalysis are larger than the original GFDL as well as the ECMWF and the NMC. A comparative study is also made with UCLA analyses over Asia in connection with the Indian monsoon. The results indicate that the qualities of both analyses are comparable. The capability of representing Madden-Julian oscillations in the reanalysis and in the ECMWF and old GFDL analysis is investigated by comparing with satellite observations. It is revealed that these oscillations are successfully reproduced by the new analysis; however, the agreement with the satellite data is not quite satisfactory. The utilization of satellite-observed wind (satobs) and aircraft data (aireps) in the data assimilation needs particular care. It appears that the quality control of these data in the GFDL reanalysis is too restrictive; in other words, the toss-out criterion of wind data is too small. A consequence of the failure to accept some single-level data turns out to be a fairly large discrepancy in representing the maximum wind speed in the analysis. It is also discussed that the current forward continuous-injection scheme is not adequate to obtain diabatic quantities for the archive.
Potemra, J. T., S. L. Hautala, et al. (2002). "Interaction Between the Indonesian Seas and the Indian Ocean in Observations and Numerical Models." Journal of Physical Oceanography 32(6): 1838-1854.
Recent measurements from six bottom-mounted gauges are used with numerical model results to study the exchange of water between the Indonesian seas and the Indian Ocean via the Lesser Sunda Islands known collectively as Nusa Tengarra. The observations are approximately three years in length, from late 1995 to early 1999, and include measurements of bottom pressure, temperature, and salinity. The locations of the gauges are at the boundaries of three straits connecting the southern Indonesian seas with the eastern Indian Ocean: the Lombok Strait, the Ombai Strait, and the Timor Passage. The magnitude of intraseasonal variations in the pressure data dominates over that of the seasonal cycle. Intraseasonal variability appears most frequently and largest in magnitude at the westernmost strait (Lombok) and decreases along the coastline to the Timor Passage. Comparison to wind data shows these intraseasonal variations to be due to Kelvin wave activity in the Indian Ocean, forced by two distinct wind variations: semiannual monsoon reversals and Madden-Julian oscillation (MJO) activity. Sea level variations from both forcing mechanisms are then adjusted by local, alongshore winds. Longer-duration model results show the observation period (1996 through early 1999) to be a time of increased ENSO-related interannual variability and of suppressed annual cycle. MJO activity is also increased during this time. These factors explain the dominance of the higher frequency signals in the pressure data and the relative lack of a distinct annual cycle. An optimal fit of model sea level variations to model through-strait transport variations is used to estimate transport variability from the observed pressure records. At each strait the optimal fit is consistent with a cross-strait geostrophic balance for transport variations in the upper 250 m.
Prakasa Rao, G. S. (1975). "Synoptic features during the break monsoon in 1965 and 1966." Indian Journal of Meteorology, Hydrology and Geophysics, Delhi 26(4): 535-537.
A prolonged break of 12 days in Aug. (4th-15th) and 10 days in July (2nd to 11th) occurred during 1965 and 1966, respectively. An examination of zonal and meridional components of wind was made during the break periods of 1965 and 1966. Zonal winds at the 150-mb level for Trivandrum, Madras, Bombay, and Nagpur are shown in a graph. During the break periods, easterlies continued to be strong and remained at the same latitude (8-13 degrees N) as in strong monsoon periods. The meridional circulation during the break period is compared with that of the mean meridional circulation. The easterlies descended and penetrated to 700 mb, and the strongest easterlies at 150 mb continued to be observed at their normal latitude of occurrence.
Prasad, K. and S. Sen (1981). "Flood meteorology of Ganga basin in Bihar: a synoptic analogue study." Mausam, New Delhi 32(4): 415-424.
A detailed survey has been made from available records of all past synoptic situations that resulted in heavy rainfall in different subcatchments of the Ganga basin in Bihar in different months of the monsoon season. Most of these spells are associated with either a low-pressure system or a break monsoon condition. While the heavy rain associated with break condition is confined to the immediate vicinity of the monsoon trough, the effect of a low-pressure system extends beyond its location. It has been possible to delineate an area for each month and each individual subcatchment where there is a low-pressure system which contains the risk of heavy rainfall in that particular subcatchment. This area has been termed the risk zone for the subcatchment.
Prasad, K. D. and S. V. Singh (1988). "Large-scale features of the Indian summer monsoon rainfall and their association with some oceanic and atmospheric variables." Advances in Atmospheric Sciences, Beijing, China 5(4): 499-513.
The summer monsoon rainfall totals for 31 meteorological subdivisions of India for the years 1901-1980 are analysed. The analysis reveals that four leading eigenvectors (EVs) are significant and account for 65% of the total variance. The spatial pattern of the first EV exhibits in phase fluctations over almost all of India. The large coefficients of this vector can be considered as representative of the conditions of large-scale flood and drought over the country. The second pattern reveals the fluctuations mostly over the north Indian region (north of 20 degrees latitude) probably in association with the western disturbances. The third pattern indicates fluctuations over northwest and northeast India in opposite phase and the fourth pattern exhibits the characteristic features of fluctuations associated with `break'. The spectral analysis of the coefficients of these EVs revealed quasi-periodicities of 2-5 years. On the basis of examination of the elements of these EVs the country has been divided into seven homogeneous regions. Rainfall indices of these regions and of the four EVs have been examined for association with some oceanic and atmospheric variables. The association is significant for the coefficients of the first EV and for the rainfall indices of central and south India. Among all the variables examined, Darwin pressure tendencies have the highest association and appear to be of special significance in prediction of the monsoon rainfall.
Puri, K. (1990). "Tropical numerical weather prediction studies for the 1987 Australian summer monsoon." Mausam, New Delhi, India 41(2): 257-264.
The performance of the ECMWF analysis-forecast system in depicting the main features of the 1987 Australian monsoon is described. The features include the mean circulation for the period and the onset and active/break periods of the monsoon. The model is successful in predicting the mean features up to 3 days ahead with the main deficiency being the marked weakening of the upper level divergent circulation with time. The analyses successfully depict most features of the monsoon circulation. Although the model is able to forecast onset up to about 1 day ahead and the subsequent episodes up to about 2 days ahead, there is a tendency in the model forecasts to weaken the low level westerly winds with time.
Puri, K., N. E. Davidson, et al. (1992). "The BMRC tropical limited area model." Australian Meteorological Magazine, Canberra, Australia 40(2): 81-104.
This paper describes a numerical weather prediction model developed at the Bureau of Meteorology Research Centre (BMRC) for the purposes of both research and real-time analysis and prediction over the Australian tropics. The model is described in some detail and its capabilities are illustrated by examining its performance in predicting the motion of two tropical cyclones, and the onset and active/break periods of the Australian summer monsoon during the Australian Monsoon Experiment (AMEX). Overall the model shows skill on both the prediction of tropical cyclone tracks and for features of the tropical circulation, such as a trend to weakening and subsequent strengthening of low-level monsoon westerly winds. However the model, in common with most others, is unable to predict the initial onset of the monsoon beyond 24 hours.
Puri, K., P. Loennberg, et al. (1990). "The ECMWF Analysis-Forecast System during AMEX." Berkshire, England, European Centre for Medium Range Weather Forecasts 166.
Part I: Performance of the ECMWF analysis-forecast system during AMEX, documents the performance of the ECMWF analysis-forecast system in depicting the mean features of the Australian monsoon circulation during AMEX and some individual features such as the onset of the monsoon, the active and break periods in the monsoon and the four tropical cyclones two of which formed within the AMEX network. In most cases only forecasts up to 72 hours are considered as the model skill in the tropics beyond this period falls off rapidly. Part II: Sensitivity of ECMWF analyses-forecasts of tropical cyclones to cumulus parameterization, presents a brief description of the cumulus convection parameterization used and the sensitivity of ECMWF analysis-forecasts of tropical cyclone to cumulus parameterization is then considered. The results indicate some sensitivity of model forecasts of tropical cyclone motion to initial conditions particularly the analyzed location.
Raj, Y. E. A. and S. M. Jamadar (1989). "Intra-annual quasi-biweekly periodicities of Indian rainfall." Mausam, New Delhi, India 40(3): 337-339.
On the basis of a study of long-period normal rainfall in India involving the spectral analysis of the rate of change of normal rainfall of several stations, it is shown that the quasi-biweekly oscillation, which is present in several parameters of monsoon rainfall, manifests itself in normal rainfall. This average feature is manifested by and large for individual years also. Thus, peaks in the rate of change of rainfall graphs for individual years, which correspond to intense rainfall activity, should have an approximate biweekly recurrence period. The weekly periodicity may have the potential to serve as a useful precursor for medium-range forecasting of rainfall.
Ram Mohan, H. S., P. Vaisala, et al. (1983). "Drought spells over north-central India during the 1979 southwest monsoon." Mausam, New Delhi 34(3): 299-302.
Widespread drought spells were observed over north-central India during the 1979 summer monsoon. The authors study the impact of drought conditions on the short-period water balances of representative stations in the region according to the bookkeeping procedure of Thornthwaite and Mather (1955). Cumulative deviation techniques have been used to delineate and categorize the intensity and duration of drought spells.
Ramamurthy, K. (1976). "Water vapour mixing ratio in the troposphere and its influence on the tropopause over India and neighbourhood." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 27(3): 275-284.
The distribution of the water-vapor mixing ratio in the troposphere over different stations in India and the three equatorial stations, Nairobi, Gan, and Singapore, throughout the year has been studied with reference to the level of the tropopause over these stations to see if there is any relationship between these two elements. The meridional distribution of the water-vapor mixing ratio along 73 and 80 degrees E long. during June and Oct. was also examined, with reference to the tropopause level along these longitudes during these two months. In addition, the distribution of the mixing ratio and the level of tropopause during active and break monsoon spells during the years 1971 through 1973 was investigated. It is concluded that there is no unique or uniform relationship between the tropopause and water-vapor mixing ratio over India and along the Equator throughout the year.
Raman, C. R. V. and Y. P. Rao (1981). "Blocking highs over Asia and monsoon droughts over India." Nature, London 289(5795): 271-273.
Severe summer droughts of the Indian subcontinent were found to accompany prolonged breaks in the southwest monsoon. Data from the years of severe drought suggest that the associated breaks were due to upper tropospheric blocking ridges over East Asia. A model is presented for the evolution of the East Asian (EABR) and the West Asian (WABR) blocking ridges. Apparently, an intense WABR, the initiator of the monsoon break, precedes the formation of the EABR. This cycle could be a way of predicting years of intense drought or monsoon activity.
Ramaswamy, C. and R. S. Pareek (1978). "Southwest monsoon over India and its teleconnections with the middle and upper tropospheric flow patterns over the Southern Hemisphere." Tellus, Stockholm 30(2): 126-135.
The paper contains an analysis of the daily synoptic charts of July 1972 between 50 degrees N and the South Pole and between 50 and 160 degrees E, with reference to the activity of the southwest monsoon (summer monsoon) over India. The month began with a spell of normally active monsoon for eight days and ended with a spell of large-scale break in the monsoon for nine days with two transitional phases in between. There was nearly simultaneous development in the middle-latitude westerlies in the middle and upper troposphere in both hemispheres, especially during the break period. Blocking ridges over and near the Tasman Sea extended far toward the SSE during the break conditions. The pronounced troughs and ridges observed in the meridional profiles along 35 degrees N and 35 degrees S, especially along the latter during break conditions, have been discussed, and certain tentative explanations are given regarding the developments in the Southern Hemisphere.
Randell, S. C., S. A. Rutledge, et al. (1994). "A modeling study on the early electrical development of tropical convection: continental and oceanic (monsoon) storms." Monthly Weather Review, Boston, MA 122(8): 1852-1877.
Numerical modeling studies of continental tropical and maritime tropical convection were conducted using the two-dimensional, nonhydrostatic, cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water vapor, cloud water, cloud ice, rain, snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. Charge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi. The goal of the numerical experiments was to examine the kinematic and microphysical differences that lead to marked differences in observed electrification between the break (continental) and monsoon (oceanic) convective regimes observed near Darwin, Australia. The break regime is associated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts of lightning accompany break period convection events. Monsoon conditions are associated with heavy rain and relatively weak convection that forms in moderate to low-CAPE environments. Very little lightning activity is normally observed in the monsoon. Three numerical simulations ranging from high- to low-CAPE conditions are presented. The results indicate that the electrification of the simulated storm critically depends on the juxtaposition of the level of charge reversal (LCR), which is in turn dependent on temperature and liquid water contents, and the particle interaction region, which is the level where ice particle collisions occur and thus where noninductive charging can take place. In the high-CAPE (break period) case, the LCR is located several kilometers below the interaction region, and strong in-cloud electric fields develop as a consequence. In the low- to moderate-CAPE (monsoon) cases, the LCR and interaction region are closely located in the vertical. As hydrometeors move across the LCR in both directions, the charge on their surfaces continually changes sign, thus preventing the development of a significant in-cloud electric field. It is further hypothesized that in conditions of zero to extremely low CAPE, the particle interaction region would be situated below the LCR, leading to the development of an inverted dipole (positive charge underlying negative charge), such as may occur in the stratiform regions of mesoscale convective systems.
Rangarajan, C., S. K. Subramanian, et al. (1979). "Synoptic study of the variations in radon activity over the Arabian Sea and the Bay of Bengal region during the Monsoon-77 period." Archiv fuer Meteorologie, Geophysik und Bioklimatologie, Ser A, Vienna 28(4): 361-374.
Measurements of radon activity conducted during the Monsoon-77 Observational Program over the Arabian Sea-Bay of Bengal region are presented and discussed along with earlier ISMEX-73 data. Levels of radon within the monsoon region show considerable variations in the range of 1-15 picocuries/m super(3) of air. The levels in the Arabian Sea showed a marked increase at latitudes similar to 20 degrees N, reaching values of 8-10 picocuries/m super(3) , thereby suggesting increased input of continental air. At lower latitudes, the levels are a few picocuries/m super(3) , which is characteristic of maritime air from equatorial regions. Continuous measurements at stationary positions in the Arabian Sea at widely separated locations show similar variations in radon levels. These variations seem to be related to monsoon disturbances and their westward movements which reduce radon levels, because of the strengthening of the southerly monsoon current, which has low radon content. In the Bay of Bengal, there is evidence of lower radon values during break monsoon conditions. This could be caused by the prevailing pressure pattern during the breaks, with isobars running in N-S direction, resulting in the movement of equatorial maritime air mass directly to the bay without going over the land mass of India. During strong monsoon conditions, the air mass enters the bay after traveling over India and, consequently, has higher radon levels. Near the coast, radon values are high, as expected.
Rao, K. G. (1986). "Sensible heat fluxes during the active and break phases of the southwest monsoon over the Indian region." Boundary-Layer Meteorology, Dordrecht, Holland 36(3): 283-294.
On the basis of a theory given by Saltzman and Ashe (1976), sensible heat fluxes are calculated for the active and break phases of the southwest monsoon over the Indian region. The conclusion drawn is that the sensible heat flux is generally larger during the break monsoon situation than during the active monsoon. The synoptic heat flux is negligible when compared with mean and diurnal heat fluxes over the Indian region even during the monsoon season.
Rao, K. G. and B. N. Goswami (1988). "Interannual variations of sea surface temperature over the Arabian Sea and the Indian monsoon: a new perspective." Monthly Weather Review, Boston 116(3): 558-568.
The interannual variation of surface fields over the Arabian Sea and Bay of Bengal are studied by using data obtained between 1900 and 1979. It is emphasized that the monthly mean sea surface temperature (SST) over the north Indian Ocean and monsoon rainfall are significantly affected by synoptic systems and other intraseasonal variations. To highlight the interannual signals, it is important to remove the large-amplitude high-frequency noise and very-low-frequency long-term trends, if any. By suitable spatial and temporal averaging of the SST and the rainfall data and by removing the long-term trend from the SST data, it is possible to show that there is a homogeneous region in the southeastern Arabian Sea over which the March-April (MA) SST anomalies are significantly correlated with the seasonal (June-Sept.) rainfall over India. A potential of this premonsoon signal for predicting the seasonal rainfall over India is indicated. It is shown that the correlation between the SST and the seasonal monsoon rainfall goes through a change of sign from significantly positive with premonsoon SST to very small values with SST during the monsoon season and to significantly negative with SST during the postmonsoon months. For the first time, it is demonstrated that heavy or deficient rainfall years are associated with large-scale coherent changes in the SST (although perhaps of small amplitude) over the north Indian Ocean. Possible reasons for the apparent lack of persistence of the premonsoon SST anomalies are also indicated.
Rao, R. R., S. V. S. Somanadham, et al. (1978). "Study of the influence of surface energy budget of north Indian Ocean on the behaviour of Indian summer monsoon." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 253-258.
The probable influence of the fluctuations in the energy budget of the north Indian Ocean surface upon the general behavior of the Indian summer monsoon during July is investigated. Normals of the energy budget components, such as monthly heat gain by the ocean, solar radiation absorbed by the ocean, and the net IR radiational exchange at the surface, according to Budyko's method; and latent heat flux and sensible heat flux, according to bulk aerodynamic method, during July, are evaluated by using the available marine climatological data over the north Indian Ocean. Deviations of energy budget components from their respective normals during the break monsoon period show significant differences, which are discussed in terms of the behavior of the summer monsoon. Charts of energy budget components are presented, and related features are discussed.
Rasch, P. J., W. D. Collins, et al. (2001). "Understanding the Indian Ocean Experiment (INDOEX) aerosol distributions with an aerosol assimilation." Journal of Geophysical Research. D. Atmospheres 106(D7): 7337-7355.
We use the aerosol assimilation procedure described by Collins et al.  to help explain INDOEX aerosol distributions. The procedure combines modeled aerosol with AVHRR satellite estimates. The result is consistent with satellite measurements, regular in space and time, and provides information where retrievals are difficult (over land, coincident with clouds, and at night). Extra information on aerosol composition, vertical distribution, and region of origin is also produced. Carbonaceous, sulfate, and sea salt aerosols agree with the in situ measurements to 10-20%. Carbonaceous aerosols were estimated to be the dominant contributor (36%) to the aerosol optical depth (AOD); dust (31%) and sulfate (26%) were also important. The residence time for sulfate and carbon is similar to 7 and similar to 8 days respectively, longer than globally averaged residence times of many modeling studies. Thus aerosols produced here during the winter monsoon may have a larger climate impact than the same emissions occurring where the residence time is shorter. Three points of entry are found for anthropogenic aerosol to the INDOEX region: a strong near surface southward flow near Bombay; a deeper plume flowing south and east off Calcutta and a westward flow originating from southeast Asia and entering the Bay of Bengal. All three plumes are strongly modulated by a low-frequency change of meteorological regime associated with the Madden Julian Oscillation. The analysis suggests that India is the dominant source of aerosol in the Arabian sea and Bay of Bengal near the surface but that Asia, Africa and the rest of world also contribute at higher altitudes. India and Asia contribute similar to 40% each to the total column mass of air reaching the Maldives, the balance of air comes from other source regions. The assimilation procedure produces an analysis that is a synergy in information about aerosols, that is not easily accessible by independent estimates from remote sensing, in situ measurements, or global transport models by themselves.
Rasmusson, E. M. (1989). "Global climate variability: interannual and intraseasonal time scales." American Association for the Advancement of Science.
In responding to internal and external forcing mechanisms, the atmosphere exhibits preferred regional and global modes of variability, which appear over and over again in generally similar forms. These teleconnections, a fundamental feature of atmospheric variability, dictate taking a global view of climate dynamics and prediction. The evolving teleconnection patterns that appear as part of two global-scale oscillations are particularly important elements of intraseasonal and interannual variability and prediction. They are the Madden-Julian (40-50 d) oscillation and the El Nino/Southern Oscillation (ENSO) cycle of coupled ocean atmosphere interactions (which evolves on the interannual time scale). Both oscillations have maximum amplitude in the Pacific trade wind-monsoon sector of the tropics and also affect other regions of the globe. Current prospects for predicting monthly and seasonal means by empirical-statistical techniques are discussed in light of improved understanding of short-term climate variability and its relation to ocean and land surface anomalies. Possible developments in short-term climate prediction are reviewed, with emphases on the possibility of the development and use of general circulation models of the coupled atmosphere-ocean-land surface climate system.
Rasmusson, E. M. (1990). "Nature of low-frequency tropical variability." Guzzi, Rodolfo.
Low-frequency variability in the tropics is surveyed with emphasis on the socio-economically important intraseasonal and interannual time scales in general and the El Nino-Southern Oscillation (ENSO) cycle in particular. The contents are as follows: 1) sources of climate variability; 2) tropical climatology; 3) the tropical annual cycle; 4) synoptic variability, i.e., cloud clusters and tropical storms, monsoon rainfall, intraseasonal variability, interannual variability: the ENSO cycle--equatorial Pacific interactions global teleconnections, and ENSO cycle time scales.
Ray, T. K. (1985). "Monsoon's response to air-sea interaction." National Seminar cum Workshop on Atmospheric Science and Engineering, Calcutta, India, Feb.
Energy exchange through the atmospheric boundary layer is a slow but steady process. Energy transfer to the atmosphere from the oceans reaches a maximum during the southwest monsoon over India. Formation of an area of high evaporation over the equatorial Indian Ocean and its movement northward has a strong bearing on the activities of the monsoon over India. High-energy transfer in the early summer or late spring indicates the early onset of the monsoon. Cross-equatorial fluxes of air and water vapor are predominantly negative, i.e., southward between 65 and 80 degrees E long. Fluxes across the Equator are much higher than across any other northern latitudes. During weak and break monsoons, the north Bay of Bengal gains heat while the north Arabian Sea loses it.
Ray, T. K. (1986). "Ocean waves and their response to monsoon." Mausam, New Delhi 37(2): 203-206.
The arrival of the southwest monsoon in the Indian seas, its onset over India, and its break and withdrawal phases reflect the manifestations of mechanical energy on the water surface. An inverse relationship between sea surface temperature and swell heights has been explained. High-period swells, which die down after the monsoon onset, appear over the Arabian Sea before onset. The height of south Indian Ocean swells and the active monsoon over India have a direct relationship. An attempt is made to predict the height of waves by use of a simple regression equation.
Ray, T. K. and H. S. Bedi (1985). "Thermodynamic and kinematic structure of the troposphere over the Arabian Sea and the Bay of Bengal during 1979 monsoon season." Mausam, New Delhi 36(4): 417-422.
The Monsoon Experiment (MONEX-79) has provided an exhaustive data set over the Indian seas during 1979 monsoon. The U.S.S.R. research ships recorded upper air data in stationary polygons formed over the Arabian Sea and the Bay of Bengal. By using these data, various parameters determining the kinematic and thermodynamic structure of the atmosphere from 1000 to 100 mb have been calculated. The comparison of these parameters during various phases of the monsoon over the Arabian Sea and the Bay of Bengal brings out interesting contrasts during the onset, active, and break monsoon periods.
Rodwell, M. J. (1997). "Breaks in the Asian monsoon: the influence of Southern Hemisphere weather systems." Journal of the Atmospheric Sciences, Boston, MA 54(22): 2597-2611.
Atmospheric model results suggest that chaotic weather systems in the Southern Hemisphere midlatitudes can trigger ``breaks'' in the Indian monsoon rainfall. Indeed, the mechanism may be able to trigger a more general break of the entire Asian monsoon. The mechanism proposed involves the injection of dry, high negative potential vorticity air from the Southern Hemisphere midlatitudes into the low-level monsoon inflow. Observations from the 1994 monsoon season tend to support this mechanism and, if true, it may imply some predictive skill for shorter-range forecasting. However, the mechanism proposed may also imply that an accurate seasonal forecast of monsoon rainfall is an impossible objective, with important consequences for the agricultural economies of the region. Results are presented from both an idealized model and a full general circulation model.
Roy, N. and S. Kaur (2000). "Climatology of monsoon rains of Myanmar (BURMA)." International Journal of Climatology, Chichester, UK 20(8): 913-928.
Based on 33 years' rainfall data of Myanmar for the summer monsoon months (June-September), the detailed rainfall climatology of the country has been studied. Seasonal rainfall series are found to approximate to a Gaussian distribution. By using the rainfall distribution and coefficient of variation, it has been possible to divide the country into five homogeneous rainfall regions. Different statistical characteristics of the seasonal, monthly and zonal rainfall, as well as the whole country's rainfall, have been determined. Analysis of interannual and intraseasonal variability highlights the fact that the correlation between the rainfall of different months and zones is rather weak. Trend and periodicity of the rainfall series have been examined by different statistical techniques, indicating little evidence of a trend. The power spectrum of the rainfall series appears to show only marginal significance at the 95% level for an 11 year cycle. The rainfall series of Myanmar shows little correspondence with neighbouring Bangladesh and Northeast India, even though all of them are influenced by similar weather systems.
Rui, H. and B. Wang (1990). "Development characteristics and dynamic structure of tropical intraseasonal convection anomalies." Journal of the Atmospheric Sciences, Boston, MA 47(3): 357-379.
The development and dynamical structure of intraseasonal low-frequency convection anomalies in the equatorial region are investigated using 10 years (1975-85) of outgoing longwave radiation (OLR) and 7 years (1979-85) of 200 and 850 mb wind data. The composite OLR anomalies for 36 cases show a four-stage development process: initiation over equatorial Africa, rapid intensification when passing through the Indian Ocean, mature evolution characterized by a weakening in the maritime continent and redevelopment over the western Pacific, and dissipation near the date line in moderate events or emanation from the equator toward North America and southeastern Pacific in strong events. A noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases. Equatorial upper- (lower-) level easterly (westerly) anomalies and associated twin anomalous anticyclonic (cyclonic) circulation anomalies couple with equatorial convection anomalies. The wind anomalies, however, generally lag convection anomalies in development and early mature phases, but nearly overlap in late mature phase and slightly lead the convection anomalies in dissipation phase. The upper-level twin cyclonic cells associated with the westerly anomalies in front of the convection travel across eastern Pacific after the convection ceases in the central Pacific, while the low-level wind anomalies die out east of the date line. The implications of the findings in relation to theoretical hypotheses on low-frequency motion are discussed.
Russell, P. B., L. Pfister, et al. (1993). "The Tropical Experiment of the Stratosphere-Troposphere Exchange Project (STEP): science objectives, operations, and summary findings." Journal of Geophysical Research, Washington, DC 98(D5): 8563-8589.
The Stratosphere-Troposphere Exchange Project Tropical Experiment (STEP Tropical) investigated stratosphere-troposphere exchange and dehydration processes in that region and season with the coldest average tropopause temperatures, the tropical western Pacific and northern Australia during the winter monsoon (January-February) of 1987. This is also a period of extensive convective activity and rainfall. In addition to this primary goal, STEP Tropical (1) extended stratospheric aircraft chemical tracer surveys to the Southern Hemisphere subtropics and mid-latitudes and (2) sampled the anticyclone that dominates the circulation over Australia in the lower stratosphere during the winter monsoon. Advanced fast-response instruments on NASA's ER-2 aircraft measured meteorological variables, stratospheric and tropospheric tracers, particles, and radiative fluxes. ER-2 sorties included six ferry flight legs across the Pacific and eleven flights in the Australian region. The 1986-1987 monsoon was atypical in that (1) its date of onset, January 14, was 3 weeks later than normal and (2) it was unusually intense and sustained once it did arrive. As a consequence, almost all the flights were conducted under monsoon conditions, with only limited sampling of premonsoon and break-monsoon continental convection. Illustrations show flight paths for each sortie on satellite images and on 100 hPa synoptic flow charts, as well as the timing of flights with respect to overall cloudiness in the Australian region. STEP Tropical results, reported in the accompanying set of papers, include (1) observational documentation of a convective scale cold trap that dries air of recent tropospheric origin to prevailing stratospheric minimum water vapor mixing ratios (3 ppmv or less); (2) indications that this drying mechanism can be effective not only in the anvils of the tallest clouds (which occur during ``break-monsoon'' conditions) but also in the anvils of tropical cyclones and monsoon mesoscale convective systems; (3) demonstration that convectively generated gravity waves account for a significant part of the momentum forcing of the semiannual and quasi-biennial oscillations in the stratosphere; and (4) documentation of an upper tropospheric source of NO sub(y) and a significant upward flux of NO sub(y) at the tropical tropopause. For monsoon convection to be an important contributor to the net transfer of tropospheric air to the stratosphere, a mechanism is required to move air from the cloud tops to significantly greater heights. STEP Tropical results suggest that gravity-wave-induced small-scale turbulence or radiative heating of anvils provides this mechanism. Observational proof of the theoretically required high heating rates remains to be obtained.
Rutledge, S. A., E. R. Williams, et al. (1992). "The Down Under Doppler and Electricity Experiment (DUNDEE): overview and preliminary results." Bulletin of the American Meteorological Society, Boston, MA 73(1): 3-16.
DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carried out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the ``maritime continent,'' experiences both monsoon and ``break'' period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potential energy) and total flash rate is presented and discussed to explain this observation.
Sabre, M., K. Hodges, et al. (2000). "Simulation of monsoon disturbances in the LMD GCM." Monthly Weather Review, Boston, MA 128(11): 3752-3771.
The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Meteorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems. The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June-July-August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.
Salby, M. L. and R. R. Garcia (1987). "Transient response to localized episodic heating in the Tropics, Pt. 1, Excitation and short-time near-field behavior." Journal of the Atmospheric Sciences, Boston 44(2): 458-498.
The dynamic response to localized, unsteady tropical heating is studied in a stochastic framework. Spectral statistics of the random wave response are derived from those of tropical convection by using the primitive equations for a spherical baroclinic atmosphere. Short-time near-field behavior emerges in the form of a transient wavepacket that disperses away from the source region. Two principal components characterize the response: 1) a projection response that matches the vertical scale of the heating, and 2) a barotropic response involving Rossby normal modes. The projection response consists of a continuum of frequencies and vertical scales centered about vertical wavelengths twice the effective depth of the heating. This scale discrimination is shown to be insensitive to variations in the heating distribution. The associated disturbance is trapped laterally about the Equator, but radiates vertically away from the source region. It corresponds to the tropical waves traditionally studied on the equatorial beta-plane. The barotropic component radiates latitudinally into middle and high latitudes, but is vertically trapped. This component of the response corresponds to planetary Rossby waves usually developed with the barotropic vorticity equation on the sphere. Because of the complementary nature of these two components, far-field tropospheric behavior is dominated by the barotropic contribution. These elements of the response are presented in both local and more conventional modal descriptions. Vertical radiation and dispersion are evaluated for several modes. The wavepacket associated with the Kelvin mode completes less than one circuit around the globe before propagating completely out of the troposphere. Higher frequency components of the projection continuum radiate out of the source region even more quickly. For short-term heating fluctuations typical of tropical convection, the response at tropopause level is in accord with classical observations of the Wallace and Kousky Kelvin wave. The fast and ultrafast Kelvin waves are secondary ingredients of the initial wave spectrum. In the case of slow transitional heating, e.g., the seasonal drift in monsoon activity between hemispheres, the Kelvin response assumes the form of a damped transient Walker circulation. This eastward migrating cell captures the salient characteristics of Madden and Julian's composite of the 40-day wave in the tropical Pacific Ocean.
Sankar-Rao, M., V. N. Lykossov, et al. (1991). "Relationships between the global general circulation and the Indian summer monsoon." Advances in Atmospheric Sciences, Beijing, China 8(2): 137-148.
The relationships between the global general circulation and the Indian monsoon during active and break phases are investigated with the help of FGGE IIIb data. It was found that the ultralong wave component positive and negative height anomalies over Tibet are associated with active and break monsoon phases respectively. This ultralong wave component has a significant effect even up to 22 degrees N over the Indian region which is the monsoon trough region. During a monsoon break, the general circulation was found to be more turbulent in the sense that more waves become energized. It was observed that during a break, blocking prevails over the Siberian region and cold air advection takes place toward the Indian region from the Siberian region, depressing the temperatures over the Indian region by about 1 degrees C. During the break, the Indian region gets connected with higher latitudes by the south winds blowing from polar Soviet regions to the Indian region. From active to break phase the zonal component weakens by about 25% from the Indian Ocean area right up to the Alaskan region, along the east coast of Asia.
Sankar-Rao, M. and S. V. Sachidananda (1987). "On global summer monsoonal drought mechanics." Proceedings of the Indian Academy of Sciences (Earth and Planetary Sciences), Bangalore 96(2): 147-167.
It is shown that, within the framework of a linear, five-level, quasi-geostrophic steady-state global model, the middle-latitude systems can always have significant influence upon the Asian summer monsoon system through the lower tropospheric monsoon westerly window region near 80 degrees E. It is hypothesized that the quasi stationarity of the middle-latitude long-wave systems results in stronger teleconnections through this window, and the consequent monsoon breaks when the phase is right.
Sastry, P. S. N. (1989). "Agroclimatic analysis for semi-arid climatic conditions." Reifsnyder, W. E. and Darnhofer, T. O.
Since development of Koppen's climatic classification, several indices have been used to delineate climatically homogeneous regions and some of them have been applied to agroclimatic classification in several countries. The delineated regional patterns, whether based on rainfall or climatic water balance which includes the soil factor, do not generally differ from one another and one technique can be considered as good as the other. Semi-arid regions like the Indian subcontinent being characterized by both spatial and temporal variability and advective conditions during the break monsoon periods, the effect of climate on different phenophases assumes importance. The need for a phenology-based analysis of rainfall probability for agroclimatic purposes for major crops is brought out.
Sastry, P. S. N. and N. V. K. Chakravarty (1984). "Assessment of atmospheric drought during monsoon cropping season." Mausam, New Delhi 35(3): 267-272.
During break periods in the southwest monsoon season in the semiarid environment of India, crops under rainfed as well as irrigated conditions are prone to the effect of atmospheric drought as distinguished from soil or agricultural drought. This situation occurring at different stages of crop growth for a week or two is not reflected in the soil water balance, but is revealed in the crop growth rates and yields. Evidence exists of energy gain through advection by evaporating surfaces of the order of 4-5 mm /day resulting in an increase in potential evapotranspiration rates by 2-3 mm/day. This situation can be traced on a macroscale to the westerlies during the break periods and on a local scale to the postmonsoon warming of surroundings with a reduced intensity. Apart from the soil budgeting approach with the limitation mentioned above, there appears to be no method by which years with widely varying amounts of total seasonal rainfall but different distribution patterns in the different growth stages, resulting in atmospheric drought, could be distinguished from one another or grouped together. This paper, while focusing attention on this aspect, illustrates an empirical technique based on sequential dry days for assessment of atmospheric drought.
Sathiyamoorthy, V., K. Mohankumar, et al. (2002). "Interannual variability of total ozone and its relation with the Asia Pacific Wave." Tellus. Series B: Chemical and Physical Meteorology 54B(3): 269-277.
Analysis of the NCEP/NCAR reanalysis wind data shows the presence of a stationary Rossby wave in the lower stratosphere during May. This wave is seen prominently below 70 hPa level, confined between 10 degree N and 50 degree N latitudes and has a zonal wave number of 6 or 7. It is an extension into the stratosphere of the Asia Pacific Wave (APW) of the troposphere. As in the troposphere, in the lower stratosphere this wave shows a phase shift of 20 degree longitude between deficient and excess Indian summer monsoon rainfall (ISMR) years. This wave has maximum amplitude at about 200 hPa. The amplitude of the wave decreases both above and below 200 hPa level. The large-amplitude portion of this wave is thus situated in the break region between the tropical and extratropical tropopauses around 30 degree N latitude. It is suggested that this large-amplitude APW exchanges the tropical and extratropical airmasses through the tropopause break, making the APW signature seen in the satellite monitored total ozone (TOMS data). APW is found to exist in the following monsoon season (June to September) with the same phase as in May and its signature is also seen in that season in total ozone.
Schmidt, F. (2000). "On normal mode aided modelling: application to Asian Monsoon." Meteorology and Atmospheric Physics, Vienna, Austria 73(3-4): 189-210.
A new concept of utilizing generalized normal mode initialization (NMI) for modelling is investigated. This NMI-balancing adapts real data to model solutions. For eigen-modes of typical (climatological) structure and a simple 1-or more-layer shallow water model, anomalous flow of special episodes or events can be identified with mode groups giving distinctive signals. If these signals are part of the model solution or of the complement of the data to the model solution and how this is changed by introducing additional physical mechanisms to the model, gives hints for improvement. In applying this to multi-scale interactions of Asian monsoon, in particular a seventeen-years background knowledge and the anomalous flow of years 1991, 1977, 1978 we have a challenging test frame. Particular topics are: (1) Mei Yu rains in China, the representation of particular aspects of them by certain mode groups, the degree of independence of the corresponding signals on other modes, and the role of dissipation; (2) detection of El Nino-Southern Oscillation (ENSO) in atmospheric (reanalysis) data minus the NMI-balanced model solution; (3) Madden-Julian Oscillations (MJO) and the introduction of diabatic processes, and finally (4) aspects of dynamic stability. From among the results which are new or we simply have become aware of we pull out that atmospheric processes generally can be represented by modes or mode groups neither straight forward nor uniquely. Introduction of some dissipation schemes to the model was not helpful but rather destroyed the realistic structure of atmospheric dynamics. A parameterization of diabatic processes passes signals present in the data but not in the NMI-balanced model solution through to such being part of the latter, in case of MJO but not yet in case of ENSO. Ocean dynamics proves to be indispensable. Nevertheless there are imprints of essential atmospheric processes hidden on certain (e.g., most unstable) modes and partially recovered by NMI-balancing.
Schubert, S. D. and M. L. Wu (2001). "Predictability of the 1997 and 1998 South Asian Summer Monsoon Low-Level Winds." Journal of Climate 14(15): 3173-3191.
The predictability of the 1997 and 1998 south Asian summer monsoon winds is examined from an ensemble of 10 atmospheric general circulation model simulations with prescribed sea surface temperatures (SSTs) and soil moisture. The simulations have no memory of atmospheric initial conditions for the periods of interest. The model simulations show that the 1998 monsoon is considerably more predictable than the 1997 monsoon. During May and June of 1998 the predictability of the low-level wind anomalies is largely associated with a local response to anomalously warm Indian Ocean SSTs. Predictability increases late in the season (July and August) as a result of the strengthening of the anomalous Walker circulation and the associated development of easterly low-level wind anomalies that extend westward across India and the Arabian Sea. During these months the model is also the most skillful, with the analyses showing a similar late-season westward extension of the easterly wind anomalies. The model shows little predictability or skill in the monthly mean low-level winds over Southeast Asia during 1997. Predictable wind anomalies do occur over the western Indian Ocean and Indonesia; however, over the Indian Ocean the predictability is artificial, because the model is responding to SST anomalies that were wind driven. The reduced predictability in the low-level winds during 1997 appears to be the result of a weaker (as compared with 1998) simulated anomalous Walker circulation, and the reduced skill is associated with pronounced intraseasonal activity that is not captured well by the model. It is remarkable that the model does produce an ensemble mean Madden-Julian oscillation (MJO) response, though it is approximately in quadrature with, and much weaker than, the observed MJO anomalies during 1997.
Sengupta, D., B. N. Goswami, et al. (2001). "Coherent Intraseasonal Oscillations of Ocean and Atmosphere during the Asian Summer Monsoon." Geophysical Research Letters 28(21): 4127-4130.
Shaik, H. A. and P. W. Bate (2000). "The tropical circulation in the Australian/Asian region: May to October 1999." Australian Meteorological Magazine, Canberra, Australia 49(1): 59-69.
A summary of the broadscale tropical circulation from 70 degrees E to 180 degrees for the six months May to October 1999 is presented. By the start of this summary period there were indications that La Nina conditions were giving way to a neutral ENSO phase (e.g. weaker cool sea-surface temperature anomalies in the equatorial Pacific and a lower southern oscillation index). However, this was short-lived and a return to La Nina conditions was subsequently observed. Evidence for this included below average atmospheric pressure and above average convection over the western half of the region and positive values of the southern oscillation index, as well as a prolonged active summer monsoon over Asia. Four major active phases of the 30 to 60-day intraseasonal oscillation were observed, with periodicity apparently fluctuating between about 35 and 55 days. A total of 23 tropical cyclones developed during the period, close to the mean for the area, and most of them in the northwestern Pacific.
Shaik, H. A. and P. W. Bate (2000). "The tropical circulation in the Australian/Asian region: November 1999 to April 2000." Australian Meteorological Magazine, Canberra, Australia 49(4): 331-342.
A summary of the broadscale tropical circulation from 70 degrees E to 180 degrees , for the six months November 1999 to April 2000, is presented. Weak to moderate La Nina conditions persisted throughout the season. Evidence fo this includes continuation of an enhanced upward branch of the Walker circulation over the Australian region, below average atmospheric pressure and above average convection over the western half of the region, positive values of the Southern Oscillation Index, a weak and narrow zone of cool sea-surface temperatures in the equatorial eastern Pacific, weak warm anomalies in the northwestern Pacific and northern Australia, as well as an active monsoon over Australia. Four major active phases of the 30 to 60-day intraseasonal oscillation were observed, with periodicity apparently fluctuating between about 45 and 55 days. A total of 22 tropical cyclones developed during the period, less than the mean for the area. Of these, only two cyclones developed in the northwestern Pacific and none in the north Indian Ocean. The onset of the southern monsoon took place during the third week of December, close to the long-term mean over northern Australia, and the monsoon was active till late April. Rainfall over northern Australia was well above average, mainly due to the active monsoon over the region.
Shaik, H. A. and P. W. Bate (2001). "The tropical circulation in the Australian/Asian region - May to October 2000." Australian Meteorological Magazine 50(2): 137-147.
A summary of the broadscale tropical circulation from 70 degree E to 180 degree , for the six months May to October 2000, is presented. Weak La Nina conditions persisted throughout the season. Evidence for this includes continuation of an enhanced upward branch of the Walker circulation over the Australian region, below average atmospheric pressure and above average convection over the summer hemisphere of the region, near-neutral values of the southern oscillation index, a weak and narrow zone of cool sea-surface temperatures in the equatorial eastern Pacific, weak warm sea-surface temperature anomalies in the northwestern Pacific and northern Australia, as well as an active monsoon over Asia. Four major active phases of the 30 to 60-day intraseasonal oscillation were observed, with periodicity apparently fluctuating between about 30 and 50 days. A total of 22 tropical cyclones developed during the period, less than the mean for the area. No cyclones formed in the south Pacific and south Indian Ocean (mean 2.8) for the season.
Sham, P. and C. P. Chang (1990). "International Conference on East Asia and Western Pacific Meteorology and Climate, Hong Kong, 6-8 July 1989." Teaneck, NJ, World Scientific Publishing Co 572.
The papers presented at this conference discussed 1) monsoon meteorology, including thermal structure and convective activities over the Tibetan Plateau, the Mei-yu season in Taiwan, lee cyclogenesis over the Yun-Gui Plateau, and dynamic aspects of the equatorial intraseasonal oscillations; 2) remote sensing and in situ measurements, including the Chinese polar orbiting satellite FY-1, VHF radar to improve satellite temperature sounding in the tropopause, dual Doppler radar weather observations and aviation weather services, tipping-bucket raingages, and the use of S-VISSR satellite imagery for operational forecasting in Hong Kong; 3) climate and general circulation, including the impact of urbanization in Hong Kong and its implications for human energy exchange and the Heihe Basin Field Experiment (HEIFE), an atmosphere-land surface interaction program; 4) mesoscale meteorology, including theory and data analysis of the microphysics of a Mei-yu case, the frontal cyclone system in southern China and Taiwan during late winter and early spring, and baroclinic instability of modified Eady waves; 5) air-sea interaction studies; 6) operational weather forecast and numerical weather prediction, including the long-range rainfall forecast methods used by the Hong Kong Royal Observatory, an overview of present typhoon forecast operations in Taiwan, and the operational global forecast system at the Central Weather Bureau in Taiwan; 7) theoretical studies on nonlinear interaction of internal waves and turbulence, multiple equilibria of a thermally forced baroclinic atmosphere, and the nature of regional cyclogenesis; and 8) typhoon studies, including typhoon formation and development.
Sharma, O. P. (1992). "Initialization of humidity in numerical weather prediction models." Sikka, D. R. and Singh, S. S.
The effect of humidity is included among the initial conditions of numerical weather prediction modeling. In order to determine to what extent the improvement in the humidity analysis and cloud-radiation interaction processes might contribute to further refinements in the forecast of monsoon on the planetary scale, a series of numerical experiments was designed to assess the impact of humidity modification/enhancement, with a control experiment being the 10-day numerical forecast tests starting on July 27, 1979, 12 GMT as the initial conditions. These conditions represented a situation of active monsoon after a break over India. Attention is given to ground temperature, threshold relative humidites, precipitation, radiative properties of clouds and the surface, the retrieval of vertical temperature and moisture profiles from remote regions using geostationary satellite data, and the use of an iterative procedure to calculate humidity enhancement. It is found that humidity modification does improve both the simulation of global tropical circulation, and the prediction of typhoon position and the associated circulation over the region of 20 degrees N, 120 degrees E, which are strongly coupled with the global tropical circulation.
Sheng, J. (1995). "The Madden-Julian oscillation in the Canadian Climate Centre general circulation model." Climate Dynamics, Berlin, Germany 12(2): 125-140.
The Madden-Julian oscillation (MJO) simulated by the Canadian Climate Centre general circulation model (CCC GCM) is identified by a principal oscillation pattern (POP) analysis and compared with that observed in the real atmosphere. The results are based upon two integrations of the CCC GCM, one with a parameterization of penetrative cumulus convection (EXP1) and the other with a moist convective adjustment scheme (EXP2). The signal of MJO can be detected in both integrations as the first POP of the 200 hPa velocity potential along the equator. The disturbances show a distinctive wave number one structure with the strongest local amplitude found in the longitudes corresponding to the region of the Asian monsoon. The phase speed of the eastward wave propagation is higher in the eastern Pacific and lower in the monsoon region where the convective activities are strongest. These features are in good agreement with the observations. The energy spectrum of the velocity potential peaks at the frequency corresponding to a period of about 38 days for EXP1, which is somewhat shorter compared to the observed periods of 40-50 days. On the other hand, two spectral peaks can be clearly identified for EXP2, one with a period of 24 days and the other with a much longer period, somewhere near 112 days. Both peaks appear statistically significant at 95% level. Long term data of the observed atmosphere show little indication of such spectral separation. The horizontal patterns identified by the POP analysis resemble to some extent the baroclinic response of tropical flow to a heat source travelling with the speed of MJO. At the upper level, Rossby wave energy propagates westward with winds generally following the height contours, whereas Kelvin wave energy propagates to the east from the heat source with strong cross-contour flow near the equator. At the lower level, the patterns are essentially reversed. The model-generated precipitation and diabatic heating are examined by compositing against the moving MJO. It is found in EXP2 that the composite heating distribution is coherent with the flow pattern only in a certain sector of the equator, depending on whether the fast or slow mode is used to determine the reference point. The composite vertical heating profile of a slower mode tends to have a maximum found at a lower level. The sensitivity of simulated MJO to the cumulus convection scheme in the model is discussed.
Shepherd, I. J. and P. W. Bate (1997). "The South Pacific and southeast Indian Ocean tropical cyclone season 1994-95." Australian Meteorological Magazine, Canberra, Australia 46(2): 143-151.
Eight tropical cyclones formed in the South Pacific and southeast Indian Ocean during the 1994-95 season, well below the long-term averages for the two basins. The 1994-95 summer monsoon was generally less well developed than normal as El Nino conditions peaked during early summer then dissipated during 1995. Above average sea-surface temperatures across most of the tropical Pacific contributed to above average tropical convection east of the date-line early in the season. The distribution of cyclone genesis was linked with active phases of the intraseasonal oscillation in most cases. Six of the season's cyclones formed in Australian longitudes (115 degrees E-160 degrees E) and two in the South Pacific, one well east of the date-line.
Shrestha, M. L. and T. Murakami (1988). "Intraseasonal fluctuations in low-level meridional winds over the Indian Ocean and monsoonal convection over South Asia." Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm 40(2): 120-132.
Some aspects of 30-60-day meridional wind fluctuations over the midlatitude Indian Ocean were investigated by using lag correlation maps for the five summers of 1979-1983. At times, the equatorial low-frequency (30-60-day) modes exhibit a systematic eastward propagation. However, they are occasionally interrupted by irregular movements or even westward propagation. This study concentrates only on those periods with a systematic eastward propagation and a well-defined tropical-extratropical atmospheric teleconnection pattern. Low-level meridional wind fluctuations in the low-frequency domain are prominent over the western Indian Ocean off the east coast of Africa. The anomalous southerly flows, which are initiated around northern Madagascar, move northward, and after crossing the Equator, contribute to the intensification of the monsoonal westerlies over South Asia, thus enhancing convection over the monsoon region. In comparison, 30-60-day southerlies (anomaly) over the eastern Indian Ocean off the west coast of Australia tend to enhance equatorial convection over the western Pacific between similar to 150 degrees E and the date line. Perturbations with periods <30 days are considered transient disturbances. Spectral analysis of transient eddy kinetic energy in the extratropical regions reveals a significant peak in the low-frequency range. This is associated with an amplitude modulation of transient disturbances. Thus, it is plausible that transient disturbances, as a group, provide energy for the enhancement of 30-60-day planetary-scale perturbations over the subtropics.
Shukla, J. (1989). "Predictability of weather and climate: long-range forecasting of Indian monsoons." American Association for the Advancement of Science.
Space-time variability of monsoon rainfall over India is described, and possible mechanisms for variability on intraseasonal and interannual time scales are suggested. Large-scale persistent anomalies of monsoon rainfall over India are associated with anomalies in planetary scale circulation and boundary conditions, and these relationships are sufficiently strong to be useful in predicting seasonal rainfall.
Shyamala, B. and G. M. Shinde (1999). "Study of synoptic systems associated with intraseasonal variability of summer monsoon -- A New Perspective." Mausam 50(1): 31-36.
An attempt has been made in this paper to identify the important synoptic situations that result in widespread rainfall activity in Maharashtra and Gujarat based on latest observational technology and develop forecasting techniques for day to day short range prediction of monsoon activity in these areas with special reference to Monsoon'96.
Sikka, D. R. and S. Gadgil (1978). "Large-scale rainfall over India during the summer monsoon and its relation to the lower and upper tropospheric vorticity." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 219-231.
The daily variation of the lower and upper tropospheric wind field for the monsoon seasons of 1972 and 1973 is investigated. The changes in the large-scale rainfall are directly related to those in the large-scale vorticity above the frictional boundary layer on a day-to-day basis throughout the two seasons; large cyclonic vorticity is associated with active spells and anticyclonic vorticity with weak monsoon spells. In the upper troposphere, the regional average of the anticyclonic vorticity is generally found to increase in active periods, decrease slightly in breaks and spectacularly toward the end of the break, and increase again during revival. The maximum value of the kinetic energy of the easterlies during a season is found to occur during the break or weak monsoon spell for the seasons studied. The major difference in the upper tropospheric circulation of the two seasons is in the meridional component, which is southerly over large regions over India in 1972 and northerly over the entire region on most days in the season of 1973. The contribution of the meridional advection of vorticity in determining the upper level divergence is significant, and its variations are correlated to those of the lower-level convergence.
Sikka, D. R. and S. Gadgil (1980). "On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon." Monthly Weather Review, Boston 108(11): 1840-1853.
An investigation is presented of the daily variation of the maximum cloud zone (MCZ) and the 700-mb trough in the Northern Hemisphere over the Indian longitudes 70-90 degrees E during April-Oct. for 1973-1977. It is found that during June-September, there are two favorable locations for a MCZ over these longitudes--on a majority of days, the MCZ is present in the monsoon zone north of 15 degrees N, and a secondary MCZ often occurs in the equatorial region (0-10 degrees N). The monsoon MCZ becomes established by northward movement of the MCZ occurring over the equatorial Indian Ocean in April and May. The secondary MCZ appears intermittently and is characterized by long spells of persistence only when the monsoon MCZ is absent. In each of the seasons studied, the MCZ temporarily disappeared from the mean summer monsoon location (15-28 degrees N) approximately four weeks after it was established near the beginning of July. It is reestablished by the northward movement of the secondary MCZ, which becomes active during the absence of the monsoon MCZ, in a manner strikingly similar to that observed in the spring-to-summer transition. A break in monsoon conditions prevails just prior to the temporary disappearance of the monsoon MCZ. Thus, it is concluded that the monsoon MCZ cannot survive for longer than one month without reestablishment by the secondary MCZ. Possible underlying mechanisms are discussed.
Singh, G. (1973). "Long dry spells of short return periods during southwest monsoon along the Konkan coast." Indian Journal of Meteorology and Geophysics, Delhi 24(1): 35-36.
Singh, N., M. K. Soman, et al. (1988). "Hydroclimatic fluctuations of the Upper Narmada catchment and its association with break-monsoon days over India." Proceedings of the Indian Academy of Sciences (Earth and Planetary Sciences), Bangalore 97(1): 87-105.
Hydroclimatic fluctuations of the Upper Narmada catchment (up to Narmadasagar dam site) have been studied by examining the time series (1901-1980) of 1) 1-10-day annual extreme rainfall; 2) seasonal total rainfall between May and Oct.; 3) the precipitation concentration index (PCI); 4) a modified version of PCI(MPCI): and 5) parameters of the periods contributing specified percentages of rainfall to the annual total. Most of these parameters followed the normal distribution and did not show any significant long-term trend. However, some dominant long-period oscillations have been noticed in extreme rainfall, seasonal rainfall, PCI, and MPCI series. Influence of break-monsoon days over India during July and Aug. on the rainfall activities of the Upper Narmada catchment has also been investigated, and salient findings are discussed.
Singh, S. V. and R. H. Kripalani (1990). "Low frequency intraseasonal oscillations in Indian rainfall and outgoing longwave radiation." Mausam, New Delhi, India 41(2): 217-222.
In this study, the interannual variations of the period and intensity of 30-40 days oscillations in monsoon rainfall are examined by utilising daily rainfall data of 290 stations for a period of 80 years. Their relationship with the overall performance of the monsoon and ENSO phenomenon is also examined. Potential of these oscillations of about 40 days period in extended range forecasting of rainfall is examined by computing the cross-correlations between the rainfall and the OLR over certain key regions and the rainfall distribution over the whole country in subsequent few pentads. Finally, concurrent correlations between the 5-day OLR and rainfall are also studied.
Singh, S. V., R. H. Kripalani, et al. (1992). "Interannual variability of the Madden-Julian oscillations in Indian summer monsoon rainfall." Journal of Climate, Boston, MA 5(9): 973-978.
The Madden-Julian oscillations are quite prominent over the Indian monsoon region and they are related with the large-scale active-break phases of the monsoon. These oscillations, however, show considerable interannual variability in period and intensity. In this study interannual variability of these oscillations has been studied by using daily rainfall data of 365 stations for 80 years (1901-80). It is found that the intensity of these oscillations is not related with the overall performance of the monsoon and the El Nino-Southern Oscillation phenomenon.
Smith, E. A., K. W. Oh, et al. (1989). "PC-based interactive imaging system designed for INSAT data analysis and monsoon studies." American Meteorological Society, Boston, Bulletin 70(9): 1105-1122.
A PC-based interactive image processing system has been developed for aiding the analysis of Indian Geosynchronous Satellite (INSAT) data for Asian monsoon studies. In view of its diminutive stature, the system has been given the name MIDGET (micro-based image display and graphics enhancement tool). Various analysis procedures involving INSAT data and other monsoon datasets are described in conjunction with the MIDGET system. These include the use of the system for monitoring monsoon evolution and the evolution and the behavior of organized tropical storms, analysis of low-frequency intraseasonal oscillations, diagnostic studies of cloudiness, retrieval of monsoon precipitation and its relationship to satellite cloudiness, and statistical prediction of monsoon cloud bands associated with low-frequency intraseasonal fluctuations of monsoon rainfall. The system is designed as a workstation terminal in supercomputer environment. The basic virtue of this system is that it is an inexpensive approach for generating a high-resolution video time-lapse of large satellite datasets in an interactive environment familiar to PC users. The motivation for developing this system derives from a new source of Indian satellite data that has been made available to U.S. scientists through archive holdings at the National Center for Atmospheric Research.
Smith, E. A. and D. R. Sikka (1987). "Coherence of satellite infrared temperatures with monsoon rainfall at preferred frequencies and the triplex behavior of the Indian summer monsoon." Meteorology and Atmospheric Physics, Vienna 37(4): 219-236.
The behavior of the Indian summer monsoon during the period 1979-1985 is examined with surface rainfall and IR satellite data to determine how well the satellite measurements mimic the episodic rainfall processes. It is shown that equivalent black-body temperatures derived from satellite-measured outgoing long-wave radiation (OLR) estimates are reliable indicators for reproducing the time-variant zonal structure of monsoon rainfall over the Indian subcontinent, but only at preferred frequencies. The Indian summer monsoon is found to exhibit a distinct three-episode cycle of active-break periods along two N-S aligned cross sections; the first, along the west Indian coast, the second, through central India up to the plains of west Uttar Pradesh. The authors use the triplex behavior of the monsoon as a framework to describe individual monsoons from 1979 to 1985. This is done in terms of the initiation or phase, amplitude, duration, and propagation of the individual episodes. Cospectrum calculations between the rainfall and satellite temperature show that significant coherence is only associated with the frequencies corresponding to specific subseasonal fluctuating modes of the monsoon: 30-50- and 10-20-day modes. The 30-50-day mode exhibits particularly strong coherence. It is shown how the behavior of the rainfall normals can be used to aid the calculation of a synthetic satellite temperature normal. Coherence at the 30-50-day mode in the cospectrum of the departure time series is also strong; coherence at the 10-20-day mode is weaker, but significant. This suggests that, although satellite-derived temperature is not a universal for rainfall, it could be used as a variable for monitoring the intra-annual behavior of the fluctuating rainfall modes of the monsoon.
So, C. H. and J. C. L. Chan (1997). "Regional and synoptic-scale features associated with inactive periods of the summer monsoon over South China." Advances in Atmospheric Sciences, Beijing, China 14(2): 223-230.
This paper presents an observation study of the physical processes responsible for the inactive period (break) of the summer monsoon over South China (SC). The break of the monsoon is defined by using the rainfall data over Hong Kong Meteorological parameters provided by the European Center for Medium-Range Weather Forecasts (ECMWF) for the period 1985-1990 are examined. Daily values of each parameter for the six years are then composited each day for the period of 5 days before to 1 day after the break. It is found that several days before the break, changes opposite to those occurred during the onset and active periods begin to take place. This suggests that a feedback mechanism is present which tends to restore the atmosphere to a more stable state. This mechanism may be initiated by the formation of convective clouds during the onset and active periods. These clouds then reduce the solar radiation to the ground, leading to a gradual drop in the temperature. This drop, together with the cooling of the atmosphere due to the large amounts of rainfall, causes the pressure over the SC region to become higher, which in turn induces a westward extension of the subtropical ridge. The decrease in temperature over SC may also shift the location of the heat source to the west, which leads to a concomitant westward shift of the convergence of the southerlies and results in less moisture-laden air reaching the SC region. The atmosphere then becomes unfavourable for heavy convection and therefore a break starts.
Song, Y. and R. Huang (1995). "The dynamical effects of divergent wind on the intraseasonal variability of the East Asian circulation." Advances in Atmospheric Sciences, Beijing, China 12(3): 259-272.
In this paper, the dynamical effects of divergent wind on the intraseasonal variability of atmospheric circulation over East Asia are analyzed by using the function of Rossby-wave source and the energy exchanging function between divergent component and rotational component of the flow. The results analyzed from the observed data show that the advection of vorticity by divergent wind caused by the heating due to the monsoon rainfall in the south to the Yangtze River and the strong convective activities around the Philippines may play an important role in the northward jump of westerly jet stream during the seasonal transition from spring circulation to summer circulation over East Asia. Due to the northward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over the Yellow River basin and northwest China and will cause the intensification of the zonal flow there. Thus, the jet stream abruptly shifts northward to North China. Moreover, the analysed results also show that the advection of vorticity by divergent wind caused by the heating due to the strong convective activities around the Philippines also plays an important role in the intraseasonal variability of the circulation over East Asia during the seasonal transition from summer to winter. With the southward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over East Asia, especially over the Yangtze-Huaihe River basin. Therefore, the jet stream gradually moves southward from North China to the Yangtze River basin.
Sontakke, A., J. Shea, et al. (2001). "Potential for long-range regional precipitation prediction over India." Mausam 52(1): 47-56.
The potential for long-range precipitation prediction over the Indian monsoon region is generally good where climate noise (i.e. variability due to daily weather fluctuations) is small as compared to the climate signal (i.e. variability due to year to year fluctuations in monthly/seasonal means) being in the tropical belt. In order to understand the potential on smaller spatial seales, the ratios of interannual variability to that associated with climate noise have been computed for precipitation of four seasons as well as SW monsoon sub-seasons/months over 1656 stations in the Indian subcontinent. Precipitation in SW monsoon has been found potentially predictable on seasonal as well as intraseasonal scale. The west coast and contiguous northwest India, part of the 'northeast India are more predictable. Potential for long-range prediction over northwest India is highest during the active monsoon period from July to September. Over eastern peninsula potential for prediction is generally found low whereas over northcentral India it is always moderate. Over northern latitudes precipitation due to western disturbances during January to May is potentially predictable. Precipitation over southeast India and Sri Lanka during October to February due to northeast (NE) monsoon shows good potential for long-range prediction. It is manifested that long-range precipitation forecasting schemes for SW monsoon season, subseasons and months and for the other seasons over India on point to regional scale have good scope by taking into account the potential predictability at the individual stations as well as at contiguous resemblance areas over the country.
Sperber, K. R. and T. N. Palmer (1996). "Interannual tropical rainfall variability in General Circulation Model simulations associated with the Atmospheric Model Intercomparison Project." Journal of Climate, Boston, MA 9(11): 2727-2750.
The interannual variability of rainfall over the Indian subcontinent, the African Sahel, and the Nordeste region of Brazil have been evaluated in 32 models for the period 1979-88 as part of the Atmospheric Model Intercomparison Project (AMIP). The interannual variations of Nordeste rainfall are the most readily captured, owing to the intimate link with Pacific and Atlantic sea surface temperatures. The precipitation variations over India and the Sahel are less well simulated. Additionally, an Indian monsoon wind shear index was calculated for each model. Evaluation of the interannual variability of a wind shear index over the summer monsoon region indicates that the models exhibit greater fidelity in capturing the large-scale dynamic fluctuations than the regional-scale rainfall variations. A rainfall/SST teleconnection quality control was used to objectively stratify model performance. Skill scores improved for those models that qualitatively simulated the observed rainfall /El Nino-Southern Oscillation SST correlation pattern. This subset of models also had a rainfall climatology that was in better agreement with observations, indicating a link between systematic model error and the ability to simulate interannual variations. A suite of six European Centre for Medium-Range Weather Forecasts (ECMWF) AMIP runs (differing only in their initial conditions) have also been examined. As observed, all-India rainfall was enhanced in 1988 relative to 1987 in each of these realizations. All-India rainfall variability during other years showed little or no predictability, possibly due to internal chaotic dynamics associated with intraseasonal monsoon fluctuations and/or unpredictable land surface process interactions. The interannual variations of Nordeste rainfall were best represented. The State University of New York at Albany/National Center for Atmospheric Research Genesis model was run in five initial condition realizations. In this model, the Nordeste rainfall variability was also best reproduced. However, for all regions the skill was less than that of the ECMWF model. The relationships of the all-India and Sahel rainfall/SST teleconnections with horizontal resolution, convection scheme closure, and numerics have been evaluated. Models with resolution greater than or equal to T42 performed more poorly than lower-resolution models. The higher resolution models were predominantly spectral. At low resolution, spectral versus gridpoint numerics performed with nearly equal verisimilitude. At low resolution, moisture convergence closure was slightly more preferable than other convective closure techniques. At high resolution, the models that used moisture convergence closure performed very poorly, suggesting that moisture convergence may be problematic for models with horizontal resolution greater than or equal to T42.
Sperber, K. R., J. M. Slingo, et al. (2000). "Predictability and the relationship between subseasonal and interannual variability during the Asian summer monsoon." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 126(568, Pt. C): 2545-2574.
The relationship between subseasonal and interannual variability of the Asian summer monsoon has been investigated through analysis of the dominant modes of variability in the 40-year NCEP/NCAR Re-analysis, with complementary satellite and surface-based precipitation data. The hypothesis that the characteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual time-scales in a systematic and therefore predictable manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires that the effects of the slowly varying components of the climate system be correctly simulated. An interannual mode of monsoon variability has been identified which is closely related to the observed seasonal mean all-India Rainfall (AIR). A counterpart of this mode has also been identified at subseasonal time-scales which projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on sub-seasonal and interannual time-scales. It has been shown that the temporal behaviour of this subseasonal mode, as described by the probability distribution function (PDF) of the principal component time series, does not show any evidence of bimodality, the shape of the PDF being Gaussian. Further, it has been shown that the mean of the PDF is systematically and significantly perturbed towards negative (positive) values in weak (strong) monsoon years as categorized in terms of the seasonal mean AIR. This translation in the mean of the PDF, rather than a change in shape of the PDF, suggests that anomalous monsoons are associated with changes in weather regimes. Further analysis has confirmed that low-frequency modulation of the basic state is primarily responsible for these shifts in the subseasonal PDFs, supporting the null hypothesis that predictability of the seasonal mean monsoon requires that the effects of the slowly varying components of the climate system be correctly simulated. Thus, model improvements to reduce systematic errors in the mean simulation and the response to low-frequency boundary forcing may improve the prospects for dynamical seasonal prediction. However, the results indicate that only a subset of the subseasonal modes are systematically perturbed either by the El Nino Southern Oscillation or in weak vs. strong monsoon years, suggesting that predictability is likely to be limited by the chaotic, internal variability of the monsoon system.
Srinivasan, J., S. Gadgil, et al. (1993). "Meridional propagation of large-scale monsoon convective zones." Meteorology and Atmospheric Physics, Vienna, Austria 52(1-2): 15-35.
Observational studies indicate that the convective activity of the monsoon systems undergo intraseasonal variations with multi-week time scales. The zone of maximum monsoon convection exhibits substantial transient behavior with successive propagating from the north Indian Ocean to the heated continent. Over South Asia the zone achieves its maximum intensity. These propagations may extend over 3000 km in latitude and perhaps twice the distance in longitude and remain as coherent entities for periods greater than 2-3 weeks. Attempts to explain this phenomena using simple ocean-atmosphere models of the monsoon system had concluded that the interactive ground hydrology so modifies the total heating of the atmosphere that a steady state solution is not possible, thus promoting lateral propagation. That is, the ground hydrology forces the total heating of the atmosphere and the vertical velcoity to be slightly out of phase, causing a migration of the convection towards the region of maximum heating. Whereas the lateral scale of the variations produced by the Webster (1983) model were essentially correct, they occurred at twice the frequency of the observed events and were formed near the coastal margin, rather than over the ocean. Webster's (1983) model used to pose the theories was deficient in a number of aspects. Particularly, both the ground moisture content and the thermal inertia of the model were severely underestimated. At the same time, the sea surface temperatures produced by the model between the equator and the model's land-sea boundary were far too cool. Both the atmosphere and the ocean model were modified to include a better hydrological cycle and ocean structure. The convective events produced by the modified model possessed the observed frequency and were gathered well south of the coastline. The improved simulation of monsoon variability allowed the hydrological cycle feedback to be generalized. It was found that monsoon variability was constrained to lie within the bounds of a positive gradient of a convective intensity potential (I). the function depends primarily on the surface temperature, the availability of moisture and the stability of the lower atmosphere which varies very slowly on the time scale of months. The oscillations of the monsoon perturb the mean convective intensity potential causing local enhancements of the gradient. These perturbations are caused by the hydrological feedbacks, discussed above, or by the modification of the air-sea fluxes caused by variations of the low-level wind during convective events. The final result is the slow northward propagation of convection within an even slower convective regime. The ECMWF analyses show very similar behavior of the convective intensity potential. Although it is considered premature to use the model to conduct simulations of the African monsoon system, the ECMWF analysis indicate similar behavior in the convective intensity potential suggesting, at least, that the same processes control the low frequency structure of the African monsoon. The implications of the hypotheses on numerical weather prediction of monsoon phenomenon are discussed.
Stendel, M. and E. Roeckner (1998). "Impacts of horizontal resolution on simulated climate statistics in ECHAM 4." Hamburg, Germany, Max Planck Institut fuer Meteorologie.
The sensitivity of a general circulation model to changes in resolution is studied using the Max Planck Institute for Meteorology (MPI) 19-level model, ECHAM4. Simulations extending over a period between 10 and 15 years, with observed sea surface temperatures as lower boundary conditions from 1979 onward, have been performed using four different horizontal resolutions, T21, T30, T42 and T106. The atmospheric time-mean state and the intraseasonal variability are compared to the European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses and a few other observational datasets. The T30, T42 and T106 simulations are similar in many respects, whereas the T21 simulation is qualitatively different. Several effects related to model resolution could be identified, such as increasing tropical upper tropospheric warming with increasing resolution. This is due to more vigorous tropical convection, larger ice water content and, hence, increasing cirrus cloud greenhouse effect. Associated with this increasing warming at higher resolution is a poleward expansion of the zonal averaged circulation regime. On the other hand, the zonally asymmetric component of the circulation, i.e., the stationary waves, improve with higher resolution. Also, higher resolution has a positive impact on regional precipitation patterns which are affected by orography such as the summer monsoon precipitation over India. Intraseasonal variability has been analyzed only for the higher resolution models, T42 and T106. Compared to the ECMWF reanalyses, both models are able to simulate the intraseasonal geopotential height variability, eddy fluxes of heat and momentum, and eddy kinetic energy with reasonable accuracy. This applies to transient eddies in both the bandpass and lowpass regime and to the stationary eddies as well. Some biases can be identified which are more or less independent of resolution. These include the mislocation of the Azores high and the overestimation of its intensity, a cold bias in the polar upper troposphere and lower stratosphere and the poleward and upward displacement of the maxima of geopotential height variability, momentum fluxes and eddy kinetic energy. An important finding is that the operational ECMWF analyses, which have been widely used for model validation, considerably overestimate the lowpass variability, as compared to the reanalyses, due to frequent changes of the forecast model and data assimilation scheme. This implies that the results from our investigations are not directly comparable to previous investigations that used operational analyses for validation.
Stringer, R. K. (1995). "The tropical circulation in the Australian/Asian regionANovember 1993 to April 1993." Australian Meteorological Magazine, Canberra, Australia 44(4): 313-322.
A summary of the broadscale tropical circulation from 70 degrees E to the date-line, for November 1992 to April 1993, is presented. Warm ENSO conditions persisted through the season, and there was an eastwards shift of the upward branch of the Walker Circulation towards the date-line. Associated with this, rainfall across central and eastern parts of Australia was generally average to below average, and downmotion anomalies were evident over the southwest Pacific. The Asian northeast winter monsoon produced weaker than normal winds over east China and the South China Sea; however, the northeast trade winds extended strongly into the western Pacific during two periods. Each of these periods coincided with the eastward movement of a broadscale intraseasonal oscillation from the Indian Ocean to the western Pacific.
Subbaramayya, I. and O. Bhanu Kumar (1976). "On the fluctuations of the monsoon trough." Symposium on Tropical Monsoons, Poona, India, Sept.
The pentad mean positions of the monsoon trough between 75 and 85 degrees E during July and August of 1965-1975 were evaluated and studied. On the average, there was no trend in the location of the trough in the two months and no significant northward movement in the mid-Aug. period, contrary to the suggestion made by Ananthakrishnan earlier. The southward displacements of the trough were caused by the traveling lows from the east at relatively lower latitudes. The northward displacements were associated with western disturbances over northwest India or recurving tropical lows. It is suggested that the Tibetan anticyclone and the N-S trough that normally exists near 70 degrees E in the upper troposphere were displaced considerably to the east and, also, possibly to the south during periods of northerly situation of the monsoon trough. Apart from the 5-10-day period fluctuations, periodic variations of one month to more than two months were observed almost every year. The disastrous droughts in 1965 and 1972 resulted from the prolonged and intense break conditions in Aug. and July, respectively. The mean latitude of the monsoon trough for both July and August in different years showed a significant 11-yr cycle that was approximately in phase with the sun-spot cycle.
Subbaramayya, I., O. Bhanu Kumar, et al. (1981). "Summer monsoon rains over south and east Asia and [the] West Pacific." World Meteorological Organization, Geneva, WMO No 578: 316-333.
This comprehensive review of the characteristics of the summer monsoon rains over south and east Asia and the West Pacific covers the onset dates of monsoon rains over the regions considered; the associated features of the atmospheric circulation; the bai-u and shurin rain seasons of Japan; the withdrawal dates of the monsoon; the characteristics and movement of the intertropical convergence zone over Southeast Asia and its relationship to the onset and withdrawal of the summer monsoon; the break monsoon over India; and homogeneity of monsoon rains over the peninsulas of India, northeastern India, and northwestern India. The authors summarize their studies on the relationship of the onset of the southwest monsoon and the upper air changes over India in the 3-yr period 1968-1970 and the results of a preliminary study of the advance of the summer monsoon over India and of the bai-u rains over southern Japan in relation to the surface flow pattern developments and the ITCZ.
Subbaramayya, I. and M. Rao (1985). "On the vagaries of the Indian southwest monsoon." Mahasagar, Bulletin of the National Institute of Oceanography, Goa, India 18(2): 179-185.
A comprehensive review of the more recent work done on the vagaries of the southwest monsoon, with special emphasis on the break monsoon and interannual variations, is presented. Importance of the Northern Hemispheric middle latitude forcing for the failure of the monsoon is stressed, and a brief review of the Southern Oscillation and El Nino and of their effect on the Indian summer monsoon is also given.
Sui, C. H. and K. M. Lau (1992). "Multiscale phenomena in the tropical atmosphere over the Western Pacific." Monthly Weather Review, Boston, MA 120(3): 407-430.
Multiscale variabilities in the atmosphere over the tropical western Pacific during the 1979 Northern Hemisphere winter are studied with an aim at identifying possible interactions between phenomena of different spatial and temporal scales. Based on the convection-index information derived from satellite measurements, two intraseasonal oscillations (ISOs) are identified within the equatorial belt between 0 degrees and 10 degrees S in the analyzed period. The two ISOs, accompanied by both rotational and divergent circulations, propagate eastward from the Indian Ocean to the western Pacific. Over the warm pool in the western Pacific, the ISOs develop into quasi-stationary systems with an enhanced rotational circulation characterized by a strong westerly jet in the lower troposphere. The ISOs appear to interact with a number of regional- and synoptic-scale phenomena in the maritime continent and western Pacific region. For example, the onset of the monsoon coincides with the arrival of the first ISO at northern Australia region (140 degrees E) in late December. The passage of ISOs in the monsoon flow are also associated with surface westerly wind outbreaks. On shorter time scales (<10 days), the ISOs appear to provide a favorable condition over the warm ocean for the development of 2-4-day disturbances that further organize mesoscale cloud clusters. In addition, the diurnal cycle provides another important forcing mechanism modulating cloud clusters, particularly over the maritime continents. There appears to be an inverse relationship between diurnal cycle and intraseasonal disturbances ( greater than or equal to 10 days) over the maritime continent; that is, periods of active intraseasonal variabilities are characterized by diminished diurnal cycles and vice versa. The possible role of these multiscale processes in the coupled ocean-atmosphere system is discussed.
Sumathipala, W. L. and T. Murakami (1986). "Large-scale aspects of active and break monsoons over Southeast Asia during the 1979 summer." Papers in Meteorological Research, Taipei, Taiwan 9(2): 105-116.
Based upon outgoing long-wave radiation (OLR) and FGGE Level 3-b wind data, the active and break phases of the Asiatic summer monsoon are investigated. During the active phase, there is strong upper outflow emanating from the monsoonal source region. Associated with this is the intensification of the N-S (Hadley) overturnings with divergent winds > 5 m sec super(-) super(1) , as well as the E-W (Walker) vertical circulation with divergent winds > 2 m sec super(-) super(1) . Thus, the mean circulation during the active phase exhibits a zonal wavenumber 1 character. In contrast, the break phase is characterized by a typical wavenumber of 2 or 3. Westerly moisture fluxes are most prominent over the Arabian Sea and decrease eastward, resulting in moisture convergence over India and Burma. Over these regions, precipitation exceeds evaporation (P-E > 0) during active phase. Conversely, moisture fluxes diverge over India producing negative P-E during break phase. Concurrently, P-E increases over the eastern North Pacific; i.e., moisture fluctuations of the eastern North Pacific have a clear out-of-phase relation with the Asian monsoon. Transient eddy kinetic energy (k*) indicates pronounced disturbance activity in the Bay of Bengal and the western Pacific east of the Philippines during active phase. During break, k* increases over the eastern North Pacific where a large number of tropical storms develop.
Sumathipala, W. L. and T. Murakami (1988). "Intraseasonal fluctuations in low-level meridional winds over the south China Sea and the western Pacific and monsoonal convection over Indonesia and northern Australia." Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm 40(3): 205-219.
Outgoing long-wave radiation (OLR), objectively analyzed daily winds (u, v), and geopotential height data for five northern winters are used to study the relationship between low-level equatorward (meridional) winds and tropical convection on the 30-60-day time scale. The general tendency of equatorial 30-60-day OLR perturbations is to propagate eastward. Occasionally, these perturbations exhibit irregular movement. This study is limited to the periods when the eastward propagation is clearly defined. Correlation maps indicate that the low-frequency modes are of global-scale character with zonal wavenumber one. The low-level 30-60-day northerly wind along the eastern edge of the Siberian high (anomaly) does not contribute to convection and monsoon activity in the Indonesian-northern Australian region. However, 30-60-day northeasterly flows of subtropical Pacific origin play a significant role in monsoon activity. An active monsoon condition with strong upper level southerly outflow from the monsoon region and an intense westerly jet near Japan is observed similar to 10 days prior to the maximum low-level northerly currents from Siberia. Fluctuations with periods <30 days are considered as transient disturbances. The square of transient meridional winds shows pronounced spectral peaks in the 30-60-day-period range. This is associated with an amplitude modulation of synoptic-scale transient disturbances, which is most pronounced in both the northern and southern Hemisphere extratropics. Perhaps transient disturbances, as a group, can make a contribution toward the enhancement and/or maintenance of extratropical low-frequency perturbations.
Suppiah, R. (1997). "Extremes of the southern oscillation phenomenon and the rainfall of Sri Lanka." International Journal of Climatology, Chichester, UK 17(1): 87-101.
Influences of extreme phases of the Southern Oscillation (SO) phenomenon, El Nino and La Nina events, on the seasonal rainfall of Sri Lanka are examined by using composite maps of seasonal rainfall and sea-surface temperature (SST) anomalies. There were 27 El Nino and 22 La Nina events, during the period from 1881 to 1990. Positive and negative rainfall anomalies during the south-west monsoon (SWM) season are associated with La Nina and El Nino events, but negative and positive rainfall anomalies are linked to La Nina and El Nino events during the second intermonsoon (SIM) season. These contrasting patterns are dominant in the dry zone of Sri Lanka. Rainfall anomalies during first intermonsoon (FIM) and north-east monsoon (NEM) seasons do not show clear contrasting patterns as in other seasons and show positive and negative values. On the basis of wettest 20 per cent, mid-20 per cent and driest 20 per cent of years of seasonal total rainfall, composite maps of SST anomalies over the Pacific and Indian Oceans were made. As in rainfall patterns, SST anomalies during FIM and NEM seasons do not show clear contrasts between El Nino and La Nina events. During the SWM season, wet (dry) years are associated with negative (positive) SST anomalies over central and eastern Pacific and west Indian Oceans, but opposite SST anomalies are found over the `maritime continent'. During the SIM season, wet (dry) years are associated with positive (negative) SST anomalies over central Pacific and west Indian Oceans and opposite SST anomalies over the `maritime continent'. Based on the results of this study and previous studies on synoptic circulation patterns, and the dominance of the intraseasonal oscillation, a plausible explanation is given for larger anomalies during the SWM and SIM seasons in Sri Lanka.
Suppiah, R. and X. Wu (1998). "Surges, cross-equatorial flows and their links with the Australian summer monsoon circulation and rainfall." Australian Meteorological Magazine, Canberra, Australia 47(2): 113-130.
Links between wind surges from the South China Sea and Australian monsoon circulation features are analysed for twelve summers from 1980-81 to 1991-92 using European Centre for Medium Range Forecasts (ECMWF) data. Composite analyses and lagged correlation methods were used to investigate the link in circulation features between these two regions. During strong surge events, winds are stronger over the South China Sea and east Asia. Anticyclonic circulations occur over east China and to the southwest of Australia during surge events. Cross-equatorial flows occur over the Indonesian region five days after surge events over the South China Sea that later influence the circulation pattern over Indonesia and northern Australia. Composites of winds in relation to monsoon onset dates show the development of surges over the South China Sea five to ten days prior to the monsoon onset. It appears that the relationship between surges and monsoon activity is strong prior to and during the onset of the Australian monsoon, but is weak once the monsoon is well established in the Australian region. Other influencing factors, such as intraseasonal oscillations, tropical cyclones and the El Nino-Southern Oscillation (ENSO) phenomenon and local thunderstorm activity, make the above-mentioned link more complex. The strongest correlation between north-south winds over the South China Sea, the west coast of Australia, east-west winds over the Timor Sea and Coral Sea was found five to ten days before the monsoon onset. Power spectra of wind components over the Timor Sea and rainfall in northern Australia for all twelve years also show dominant peaks at five to ten days. These results suggest that the periodicity around a five to ten-day period could arise from synoptic-scale circulation patterns associated with surges over the South China Sea. The relationship between the South China Sea winds and northern Australian rainfall is negative, but weak. The relationship between Timor Sea winds and northern Australian rainfall is positive and strong. In general, the onset of the monsoon is associated with a steady increase in westerly winds and rainfall which follow stronger northerlies over the South China Sea. This relationship does not always exist. In some years heavy rainfall preceded strong winds and in others heavy rainfall spells were not observed, although there were strong relationships between wind components between the South China Sea and the Timor Sea.
Tanaka, M. (1983). "Interaction between the active-break cycle of the summer monsoon and the circulation in Eurasia and the western Pacific." Meteorological Society of Japan, Tokyo, Journal 61(3): 455-463.
Synoptic analyses were conducted of the major active-break cycle of the summer monsoon and its interaction with the circulation in Eurasia and the western Pacific (10-70 degrees N, 20-180 degrees E) in a MONEX year (1979) and are compared to those of recent years (1964-1974). The series of five-day mean 500-mb height and sea level pressure data were used to monitor the change in the monsoon and adjacent circulations. The weekly rainfall data for the meteorological subdivision in India for 1964-1974 and 1979 were used to monitor the major active-break cycle of the summer monsoon rainfall in central India. Evidence is shown that development of the major break-monsoon in central India during the high summer period (50 days from July 15 to Sept. 2) is deeply associated with the circulation in the middle latitudes of Eurasia and the western North Pacific near Kyushu Island. The major break-monsoon during the high summer is preceded by the development of a 500-mb ridge near the Caspian Sea (50 degrees N, 50 degrees E). If a sharp drop in the sea level pressure at Naze (28 degrees 23'N, 129 degrees 30'E) to a value below the five-day mean of 1004 mb (often associated with major typhoons moving north near 30 degrees N, 120-130 degrees E) is observed, the major break-monsoon in central India is shown to begin within one week of this event.
Tanaka, M. (1992). "Intraseasonal oscillation and the onset and retreat dates of the summer monsoon over East, Southeast Asia and the Western Pacific region using GMS high cloud amount data." Journal of the Meteorological Society of Japan, Tokyo, Japan 70(1B): 613-629.
The seasonal cycle and the onset and retreat dates of the summer monsoon in East Asia, Southeast Asia and the Western Pacific region are analyzed using 12-year (April 1978-December 1989) means of the 5-day mean 1-degree latitude-longitude gridded GMS high-cloud-amount data. An analysis of these data showed the detailed seasonal cycles of two convective zones (ITCZ and the Baiu front) of the summer monsoon cloud defined by the regions with more than 30 (25 for Baiu front) percent of the mean high-cloud amount. The clouds associated with the ITCZ were observed to increase in Southeast Asia during May and subsequently spread to the South China Sea and the Western Pacific region in a series of sudden expansions during June and July. The Baiu front branch is preceded by an increase in the high-cloud amount along the polar front located near 30 degrees N just south of Japan around 26 April. During the middle of May, this cloud band rapidly moved south to 20-25 degrees N. This period coincides with the onset of the Baiu season in Okinawa. Subsequently, this cloud band moves north to mainland Japan and the Yellow Sea with a series of sudden northward advances during June and July. These abrupt changes are associated with the phase-locking between the intraseasonal oscillation and the seasonal cycle of the monsoon clouds. The onset and retreat dates derived from this study fill the major gaps in these dates over Southeast Asia and the adjacent Western Pacific region where the large area of the ocean prevented analysis of the onset (retreat) dates based on rainfall data.
Tang, D., J. Wang, et al. (1994). "Relation of South China precipitation to large scale water transporting flows and atmospheric heating fields." Journal of Nanjing Institute of Meteorology, Nanjing, China 17(2): 148-152.
Using ECMWF/WMO daily data of the summer of 1982, the relation of South China rainfall to large-scale water transporting flows and the atmospheric heating environment is investigated. Results show that the intraseasonal (30-60 day) oscillation of the flows from the Bay of Bengal and the western Pacific can modulate South China precipitation and that the rainfall is well correlated with the atmospheric heating field. After the establishment of summer monsoon the intense rainfall over South China is highly associated with the heating condition of the region, the intensification (weakening) of the apparent heat source and the moisture sink corresponding to the abundant (deficient) precipitation in this part of China.
Tang, T. Y. and Y. J. Yang (1993). "Low frequency current variability on the shelf break northeast of Taiwan." Journal of Oceanography, Sendai, Japan 49(2): 193-210.
A buoy-mounted Acoustic Doppler Current Profiler was deployed on the shelf break off the northeast coast of Taiwan to monitor current variations in the upper ocean. The acquired data show that the flow in the upper water column was initially southwest and then abruptly turned northwest. This abrupt change occurred in mid-October, starting from the surface layer and then gradually extending to the deeper layer. In contrast with this flow, the flow in the lower water column was southwest over the entire record, but its amplitude was reduced after the middle of October. The abrupt change of current from southwest to northwest is related to the intrusion of Kuroshio. Examination of two CTD casts showed the salinity of the upper ocean to have increased after the directional shift in mid-October, further indicating the Kuroshio intrusion. The sea level data at Keelung provided other evidence for the intrusion of Kuroshio. The sea level descended as the intrusion occurred and kept the low value until the end of the record. The northwest flow, which carried the water away from the northern coast of Taiwan, is responsible for this descent. Although the intrusion of Kuroshio was mainly confined to the upper ocean, it did have influence on the whole water column. Examination of the wind record at Pengchiayu showed that the time of Kuroshio intrusion was not coincident with the intensification of the northeasterly monsoon. The local wind and the current at 20 m were incoherent. Both the variation of Kuroshio current and the fluctuation of Kuroshio path may be responsible for the variation of the local current. Since the intrusion of Kuroshio has a weak relationship with local wind variation, it appears to be induced by non-local factors.
Tomas, R. A. and P. J. Webster (1997). "The role of inertial instability in determining the location and strength of near-equatorial convection." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 123(542): 1445-1482.
There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross-equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea-level pressure (SLP) and warmest sea-surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross-equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain the location of the convection. In this situation, the zero absolute vorticity contour ( eta =0) also lies in the summer hemisphere. Therefore, between the equator and the eta =0 contour is a region of locally-anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low-level divergence-convergence doublet centred about the eta =0 contour which is the mitigating response to the inertial instability. The associated latitude-height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour. Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross-equatorial pressure gradient exists, the eta =0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude-height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of eta =0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off-equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near-equatorial convection are listed. Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off-equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the eta =0 contour into the summer hemisphere are discussed and an explanation for the existence of the low-level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward-moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.
Tsou, C.-H., W.-S. Kau, et al. (2000). "The study of intraseasonal oscillation in South China Sea and East Asian summer monsoon by using wavelet analysis." Atmospheric Sciences, Taiwan, Republic of China 28(1): 27-46.
Intraseasonal oscillation is a common characteristic of atmospheric motion. This oscillation is part of the atmospheric internal dynamics. Through the observations and studies of the low-frequency oscillation of atmospheric activity, the study of intraseasonal oscillation (ISO) has gradually been emphasized. The purpose of the study of low frequency oscillations is to improve the forecast of long term weather-short term climate and to provide theoretical understanding and numerical weather prediction guidance. The ISO is first found in tropical areas. The activity of ISO convection is not the same everywhere in the tropics; it has very localized characteristics. At different locations and time periods, the propagation and intensity of ISO can be very different. The most significant area of the ISO in the tropics is in the Asian Monsoon region. The Asian summer monsoon is first established in the South China Sea (SCS). The SCS is located in southeastern Asia, it connects east and south Asia, while at the same time, the East Asian monsoon and south Asian monsoon both are interactive at this area. Therefore, SCS is important not only as a key area to study the onset of the Southeast Monsoon but also the evolution of the Asian summer monsoon. In this paper, we will study the ISO in SCS area in order to understand the onset and evolution of the East Asian Summer Monsoon (EASM). There are several filter methods that can be used to study ISO. However, the ISO in the monsoon area is very localized and only takes place at special instance. Therefore, a localized signal needs to be isolated. To reach this purpose, we use a wavelet analysis developed by Dubechies (1988). The primary dataset used in the present analysis is satellite-derived outgoing long wave radiation (OLR) by CDC. The OLR data available to us cover 17 years (from 1979 to 1995) in the period of the 1st of April to the 8th of August. The climatologically mean OLR in the Northern Hemisphere summer monsoon region show 3 strong convective areas. These areas are, the west coast of India, Bay of Bengal (BB), and SCS. By using wavelet analysis we can also find the 3 maximum total variances areas, which are, coincide with the above areas. The high variance values of India and BB are contributed by the seasonal variance, however in the SCS, the seasonal variance and 30-60 day variance are equally important (at the same time 30-60 day variance is greater than 10-20 day variance). By more detailed analysis of the propagation of ISO in SCS, we can conclude the following features during the onset of EASM. These are: 1) the 30-60 day oscillations propagate eastward at the beginning of May, from Arab Sea. They enhance at BB when they arrive, and then move continually eastward to SCS around mid-May. (2) The 30-60 day oscillations originated at 160 degrees E move continually westward since the beginning of May. Deep convection will develop when they arrive to Western Pacific. Afterward, these oscillations will keep on moving westward and join the eastward propagating 30-60 day oscillations from BB at SCS. (3) At 110-120 degrees E, there are 30-60 day oscillations that propagate southward from higher latitude and northward from equator, and both are joined together at SCS during mid-May. (4) The 10-20 day oscillations in SCS become evident and develop locally in mid-May. All the above four features are joined together at SCS during the onset period of EASM. The onset of EASM is signaled by an abrupt change in large-scale circulation. The subtropical Pacific anticyclone will suddenly retreat eastward from SCS. While at the same time, the low-level cyclonic circulation and the upper-level anticyclone establish in south Asia. By more detailed studying the relationship between the 30-60 day oscillations and the onset of EASM, we can find that the climatologically averaged unfiltered OLR abruptly decreased in mid-May corresponding to the climatologically onset date of EASM. A prominent negative signal occurring in mid-May, can be easily detected from both the 30-60 day and 10-20 day oscillations. From NCEP reanalysis data we can also see that the upper-level anticyclone in South Asia rapidly moves northward to the north of 15 degrees N. A rapid intensification of the lower westerly and upper easterly monsoon circulation also occur at the same period as the 30-60 day oscillations turns negative. This implies that the 30-60 day oscillations play an important role on the onset of EASM. To improve the predictability of the summer monsoon, the propagation and intensity of the ISO may need to be adequately studied and a wavelet transform which provided a local value of amplitude and phase is a suitable tool for this purpose.
Unninayar, M. S. and T. Murakami (1978). "Temporal variations in the Northern Hemispheric summer circulations." Indian Journal of Meteorology, Hydrology & Geophysics, Delhi 29(1/2): 170-186.
Changes in kinetic energy averaged over the tropical (5.0 degrees S-19.6 degrees N), subtropical (24.2-37.1 degrees N), and mid-latitude (41.0-48.1 degrees N) belts were examined for the 92-day period from June through Aug. 1970. Systematic fluctuations (10-15-day period) were observed in the eddy and zonal mean kinetic energies of the 200-mb tropical belt. Mid-latitude periodicity was longer at similar to 20 days. Short-period ( similar to 5-day) fluctuations were stronger at 700 than at 200 mb, over the tropical belt. Abnormal transitions were observed to begin approximately July 12, with a simultaneous decrease in 200-mb eddy kinetic energy at all latitudes, 700-mb eddy kinetic energy over the tropical belt, and tropical baroclinic activity as determined from cloudiness; all reached minimum values at approximately July 20-24. Tropical zonal mean kinetic energy increased to maximum values in the same period. During July 20-24, the tropospheric circulation displayed singularly different characteristics compared to normal, and break monsoon conditions prevailed over India. The zones of cloudiness and upper divergence maxima over the monsoon region shifted northward to the Tibetan area, while dry weather and abnormal upper convergent wind inflow was experienced over southeast Asia. A conspicuous and anomalous excitation of convective activity over tropical regions in the western North and South Pacific was associated with pronounced upper (lower) tropospheric anticyclonic (cyclonic) circulations. Consequently, 200-mb equatorial easterlies over the western and central Pacific were unusually strong. Concurrently, the anticyclone over the Mexican region reached maximum intensity and was displaced to the north of its normal position.
Vecchi, G. A. and D. E. Harrison (2002). "Monsoon Breaks and Subseasonal Sea Surface Temperature Variability in the Bay of Bengal." Journal of Climate 15(12): 1485-1493.
The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with 'active' periods of heavy rain interrupted by drier 'break' periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1 '-2 'C and local changes can exceed 3 'C over 2 weeks; these changes are as large as those seen in the local climatological seasonal cycle. This raises the possibility that air-sea interaction may be a significant factor in monsoon variability; the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation. A schematic coupled air-sea oscillator mechanism is offered for further study, in which oceanic changes play a dynamical role in monsoon variability.
Vernekar, A. D. and Y. Ji (1999). "Simulation of the onset and intraseasonal variability of two contrasting summer monsoons." Journal of Climate, Boston, MA 12(6): 1707-1725.
To simulate the onset and intraseasonal variability of summer monsoons, the National Centers for Environmental Prediction Eta Model (80 km, L38) is nested in the Center for Ocean-Land-Atmosphere Studies GCM (R40, L18). The region of the Eta Model is (30 degrees S-50 degrees N and 30 degrees -140 degrees E), which includes the Indian, Chinese, and Southeast Asian monsoons. The summer monsoons of 1987 and 1988 are simulated by integrating the nested model from mid-April to the end of September, prescribing the seasonal variations of SST of the respective years. The summer monsoons of 1987 and 1988 were extreme. In 1987, an El Nino year, the Indian monsoon rainfall was far below normal but over southeast China the rainfall exceeded normal. In contrast, in 1988, a La Nina year, Indian monsoon rainfall was far above normal but the rainfall over southeast China was below normal. The Eta Model was able to simulate the typical observed features of the monsoon onset, that is, an abrupt increase in the precipitation rate as well as in the strength of the circulation. The simulated onset dates for 1987 and 1988 were in good agreement with observations. The Eta Model was also able to simulate the observed circulation features of the break and active periods during these two years. To investigate the contrasting characteristics of the Indian and the Chinese monsoons, for these two years the following hypothesis, largely based on observational evidence, is verified. There are two preferred locations of ITCZ: one over the warm waters of the equatorial Indian Ocean and the other over the heated continent in the vicinity of the seasonal monsoon trough. There is a northward migration of the convective precipitation bands from the equatorial ITCZ to the continental ITCZ with the timescale of a few weeks. There exists an inverse relationship between the strength of the two ITCZs. During an El Nino year, sea level pressure over the Indian subcontinent and over the Maritime Continent increases. Consequently, the ITCZ over the Indian subcontinent and over the Maritime Continent weakens and the ITCZ over the equatorial Indian Ocean, Southeast Asia, and southeast China strengthens. The Eta Model simulated circulations are in support of the hypothesis. The simulations also show that there is a northward migration of convective precipitation bands from the equatorial ITCZ to the continental ITCZ.
Vernekar, A. D., V. Thapliyal, et al. (1993). "Global structure of the Madden-Julian Oscillations during two recent contrasting summer monsoon seasons over India." Meteorology and Atmospheric Physics, Vienna, Austria 52(1-2): 37-47.
The global nature of the Madden-Julian Oscillations (MJOs) have been investigated by applying a frequency filter to daily data for the summer monsoon months (June to September) during two contrasting years--1987, a deficient monsoon year and 1988, an excess monsoon year. Several meteorological parameters at five levels in the troposphere have been examined. Regions with large amplitude of these oscillations are isolated for each year. The results indicate that the global spatial distribution of these oscillations is more in a deficient year than in an excess year, in particular over the Indian subcontinent and the El Nino Southern Oscillation (ENSO) regions. The principal modes of variability during these two years have been investigated through Empirical Orthogonal Functions (EOFs). The first two eigenmodes of 850 hPa zonal wind explain nearly 50% of the variance. The dipole type of structure between the Indian and the Pacific region is more apparent in 1987 than in 1988. Time-longitude cross sections of the filtered zonal wind over the equatorial regions clearly show that eastward propagation is detected in 1987, but is virtually absent in 1988. It is also seen that the 30-60 day filtered winds are stronger during the monsoon of 1987 than in 1988.
Vincent, D. G., A. Fink, et al. (1998). "High- and low-frequency intraseasonal variance of OLR on annual and ENSO timescales." Journal of Climate, Boston, MA 11(5): 968-986.
Using 20 yr of outgoing longwave radiation observations, the complex behavior of the higher- (6-25-day) and lower- (25-70-day) frequency bands of tropical intraseasonal convective oscillations is investigated. Emphasis is given to the mean annual cycle and interannual variability of both hands and to the interaction between the two bands. The focus with regard to the interannual variability within each band is on the warm and cold events associated with the El Nino-Southern Oscillation (ENSO) cycle. The study encompasses the tropical and subtropical Indian and Pacific Oceans (including Australia). The strongest intraseasonal signals are, for the most part, aligned with the intertropical convergence zone (ITCZ) and South Pacific convergence zone. In some cases, the 6-25-day signal is not collocated with the Madden-Julian oscillation (MJO) signal and/or occurs remotely from the ITCZ. In these cases, the higher-frequency intraseasonal convective perturbations are associated with phenomena independent from the MJO, such as easterly waves, monsoon depressions, typhoons, or circulations involved in tropical-extratropical interactions. Over the equatorial eastern Indian Ocean, strong activity in both bands persists throughout the year, but the bands are found to be anticorrelated, regardless of the ENSO phase. The effect of ENSO timescales is further examined by looking at December-February anomalies for five El Nino and two La Nina events during this 20-yr sample. A well-defined response of the two bands is restricted to the northwestern and central Pacific. Over the northwestern Pacific Ocean, the two bands complement one another with suppressed (enhanced) convection occurring during El Nino (La Nina) events. Both bands also complement each other over the equatorial central Pacific but are out-of-phase with those in the western Pacific on ENSO timescales. In contrast, over the Australian monsoon region and the eastern Indian Ocean, neither band shows a uniform response in terms of anomalous activity when the latest five ENSO warm events, 1977-78, 1982-83, 1986-87, 1991-92, and 1992-93, are considered.
Volodin, E. M. (1993). "Simulation of intraseasonal variability of the Indian summer monsoon." Russian Meteorology and Hydrology, New York, NY 5: 37-43.
Probable causes of the variability of the Indian summer monsoon obtained in numerical simulations with an atmospheric general circulation model are considered. Special attention is paid to the interaction of the monsoon variations qualitatively resembling a change in the active monsoon-break phases. Responsible for the variability are cold air transport from midlatitudes to the north of the monsoon area and the convective oscillation in the tropics (similar to the 40-50-day oscillation known from observations).
Vu Boi, K. (1978). "Use of correlation fields in relating spells of monsoon and break-monsoon in North Vietnam to midtropospheric circulation over Asia in winter." Idojaras, Budapest 82(6): 313-319.
The correlations between the deviations of five-day temperatures from monthly average temperatures in Hanoi and the deviations of five-day, 500-mb heights from monthly average 500-mb heights at numerous stations in East and Southeast Asia in winter (from Nov. to May) were used to relate the spells of monsoon and break-monsoon in North Vietnam to the midtropospheric circulation over East and Southeast Asia in winter. For the monsoon and break-monsoon processes in winter in North Vietnam, three main centers of action were found in the 500-mb level circulation over East and Southeast Asia: 1) the East Asia coast center, 2) the East Ural center, and 3) the Bengal center. The East Asia center of action, characterizing the southward extension of the East Asia main trough activated through the winter half-year (Nov.-April), has strong intensity and systematic structure in Jan.-March. In the early (Nov.-Dec.) and late (April-May) winter period, the center has weaker intensity and the airflow appears to be more zonal. The Siberian blocking monsoon situation for the period May-June is built up at about 40 degrees N and usually extends from 60 degrees E to Baikal. For Feb.-March, the blocking high is more meridional and is linked with the warm ridge developed from the Arabian Sea. In April-May, the blocking situation is almost broken down and is replaced by the small moving trough situation. In Nov.-Dec., the Bengal center represents the subtropic anticyclone center during the monsoon spell; for Jan.-March, it represents the formation of the Bengal trough during the break-monsoon spell. For the late period (April-May), the negative correlation center in Bengal disappears, and there appears a secondary positive correlation center in the subtropical westerlies in India and Burma, showing the activity of the small upstream troughs during the monsoon spell.
Wang, B. (1988). "Dynamics of tropical low-frequency waves: an analysis of the moist Kelvin wave." Journal of the Atmospheric Sciences, Boston 45(14): 2051-2065.
Stability of the equatorial atmosphere to a quasi-zonal, low-frequency (order of 10 super(-) super(6) sec super(-) super(1) ) disturbance is investigated by using a model that consists of a two-layer free atmosphere and well-mixed boundary layer. The inclusion of boundary layer convergence leads to a circulation-dependent heating more nearly in accord with the behavior of numerically simulated low-frequency waves. Slowly eastward-moving unstable waves were found in a parameter regime stable to inviscid wave-CISK. The instability depends crucially upon the vertical distribution of the moist static energy of the basic state. A derived instability criterion suggests that amplification occurs when the condensational heating supported jointly by interior wave convergence and frictional convergence dominates over the dissipations because of long-wave radiation and boundary layer viscosity. The unstable waves exhibit preferred planetary scales. Since the vertical distribution of moist static energy of the basic state is closely related to sea surface temperature (SST), with increasing SST the growth rate increases for all wavelengths, but the preferred scales shift to a shorter wavelength. The boundary layer convergence plays an important role in spatial scale selection. It not only suppresses unbounded growth of short waves but also couples barotropic and baroclinic components in such a way that the generation of wave-available potential energy is most efficient for planetary scales. In the presence of boundary layer friction, the amplitude of an unstable wave in the zonal wind (or geopotential) at the upper level is significantly larger than that at the lower level. The phases between the two levels differ by similar to 180 deg. The slow eastward movement results mainly from the interior wave-induced reduction of static stability and thermal damping. Behaviors of model unstable waves appear to resemble the observed 40-50-day mode in many aspects, such as vertical structure, energy source, zonal phase propagations, characteristic zonal and meridional scales, and its relation to SST. Limitations of the model are also discussed.
Wang, B. and J. Chen (1989). "On the zonal-scale selection and vertical structure of equatorial intraseasonal waves." Quarterly Journal of the Royal Meteorological Society, Berkshire, England 115(490): 1301-1323.
Observed tropical intraseasonal oscillations exhibit a preferential zonal scale of wavenumber one or two and a baroclinic structure with largest flow field amplitude in the upper troposphere. Unstable interaction of boundary layer friction-induced moisture convergence with condensational heating could cause the scale selection and result in the asymmetric baroclinic structure by means of coupling of vertical normal modes. A vertically continuous linear model describing two-dimensional equatorial motion demonstrates this feedback mechanism and permits detailed study of vertical structure of unstable moist Kelvin waves.
Wang, B. and T. Li (1994). "Convective interaction with boundary-layer dynamics in the development of a tropical intraseasonal system." Journal of the Atmospheric Sciences, Boston, MA 51(11): 1386-1400.
Tropical boundary-layer flows interact with the free tropospheric circulation and underlying sea surface temperature, playing a critical role in coupling collective effects of cumulus heating with equatorial dynamics. In this paper a unified theoretical framework is developed in which convective interaction with large-scale circulation includes three mechanisms: convection-wave convergence (CWC) feedback, evaporation-wind (EW) feedback, and convection-frictional convergence (CFC) feedback. We examine the dynamic instability resulting from the convective interaction with circulation, in particular the role of CFC feedback mechanism. CFC feedback results in an unstable mode that has distinctive characteristics from those occurring from CWC feedback or EW feedback in the absence of mean flow. The instability generated by CFC feedback is of low frequency with a typical growth rate on an order of 10 super(-) super(6) s super(-) super(1) . The unstable mode is a multiscale wave packet; a global-scale circulation couples with a large-scale (several thousand kilometers) convective complex. The complex is organized by boundary-layer convergence and may consist of a few synoptic-scale precipitation cells. The heating released in the complex in turn couples the most Kelvin wave and the Rossby wave with the gravest meridional structure, forming a dispersive system. The energy propagates slower than the individual cells within the wave packet. A transient boundary layer is shown to favor planetary-scale instability due to the frictionally created phase shift between the maximum vertical motion and the heating associated with boundary-layer convergence. The implications of the theory to the basic dynamics of tropical intraseasonal oscillation are discussed.
Wang, B. and H. Rui (1989). "Activity of transient tropical intraseasonal convective anomalies: 1975-1985." International Meeting on Statistical Climatology, 4th, Rotorua, New Zealand.
The intraseasonal convection anomaly (ICA), which either causes or is associated with the tropical intraseasonal oscillation, is more meaningfully identified from the dynamical point of view of relatively raw data containing variations in multiple time and spatial scales. The evolution of individual ICAs is analyzed by using pentad mean anomaly (PMA) maps of ten-year OLR (outgoing long-wave radiation) for 1975-1985 (no 1978) and seven-year (1979-1985) wind data from the European Center for Medium-Range Weather Forecasting. Of 122 ICA events identified, 62% were eastward moving events. In terms of intensity, 88% of strong and moderate events are eastward moving; they are dominant intraseasonal modes and they display three major tracks. The equatorial Indian Ocean and Western Pacific ITCZ are the preferred geographical locations of their development. Independent northward propagation, which is not associated with eastward propagation, is closely related to the summer monsoon activity over these regions. Activity of ICA manifests clear seasonal dependence which tends to be related to the march of the SST maximum in the Tropics. The dynamic effect of the Equator, the thermal dynamic effect of sea surface temperature, and the influence of the monsoonal flow are the basic factors determining the synoptic-climatological features of the ICA.
Wang, B. and H. Rui (1989). "Some dynamic aspects of the equatorial intraseasonal oscillations." Sham, P. and Chang, C. P.
The slowly eastward-moving equatorial convection and circulation anomaly is a major mode of the tropical intraseasonal oscillations. The dynamics of this mode may be understood in terms of moist equatorial wave dynamics. A linear semi-geostrophic model on equatorial Beta-plane is used to study the behavior of equatorial low-frequency motions. The unstable interaction of boundary layer frictional moisture convergence with condensational heating could generate efficiently eddy available potential energy for moist Kelvin mode but not for long Rossby modes. The growing mode is rooted in a moist Kelvin wave but modified through coupling with a long Rossby wave of the lowest meridional index. The horizontal mode coupling makes the growing mode have a horizontal structure bearing similarity to both Kelvin and Rossby modes. It favors the amplification of long planetary scales and slows down the eastward movement. It also suppresses unrealistically fast growth of the uncoupled Kelvin waves by creating substantial meridional flows which induce kinetic energy destruction.
Wang, B. and H. Rui (1990). "Dynamics of the coupled moist Kelvin-Rossby wave on an equatorial beta -plane." Journal of the Atmospheric Sciences, Boston, MA 47(4): 397-413.
A simple theoretical analysis on the stability of a resting tropical atmosphere to semigeostrophic perturbations is given using a free atmosphere-boundary layer coupled model on an equatorial beta -plane. An unstable mode emerges when sea surface temperature is higher than a critical value. The growing mode is a moist Kelvin wave modified through coupling with a Rossby wave of the lowest meridional index. The modified Rossby modes, however, remain damped even for high SST. The unstable mode selection can be explained in terms of wave energy generation due to the latent heating induced by frictional moisture convergence. The horizontal mode-coupling has profound impacts on wave instability. It favors the amplification of long planetary-scale waves, slows down eastward propagation, and suppresses unrealistically fast growth of the uncoupled moist Kelvin mode by creating substantial meridional flows. These effects make the coupled unstable mode more resemble observed equatorial intraseasonal disturbances. The results also demonstrate that when maximum SST moves from the equator to 7.5 degrees N, the growth rate of the unstable wave is significantly reduced, suggesting that the annual march of the ``thermal equator'' and associated convective heating is likely responsible for annual variations of the equatorial 40-50 day wave activity.
Wang, B. and R. Wu (1997). "Peculiar temporal structure of the South China Sea summer monsoon." Advances in Atmospheric Sciences, Beijing, China 14(2): 177-194.
Beijing located at the junction of four major components of the Asian-Australia monsoon system (the Indian, the western North Pacific, the East Asian subtropical, and the Indonesian-Australian monsoon), the monsoon climate over the South China Sea (SCS) exhibits some unique features. Evidences are presented in this paper to reveal and documents the following distinctive features in the temporal structure of the SCS summer monsoon: (1) pronounced monsoon singularities in the lower tropospheric monsoon flows which include the pre-onset and withdrawal easterly surges and the southwesterly monsoon bursts at Julian pentad 34-35 (June 15-24) and pentad 46-47 (August 14-23); (2) four prominent subseasonal cycles (alternative occurrences of climatological active and break monsoons); (3) considerably larger year-to-year variations in convective activity on intraseasonal time scale compared to those over the Bay of Bengal and the Philippine Sea; (4) the redness of the climatological mean spectrum of precipitation/deep convection on synoptic to intraseasonal time scales in the central SCS; (5) a remarkable asymmetry in the seasonal transitions between summer and winter monsoons and an extremely abrupt mid-May transition (the outburst of monsoon rain and the sudden switch in the lower troposphere winds from an easterly to a westerly regime); (6) the bi-modal interannual variations of summer monsoon onset (normal and delayed modes). In addition, the monsoon rainfall displays enormous east-west gradient over the central SCS. Possible causes for these features are discussed. A number of specific science questions concerning some of the peculiar features are raised for the forthcoming SCS monsoon experiment to address.
Wang, B. and X. Xie (1997). "A model for the boreal summer intraseasonal oscillation." Journal of the Atmospheric Sciences, Boston, MA 54(1): 72-86.
The tropical intraseasonal oscillation (ISO) exhibits pronounced seasonality. The boreal summer ISO is more complex than its winter counterpart due to the coexistence of equatorial eastward, off-equatorial westward, and northward propagating, low-frequency modes and their interactions. Based on observational evidence and results obtained from numerical experiments, a mechanism is proposed for the boreal summer ISO in which the Northern Hemisphere summer monsoon (NHSM) circulation and moist static energy distribution play essential roles. With a climatological July mean basic state, the life cycle of model low-frequency waves consists of four processes: an equatorial eastward propagation of a coupled Kelvin-Rossby wave packet, an emanation of moist Rossby waves in the western Pacific, a westward propagation and amplification of the Rossby waves in South Asian monsoon regions, and a reinitiation of the equatorial disturbances over the central Indian Ocean. The life cycle spans about one month and provides a mechanism for self-sustained boreal summer ISO. Analyses of the model experiments reveal that the monsoon mean flows and spatial variation of moist static energy trap equatorial disturbances in the NHSM domain. The reduction of moist static energy over the eastern central Pacific suppresses equatorial convection, leading to disintegration of the equatorial Kelvin-Rossby wave packet and the emanation of Rossby waves in the western North Pacific. Strong easterly vertical shears and seasonally enhanced boundary layer humidity in the NHSM further amplify the Rossby waves (of the gravest meridional mode), making their structures highly asymmetric about the equator. The intensified Rossby waves start to stall and decay when approaching the Arabian Sea due to the ``blocking'' of the sinking dry air mass over North Africa, meanwhile triggering equatorial convection. The mean Hadley circulation plays a critical role in reinitiation of the equatorial Kelvin-Rossby wave packet over the equatorial Indian Ocean.
Wang, B. and X. Xie (1998). "Coupled modes of the warm pool climate system. Part I: The role of air-sea interaction in maintaining Madden-Julian Oscillation." Journal of Climate, Boston, MA 11(8): 2116-2135.
Over the warm pool of the equatorial Indian and western Pacific Oceans, both the climatological mean state and the processes of atmosphere-ocean interaction differ fundamentally from their counterparts over the cold tongue of the equatorial eastern Pacific. A model suitable for studying the coupled instability in both the warm pool and cold tongue regimes is advanced. The model emphasizes ocean mixed layer physics and thermodynamical coupling that are essential for the warm pool regime. Different coupled unstable modes are found under each regime. In contrast to the cold tongue basic state, which favors coupled unstable low-frequency SST mode, the warm pool regime (moderate mean surface westerlies and deep thermocline) is conducive for high-frequency (intraseasonal timescale) coupled unstable modes. The wind-mixed layer interaction through entrainment/evaporation plays a central role in the warm pool instability. The cloud-radiation feedback enhances the instability, whereas the ocean wave dynamics have little impact. The thermodynamic coupling between the atmosphere and ocean mixed layer results in a positive SST anomaly leading convection, which provides eddy available potential energy for growing coupled mode. The relatively slow mixed layer response to atmospheric forcing favors the growth of planetary-scale coupled modes. The presence of mean westerlies suppresses the low-frequency SST mode. The characteristics of the eastward-propagating coupled mode of the warm pool system compares favorably with the large-scale features of the observed Madden-Julian Oscillation (MJO). This suggests that, in addition to atmospheric internal dynamic instability, the ocean mixed layer thermodynamic processes interacting with the atmosphere may play an active part in sustaining the MJO by (a) destabilizing atmospheric moist Kelvin waves, (b) providing a longwave selection mechanism, and (c) slowing down phase propagation and setting up the 40-50-day timescale.
Wang, B. and X. Xu (1997). "Northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation." Journal of Climate, Boston, MA 10(5): 1071-1085.
Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities--monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations. The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May-July), and westward along 15 degrees N from 170 degrees E to the Bay of Bengal during August and September. The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October. The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.
Wang, B. and Y. Xue (1992). "Behavior of a moist Kelvin wave packet with nonlinear heating." Journal of the Atmospheric Sciences, Boston, MA 49(7): 549-559.
The effects of nonlinear (positive only or conditional) heating on moist Kelvin waves are examined with a simple equatorial zonal-plane model describing the gravest baroclinic mode. The unstable perturbation subject to nonlinear heating emerges as a wave packet. A typical amplifying, eastward-moving wave packet is characterized by an asymmetric structure: 1) the ascending branch (wet region) is much narrower than the two descending ones (dry region); and 2) the circulation cell to the east of the wet region center is smaller and stronger than its counterpart to the west of the center. The wet-dry asymmetry is primarily caused by the nonlinear heating effect, while the east-west asymmetry is a result of the movement of the wave packet relative to mean flow. The existence of Newtonian cooling and Rayleigh friction enhances the structural asymmetries. The unstable wave packet is characterized by two zonal length scales: the ascending branch length (ABL) and total circulation extent (TCE). For a given basic state, the growth rate of a wave packet increases with decreasing ABL or TCE. However, up to a moderate growth rate (order of day super(-) super(1) ) the energy spectra of all wave packets are dominated by zonal wave number one regardless of ABL size. In particular, the slowly growing (low frequency) wave packets normally exhibit TCEs of planetary scale and ABLs of synoptic scale. Observed equatorial intraseasonal disturbances often display a narrow convection region in between two much broader dry regions and a total circulation of planetary scale. These structure and scale characteristics are caused by the effects of nonlinear heating and the cyclic geometry of the equator. It is argued that the unstable disturbance found in numerical experiments (e.g., Lau and Peng; Hayashi and Sumi) is a manifestation of the nonlinear wave packet.
Wang, B. and Q. Zhang (2002). "Pacific-East Asian Teleconnection. Part II: How the Philippine Sea Anomalous Anticyclone is Established during El Nino Development." Journal of Climate 15(22): 3252-3265.
The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Nino to east Asian climate during the mature and decay of an El Nino (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Nino; its strength increases with the El Nino intensity and its sign reverses during a La Nina. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment. The development of the PSAC is attributed to combined effects of the remote El Nino forcing, tropical-extratropical interaction, and monsoon-ocean interaction. The developing El Nino induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical-extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Nino-induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind-evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphere -ocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.
Wang, J. and J. He (1991). "The 30-50-day oscillation characteristics of monsoon circulation over East Asia and Australia during the winter of 1982-1983." Journal of Tropical Meteorology, Guangzhou, China 7(2): 104-112.
In this paper, the evolution features of monsoon circulation over East Asia and Australia are investigated using ECMWF gridpoint data and OLR for Nov. 16, 1982-March 15, 1983. An analysis is carried out of monthly mean wind fields during the prevailing monsoon period and a comparison of flows is also made between monsoon active and break phases over Australia. The results show that the evolution of monsoon circulation in winter exhibits quasi 30-50-day oscillations. The major features are the variations in intensity of local Hadley circulation cell as well as an east-west movement of the Australian monsoon trough and of the Pacific High over the Northern Hemisphere.
Warner, C. (1982). "Mesoscale features and cloud organization of 10-12 December 1978 over the South China Sea." Journal of the Atmospheric Sciences, Boston 39(7): 1619-1641.
Aircraft data from Winter MONEX were combined with other data to study mesoscale features and organization of cumulus clouds on Dec. 10-12, 1978. A moderate cold surge in the northeasterly monsoon flow, toward cloudiness in an equatorial trough off Borneo, peaked on Dec. 11. Clouds in the northeasterly monsoon flow were similar to those in the trades, with variations in convective regime on length scales on the order of 100 km. Marked midtropospheric subsidence was accompanied by low-level divergence near 20 degrees N. On Dec. 10, anvil clouds near Borneo expanded; cumulus congestus and cumulonimbus formed on the periphery of this area. The approach of the low-level northeasterlies to the area of anvils was marked by a diminution of subsidence, conditional instability, and a weak field of low-level convergence, with randomly organized cumulus of increasing height. A low-level easterly jet was found in this transition zone, downstream from cloudiness over the Philippines. South of Vietnam, a clear area was associated with low air temperatures, not subsidence. Congestus and cumulonimbus clouds formed near the eastern coast of the Malay Peninsula. Cloud streets were seen from 19 degrees N lat. to the Malaysian coast (with a break south of Vietnam). These clouds were confined below the level of an inflection point in the profile of winds normal to the street direction. Greatest spacings of streets occurred with greatest vertical shears of the cross winds. Cloud number densities were more closely related to the instability of the vertical stratification than to any other parameter. Cross-wind organization of clouds occurred in circumstances of unstable stratification and, apparently, of net ascent. Alignment of clouds was at an angle to the directions of winds and vertical wind shears. It is inferred that, when convergence was strong, deep clouds formed along lines of convergence in the surface streamlines.
Watterson, I. G. (2002). "The sensitivity of subannual and intraseasonal tropical variability to model ocean mixed layer depth." Journal of Geophysical Research. D. Atmospheres [J. Geophys. Res. 107: D1-D2.
The influence of air-sea interaction on subannual and intraseasonal tropical variability is explored through analysis of three long simulations of the Commonwealth Scientific and Industrial Research Organisation atmospheric general circulation model (GCM) with differing ocean specifications: a coupled ocean GCM, a simple 50-m mixed layer model, or climatological sea surface temperatures (SST); together with 50-year simulations with mixed layer depths of 10 m and 20 m. The analysis focuses initially on a signal similar to a Madden-Julian Oscillation (MJO) contained in the first two empirical orthogonal functions (EOF) of monthly anomalies of tropical 807-hPa winds in January in the coupled model. Time-lag regression is used to demonstrate that these patterns propagate eastward, although at only half the speed of the MJO, and induce perturbations to the Australian monsoon. The specified SST model shows no such propagation. Similar results are then obtained using daily data filtered to retain subannual periods. The eastward propagation speed is faster in the shallower mixed layer cases, with the 10-m case producing speeds close to observations. In the interactive models, surface energy fluxes force SST anomalies propagating ahead of the EOF convergence. These fluxes are largely consistent with evaporation perturbed by wind anomalies to the monsoon westerlies, augmented by solar radiation. The SST anomalies then further perturb the winds, as is confirmed by a separate SST perturbation experiment. From the examination of other seasons, it is seen that air-sea interaction generally enhances the amplitude of the MJO-like patterns. It also enhances their eastward propagation along westerly wind bands. Analysis of zonal wave number one winds confirms the strong sensitivity to mixed layer depth in the amplitude and period of the eastward propagating component, particularly during September through February. The results suggest that air-sea interaction may be important to the MJO, provided that the SSTs are sufficiently responsive to the atmospheric patterns.
Webster, P. J. (1983). "Mechanisms of monsoon low-frequency variability: surface hydrological effects." Journal of the Atmospheric Sciences, Boston 40(9): 2110-2124.
Observations indicate that monsoon systems are characterized by orderly large-scale and low-frequency variations. With a time scale of two weeks and sometimes longer, regions of ascending motion are observed to form to the north of the Equator and propagate slowly northward across Southeast Asia. The propagation appears to be associated with the active-break sequence of the summer monsoon, which acts as a local modulator on the activity of the synoptic-scale disturbances. A zonally symmetric, nonlinear, two-layer model containing an interactive ocean and a continent poleward of 18 degrees N is used to investigate the mechanisms that produce the observed low-frequency variability. Only when a full hydrologic cycle is considered does the model produce variations that resemble the observed structures. Mechanisms are traced to include the interaction of the components of the total heating function. The sensible heat input in the boundary layer, although considerably smaller than the other heating components, destabilizes the atmosphere ahead of the ascending zone, permitting the moist convective heating component to move northward slightly ahead of the band of precipitation. The poleward encroachment of these components of the heating forces the vertical velocity, which is proportional to the total heating, to move poleward also. The poleward movement is aided by the evaporative cooling of the precipitation-moistened ground on the equatorial side of the rising motion, which reduces the sensible heat input and effectively stabilizes the troposphere and, thus, reduces the convective heating in that sector while at the same time reducing the latent heat flux. The time scale of the event is determined by the rate of evaporative drying behind the ascent and the formation of a new zone of ascent in the vicinity of the coastal margin. A schematic representation of heating intercomponent interaction and dynamic feedback is given, and the generality of the mechanism to other observed situations is considered. The hypotheses developed and tested in this study underline the importance of the role of ground-hydrology-related processes in large-scale atmospheric dynamics.
Webster, P. J., E. F. Bradley, et al. (2002). "The JASMINE Pilot Study." Bulletin of the American Meteorological Society 83(11): 1603-1630.
The methods and initial results of an extensive pilot study, the Joint Air-Sea Monsoon Interaction Experiment (JASMINE) held in the Indian Ocean during the summer of 1999, are described. The experimental design was based on the precept that the monsoon sways back and forth from active to inactive (or break) phases and that these intraseasonal oscillations are coupled ocean-atmosphere phenomena that are important components of the monsoon system. JASMINE is the first comprehensive study of the coupled ocean-atmosphere system in the eastern Indian Ocean and the southern Bay of Bengal. Two research vessels, the NOAA ship Ronald H. Brown and the Australian research vessel Franklin, totaled 52 days of surveillance in April-June and September, with 388 conductivity-temperature-depth (CTD) casts and 272 radiosonde ascents. In addition, both ships carried identical flux systems to measure the ocean-atmosphere interaction. The Brown had five radar systems and profilers, including a cloud radar and a Doppler C-band rain radar.Active and break periods of the monsoon, and the transitions between these phases, and the onset of the 1999 South Asian summer monsoon occurred during JASMINE. The undisturbed and disturbed periods had vast differences in the net heating of the ocean, ranging from daily averages of +150 W m-2 during the former to -100 W m-2 in the latter. Accompanying these changes in the monsoon phase were distinct states of the upper ocean and the atmosphere, including complete reversals of the near-equatorial currents on the timescales of weeks. Diurnal variability occurred in both phases of the monsoon, particularly in near-surface thermodynamical quantities in undisturbed periods and in convection when conditions were disturbed. The JASMINE observations and analyses are compared with those from other tropical regions. Differences in the surface fluxes between disturbed and undisturbed periods appear to be greater in the monsoon than in the western Pacific Ocean. However, in both regions, it is argued that the configuration of convection and vertical wind shear acts as a positive feedback to accelerate low-level westerly winds. Outstanding questions and tentative plans for the future are also discussed.
Webster, P. J., V. O. Magana, et al. (1998). "Monsoons: processes, predictability, and the prospects for prediction." Journal of Geophysical Research, Washington, DC 103(C7): 14451-14510.
The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programme's (WCRP) Global Ocean-Atmosphere-Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian-Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO-monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.
Wen, Z. and B. Liang (1992). "Variation characteristics for the low frequency oscillation in the tropical region as revealed from outgoing longwave radiation." Journal of Tropical Meteorology, Guangzhou, China 8(2): 142-150.
In this paper, we have made a preliminary study on the variation features for the Low-Frequency Oscillation (LFO) in the tropical region. The results are as follows: 1) Activity areas for the 30-60 day oscillation of the OLR data are located in the North Indian Ocean, the subtropical and the tropical western Pacific Ocean. The integrated power spectra for the 30-60 day period range is large during normal years, and small during El Nino years. 2) A large difference for LFO exists between summer and winter with its large inter-annual variability being mainly confined to the equatorial region during winter and to the Indian Ocean and western Pacific during summer. 3) The meridional propagation of LFO over the western Pacific and Indian Oceans is obvious and the zonal propagation of LFO over the equatorial region is mainly concentrated in the northern summer. 4) The intensity of filtered 30-60 day OLR fields can reflect the ``active'' and ``break'' phases for the Indian monsoon.
Williams, E. R., S. A. Rutledge, et al. (1992). "A radar and electrical study of tropical ``hot towers''." Journal of the Atmospheric Sciences, Boston, MA 49(15): 1386-1395.
Radar and electrical measurements for deep tropical convection are examined for both ``break period'' and ``monsoonal'' regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the ``break period'' regime are associated with 10-20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1 degrees -3 degrees C) in wet-bulb potential temperature between land and sea may account for the order-of-magnitude contrast in recently observed land-ocean lightning activity.
Wu, G. and Y. Zhang (1998). "Tibetan Plateau forcing and the timing of the monsoon onset over South Asia and the South China Sea." Monthly Weather Review, Boston, MA 126(4): 913-927.
Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Finally, the timing of the onset of the Asian monsoon in 1989 was explored. It was shown that the onset of the Asian monsoon occurs when the warm or rising phase of different low-frequency oscillations reach the ``East Asian monsoon area'' (EAMA) concurrently. These include the warm phase of the eastward propagating two- to three-week oscillation (TTO) of the upper-layer temperature in middle latitudes, the rising phase of the northward propagating Madden-Julian oscillation of the southern tropical divergence, and the rising phase of the westward propagating TTO of the western Pacific divergence. It was concluded that the timing of the Asian monsoon onset is determined when the favorable phases of different low-frequency oscillations are locked over the EAMA.
Wu, M. L. C., S. Schubert, et al. (1999). "The development of the South Asian summer monsoon and the intraseasonal oscillation." Journal of Climate, Boston, MA 12(7): 2054-2075.
Fourteen years (1980-93) of National Aeronautics and Space Administration reanalysis data are used to document and study the variability in the development of the South Asian summer monsoon associated with the Intraseasonal Oscillation (ISO). The focus is on the coupling of the large-scale upper-level divergent circulation with the low-level southwesterlies and the associated developing regions of moisture convergence and precipitation, which serve to define the onset times of the various regions of the South Asian monsoon. The impact of the ISO on the development of the low-level southwesterlies is both local and remote, and depends on the strength and phasing of the ISO with the seasonal cycle. Of the 14 yr examined here, 6 showed a strong contribution to the northeastward progression and onset of the monsoon rains over India. In these cases, the ISO is initially (about 2 weeks prior to onset of rains over India) out of phase with, and therefore suppresses, the seasonal development of the regions of large-scale rising and sinking motion. As the ISO moves to the northeast, the rising branch enters the Indian Ocean and acts to enhance the latent heating in the region of the emerging Somali jet. At low levels the response takes the form of an anticyclonic circulation anomaly over the Arabian Sea, and a cyclonic circulation anomaly to the south, which acts to inhibit the eastward progression of the Somali jet. As the ISO moves in phase with and enhances the seasonal mean upper-level divergent circulation, there is an abrupt and intense development of the southwesterly winds leading to an unusually rapid northeast shift and intensification of the monsoon rains over India and the Bay of Bengal. The general northeast progression of the anomalies may be viewed as an initial suppression and then acceleration of the ``normal'' seasonal cycle of the monsoon.
Wu, M. L. C., S. Schubert, et al. (2002). "Forced and Free Intraseasonal Variability over the South Asian Monsoon Region Simulated by 10 AGCMs." Journal of Climate 15(20): 2862-2880.
This study examines intraseasonal (20-70 day) variability in the South Asian monsoon region during 1997/98 in ensembles of 10 simulations with 10 different atmospheric general circulation models. The 10 ensemble members for each model are forced with the same observed weekly sea surface temperature (SST) but differ from each other in that they are started from different initial atmospheric conditions. The results show considerable differences between the models in the simulated 20-70-day variability, ranging from much weaker to much stronger than the observed. A key result is that the models do produce, to varying degrees, a response to the imposed weekly SST. The forced variability tends to be largest in the Indian and western Pacific Oceans where, for some models, it accounts for more than a quarter of the 20-70-day intraseasonal variability in the upper-level velocity potential during these two years. A case study of a strong observed Madden-Julian oscillation (MJO) event shows that the models produce an ensemble mean eastward-propagating signal in the tropical precipitation field over the Indian Ocean and western Pacific, similar to that found in the observations. The associated forced 200-mb velocity potential anomalies are strongly phase locked with the precipitation anomalies, propagating slowly to the east (about 5 m s-1) with a local zonal wavenumber-2 pattern that is generally consistent with the developing observed MJO. The simulated and observed events are, however, approximately in quadrature, with the simulated response leading by 5-10 days. The phase lag occurs because, in the observations, the positive SST anomalies develop upstream of the main convective center in the subsidence region of the MJO, while in the simulations, the forced component is in phase with the SST. For all the models examined here, the intraseasonal variability is dominated by the free (intraensemble) component. The results of the case study presented here show that the free variability has a predominately zonal wavenumber-1 pattern, and has propagation speeds (10-15 m s-1) that are more typical of observed MJO behavior away from the convectively active regions. The free variability appears to be synchronized with the forced response, at least during the strong event examined here. The results of this study support the idea that coupling with SSTs plays an important, though probably not dominant, role in the MJO. The magnitude of the atmospheric response to the SST appears to be in the range of 15%-30% of the 20-70-day variability over much of the tropical eastern Indian and western Pacific Oceans. The results also highlight the need to use caution when interpreting atmospheric model simulations in which the prescribed SST resolves MJO timescales.
Wu, R. and B. Wang (2000). "Interannual variability of summer monsoon onset over the western North Pacific and the underlying processes." Journal of Climate, Boston, MA 13(14): 2483-2501.
Climatological summer monsoon onset over the South China Sea (SCS) and the western North Pacific (WNP) (defined as the region of 10 degrees -20 degrees N, 120 degrees -160 degrees E) displays three distinct stages. Around mid-May, monsoon rain commences in the SCS and the Philippines. In early to mid-June, the monsoon rain extends to the southwestern Philippine Sea. After mid-July, the rainy season starts in the northeastern part of the WNP. The onset anomaly, however, displays an in-phase interannual variation across the entire WNP domain. The standard deviation of the onset date increases eastward from 3 pentads in the SCS to 5 pentads in the northeastern part of the domain. The large onset variability in the WNP is mainly attributed to large year-to-year changes of the seasonal cycle. The role of the intraseasonal oscillation is secondary but important especially in the SCS region. The El Nino-Southern Oscillation (ENSO)-related thermal contrast between the WNP and the equatorial central Pacific modulates significantly the seasonal migration of the monsoon trough, the subtropical high, and the convection zone over the WNP during late spring-early summer in the ENSO decay phase. Thus, ENSO plays a dominant role in the interannual variation of the WNP summer monsoon onset. The general circulation model results suggest that during El Nino events, the warm SST anomalies in the equatorial eastern-central Pacific play a major role in generation of large-scale upper-level convergence and descent anomalies over the WNP. Meanwhile, the cold SST anomalies in the WNP induce lower-level anticyclonic wind anomalies and reduce convective instability. Both the remote and local SST forcing are important for delaying the seasonal movement of the monsoon trough and the western Pacific subtropical high and hence the onset of the monsoon rain. In the La Nina case, the local warm SST anomalies in the WNP are more important than the cold SST anomalies in the equatorial eastern-central Pacific in the generation of lower-level cyclonic wind anomalies and enhancement of convective instability.
Xu, J., J. He, et al. (1993). "Role in moisture transportation of seasonal mean and intraseasonal oscillations in Asian summer monsoon area: long-term average characteristics." Acta Meteorologica Sinica, Beijing, China 7(3): 347-354.
By use of the May-September 1980-1986 ECMWF daily data of u, v, r and T at 850 hPa, a comparative analysis is performed of basic features of moisture transportation at seasonal mean, quasi-40-day, -biweekly, and -weekly oscillations, indicating that the seasonal mean transfer plays a decisive role in the moisture flux over the Asian monsoon region, displaying the integer of the monsoon systems there in character; that the transport related to these tropical intraseasonal oscillations are of equal importance in the monsoon period except the difference in their behaviors, i.e., the transfer shows considerable relative independence in the south- and east-Asian systems; and that the transport at all these intraseasonal oscillations is found to be feeble at equatorial latitudes with little or no influence on each other for both hemispheres.
Yan, H.-m. and Z.-n. Xiao (2000). "The numerical simulation of the Indian Ocean SSTA influence on climatic variations over Asian monsoon region." Journal of Tropical Meteorology, Guangzhou, China 16(1): 18-27.
Based on IAP9L, the effects of the sea surface temperature anomaly (SSTA) in Indian Ocean on the climate over Asian monsoon region are studied. The results indicated that the warmer (colder) SSTA in equatorial Indian Ocean would resulted in the wave trains which are similar to the teleconnection patterns PNA and EAP in middle and high latitude region. Furthermore, SSTA in Indian Ocean may result in significant effects on the weather and climate over Asian monsoon region. With the warmer (colder) SSTA forcing, the Asian summer monsoon will break out later (earlier) and withdraw earlier (later) than usual, and it will last shorter (longer) and the intensity is weaker (stronger) than the normal.
Yang, S. (1996). "ENSO-snow-monsoon associations and seasonal-interannual predictions." International Journal of Climatology, Chichester, UK 16(2): 125-134.
Relationships between the El Nino-Southern Oscillation (ENSO), Eurasian winter snow cover (EWSC), and Indian summer monsoon rainfall (ISMR) are studied. The information provided may be useful for improving seasonal-interannual predictions of the coupled ocean-atmosphere-land system. Results show that more extensive EWSC occurs during El Nino winters rather than La Nina winters. A precursory signal of snow variability can be found in the Southern Oscillation Index. An inverse relationship between EWSC and ISMR, reported in previous studies, is generally confirmed by this study. However, an important finding is that the general snow-monsoon relationship is disrupted by El Nino events. Considering both ENSO's direct impact on ISMR and its role in the break-down of the EWSC-ISMR relationship, it may be reasonable to say that ENSO plays a more important role than EWSC in influencing the variability of the Indian monsoon.
Yang, S. and E. A. Smith (1999). "Four-dimensional structure of monthly latent heating derived from SSM I satellite measurements." Journal of Climate, Boston, MA 12(4): 1016-1037.
Time-space distributions of mean monthly latent heating estimated from Special Sensor Microwave/Imager (SSM/I) passive microwave satellite measurements using the Florida State University precipitation profile retrieval algorithm over ocean regions are investigated for the 1992 annual cycle. The space domain is considered in both horizontal and vertical coordinates, with vertical retrieval made possible by the profiling design of the rain algorithm and the underlying relationship between the vertical derivatives of equivalent liquid water mass fluxes and latent heat release. Comparisons of the retrieved mean monthly rainfall and rain frequency to climatological datasets and atoll rain gauge measurements indicate reasonable agreement except at latitudes above 40 degrees where the satellite values are low biased relative to the climatologies. The horizontal distributions of mean monthly latent heating show that the locations of maximum heating lie in the vicinity and along the axes of well-documented large-scale convergence areas, particularly the intertropical convergence zone (ITCZ) and its transient offshoots, the South Pacific convergence zone (SPCZ), the tropical monsoon systems, and the middle-latitude storm tracks. The vertical distributions show that maximum heating rates of 5 degrees C day super(-) super(1) are located near the 5-km height level with positive heating extending to the top of the troposphere in the Tropics. Convection shifts associated with the 1992 El Nino-Southern Oscillation (ENSO) episode are well represented in the latent heating field. The seasonal variations of the ITCZ, SPCZ, and monsoon systems are clearly evident. The intraseasonal oscillation of latent heating during the northward propagation of the summer Indian monsoon is also a well-defined feature. Finally, the evolution of the Walker circulation is clearly depicted for both active and inactive ENSO conditions throughout 1992. Emphasis is given to comparison and contrast of the SSM/I-derived heating fields to results given in the published literature. Many of the stationary and transient features appearing in the retrievals are consistent with previous studies concerning cloudiness, convection, and rainfall over low latitudes, with the exceptions stemming from specific features of the 1992 ENSO event. Therefore, the study provides a framework for using SSM/I measurements as a means to estimate the four-dimensional structure of latent heating over the tropical-subtropical oceans. Since the details of these structures are of considerable importance to the Earth's weather-climate system both in terms of forcing and response, and by virtue of the design of a rain profiling algorithm, these results are presented as a necessary first step in seeking to use satellite measurements to obtain the most important component of the diabatic heating field.
Yang, S., Q. Zhu, et al. (1993). "Low-frequency oscillation and mechanism of vertical circulation of eastern Asian monsoon." Acta Meteorologica Sinica, Beijing, China 7(2): 176-185.
Analysis is performed of low-frequency oscillation (LFO) and its relation to monsoon by means of ECMWF numerical prediction data in the period 1 June to 30 September 1984, indicating that remarkable local LFO exists in the vertical meridional and equatorial zonal circulations. A preliminary discussion is made of the origin of the LFO of the east Asian summer monsoon meridional circulation in the LFO of the mid and upper troposphere vertical motion around 30 degrees S. The LFOs in the meridional circulations of both hemispheres are linked together by the LFO of the meridional circulation. Finally the possible relation between the tropical monsoon LFO and Mei-yu (plum rain).
Yasunari, T. (1979). "Cloudiness fluctuations associated with the Northern Hemisphere summer monsoon." Meteorological Society of Japan, Tokyo, Journal 57(3): 227-242.
The broad-scale fluctuations of cloudiness over the Eastern Hemisphere during the northern summer monsoon were investigated by using daily satellite mosaic pictures taken from June 1 to Sept. 30, 1973. Spectral analysis revealed two dominant periodicities, similar to 40 and 15 days. Cross-spectral, time-sectional, time-lag correlation and phase-lag vector analysis were applied to reveal the characteristics of these two modes in the time-space field. The fluctuation of 40-day periods shows marked northward movement of cloudiness from the equatorial zone to the mid-latitudes ( similar to 30 degrees E) over the whole Asian monsoon area, and southward movement over Africa and the central Pacific. The northward movement is most apparent over the India-Indian Ocean sector. The fluctuation of this mode is associated with the major, active, break cycle of the monsoon over the whole Asian monsoon area. The fluctuation of 15-day periods shows similar features to that of 40-day periods, but includes two clockwise rotations: one over India and Southeast Asia and the other over the western Pacific. A southward movement from the equatorial zone to the Southern Hemisphere middle latitudes is also prominent to the east and west of Australia. The fluctuation of this mode seems to correspond with the movements of equatorial, monsoon (or tropical), and westerly disturbances. It is suggested that the fluctuation of 40-day periods may be closely related to the global-scale zonal oscillation in the equatorial zone and that of 15-day periods may exist as a result of meridional wave interactions.
Yasunari, T. (1981). "Influence of the Southern Hemisphere circulations on the active-break cycle of the Indian summer monsoon." Japan. National Institute of Polar Research, Tokyo, Memoirs, Special Issue, Oct(19): 223-233.
During the northern summer monsoon period, the cloudiness fluctuation over and around India shows a predominant periodicity of 30-40 days as a major active-break cycle of monsoon activity, and this fluctuation appears as a northward phase shift of maximum (or minimum) cloudiness from the equatorial Indian Ocean toward the Himalayas (T. Yasunari, Journal of the Meteorological Society of Japan, 57: 227, 1979; Ibid., 58: 225, 1980). It is also known that the northward movement of the cloudiness with this periodicity is triggered by the cold air outbreak toward the Equator, associated with the large-scale westerly wave motions such as an index cycle in the Southern Hemisphere midlatitudes (T. Yasunari, Journal of the Meteorological Society of Japan, 59: 336, 1981). In addition, the analysis of the temperature fluctuation at 500 mb over Showa Station, East Antarctica, shows the dominant periodicity of the same period range (30-40 days), especially in the winter season. These results lead to a tentative idea that the major active-break cycle of Asian summer monsoon is closely linked with the hemispheric-scale wave motions in the Southern Hemisphere, including the circumpolar vortex over Antarctica.
Yasunari, T. (1981). "Structure of an Indian summer monsoon system with around 40-day period." Meteorological Society of Japan, Tokyo, Journal 59(4): 336-354.
Through the analysis of the geopotential and wind fluctuation during the summer monsoon season over and around India, the predominant periodicity of about 40 days was confirmed, especially in the lower and upper troposphere, as a major active-break cycle of the monsoon activity. As well as the cloudiness fluctuation of the same period (Yasunari, 1979; 1980), these elements, except the geopotential height in the upper level, showed a marked northward propagation from the Equator toward the Himalayan region. The geopotential height at 200 mb showed, in contrast, a standing oscillation over India. Cross-spectral analyses revealed that the vertical structure of the atmosphere is gradually modified during the northward phase shift of the anomaly cloudiness with the 40-day period. This modification is most apparent in the temperature field. The modification of the vertical structure from the south to the north over India seems to be attributed to the gradual enhancement of cumulus convection from the south to the north. It is also suggestd that this cycle is caused by the periodic cold air outbreak over the middle and upper troposphere of the equatorial zone, which may be associated with the index cycle of the westerly wave movements in the Southern Hemisphere. Finally, a multicell structure of the anomalous Hadley circulation regime over India through the Indian Ocean was proposed, which gives reasonable interpretations on various aspects of the active-break cycle of monsoon activity with this mode.
Yasunari, T. (1986). "Low-frequency interactions between the summer monsoon and the Northern Hemisphere westerlies." Journal of the Meteorological Society of Japan, Tokyo 64(5): 693-708.
Intercorrelations between the active/break cycles of the Indian summer monsoon and the circulation change in the northern middle- and high-latitude westerlies, particularly relevant to the low-frequency (30-50-day period) mode, are investigated. Empirical orthogonal functions and composite analysis revealed the standing-type E-W oscillations of the geopotential height field between central and far-east Asia with the node over Tibet. Lag correlations between the monsoon trough and the 500-mb heights in the Northern Hemisphere suggest that this E-W oscillation is part of the response of the midlatitude westerlies to the northward-moving monsoon heat source. The response in the higher latitudes seems to reach its maximum when the heat source approaches the southern periphery of the westerlies; i.e., near the break phase of the monsoon. A plausible mechanism of this interaction between the monsoon and the westerly flow in the higher latitudes is also discussed.
Yu, S. and Z. Ku (1993). "The comparative analysis of the anomanously advancing and retreating of subtropical high in western Pacific Ocean." Journal of Tropical Meteorology, Guangzhou, China 9(1): 12-19.
The western Pacific subtropical high, an intraseasonal regime that features medium-term fluctuation in longitudinal location, was anomanously more to the south from July 29 to August 16, 1980, and anomanously more to the north from July 16 to August 12, 1983. In this paper, the two processes are investigated in comparison with each other in terms of their circulation characteristics and heating difference and further on, the relationships between the East Asian monsoon and (1) convection, and (2) the advancement /retreat of the subtropical high, is discussed.
Yu, S. and Z. Qian (1992). "Features and probable mechanism of the medium-range variation of East Asia summer monsoon circulation system." Quarterly Journal of Applied Meteorology, Beijing, China 3(1): 114-119.
Spectral analysis has shown that every member of the East Asia summer monsoon system has a feature of quasi-biweekly period oscillation. They are closely correlated with each other and constitute a self-adjusting mechanism, consequently the East Asia summer monsoon circulation system presents a medium-range variation process.
Yu, S. and W. Yang (1995). "Diagnostic study of intraseasonal anomalous progression and retrogression of subtropical high over western Pacific." Journal of Tropical Meteorology, Guangzhou, China 11(3): 214-222.
By means of diagnostic study on two intraseasonal progressive retrogressive processes of anomalous subtropical high in West Pacific using daily 2.5 degrees x 2.5 degrees grid point data of ECMWF in July and August, 1980 and 1983, a number of interesting facts are revealed. The anomalous progression and retrogression of this high is intraseasonally teleconnected with the one in the eastern Pacific, shown as low-frequency waves propagating westward along a latitudinal wave train across the Northern Pacific; the same oscillatory displacement of the eastern subtropical high is again triggered by the variation of convergent sink of upper tropospheric divergent wind field in the eastern Pacific, resulted from anomalous heating through the monsoon area in South Asia and trade-wind zone in the Pacific Ocean.
Yuji, K., H. Watanabe, et al. (1999). "Current variability at the Pacific entrance of the Indonesian Throughflow." Journal of Geophysical Research, Washington, DC 104(C5): 11021-11035.
Current variability at the Pacific entrance of the Indonesian Throughflow is investigated using direct current and hydrographic measurements. Two moorings with three current meters (depths of 350, 550, and 1050 m) and one conductivity-temperature-depth profiler (260 m) were deployed at 4 degrees 1'N, 127 degrees 31'E and 3 degrees 11'N, 128 degrees 27'E between Talaud Islands and Morotai Island (Indonesia) from February 1994 to June 1995. Data from four hydrographic surveys conducted mainly between Mindanao and New Guinea from 1994 to 1996 are also used. The onset of a strong northwestward flow was observed at the southern mooring during boreal winter. In contrast, a southwestward flow containing salty South Pacific water was observed there during boreal summer. This current pattern change matched monsoon change around the mooring sites, suggesting that this variability is a seasonal signal in this region. This current change may occur because of the meridional shift of the Halmahera Eddy associated with an enlargement/diminishment of the Mindanao Dome. Our observation result during summer (the southwestward flow with the South Pacific water at the southern mooring) suggests that the Maluku Sea is one of the eastern routes of the Indonesian Throughflow. The current data also revealed that intraseasonal variability occurs in 50-day oscillations. Because the coherence between wind variability in the tropics with a period of 40-50 days (Madden-Julian Oscillation) and current variability with this period are >0.4, it is possible that the 50-day oscillation in the ocean current is induced by wind variability associated with the Madden-Julian Oscillation. The ocean eddy activity with an intrinsic period in this region may also be related to this 50-day oscillation.
Zeng, Q., Y. Dai, et al. (1998). "Simulation of the Asian monsoon by IAP AGCM coupled with an advanced land surface model (IAP94)." Advances in Atmospheric Sciences, Beijing, China 15(1): 1-16.
In this paper, the global and regional features of the seasonal variation of general circulation, and especially the Asian monsoon simulated by the Institute of Atmospheric Physics Two-level AGCM coupled with a sophisticated land-surface model (IAP94-GCM) are presented and compared with the observation. The comparison is made by using the equilibrium multiyear seasonal cycle climate from a 100-year integration. In the integration sea surface temperature (SST) and sea ice are taken from the observed climatological data (with seasonal variation) because our purpose is to see the improvement of simulation due to the coupling with an advanced land surface model. Overall, the IAP94-GCM provides a reasonably realistic simulation of the interseasonal and intraseasonal climatology of the Asian monsoon and yields an important information that sheds light on the thermal underpinning and the thermodynamics of the seasonal and even multiscale variabilities associated with the Asian summer monsoon.
Zhang, R. and S. Yu (1992). "On the dynamic mechanism of biweekly oscillation of subtropical high for the western Pacific in summer." Journal of Tropical Meteorology, Guangzhou, China 8(4): 306-314.
The biweekly mechanism of east-west oscillating movement of the subtropical western Pacific high is studied using the dynamic method. The results show that circulation conditions for East Asia are such that the synoptic biweekly CISK-wave can be initiated by thermal forcing due to latent heat condensation from monsoon rain in continental East Asia and tropical rain in the South China Sea and western Pacific. There are what is called ``self-adjusting'' effects in the wave kinetic energy in its dispersive variation towards the subtropical East Asia continent, resulting in extension and/or retreat of the subtropical high in the western Pacific.
Zhang, Y., T. Li, et al. (2002). "Onset of the Summer Monsoon over the Indochina Peninsula: Climatology and Interannual Variations." Journal of Climate 15(22): 3206-3221.
The temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the Indochina Peninsula were studied using the observed daily rainfall at 30 stations and the NCEP-NCAR reanalysis from 1951 to 1996. The climatological monsoon onset over Indochina is on 9 May, with a standard deviation of 12 days. The monsoon onset is characterized by the pronounced northeastward progression of the low-level southwesterlies over the Indian Ocean and the intensification and northward extension of the tropical convection from Sumatra. It coincides with the weakening of the midlatitude westerly over south Asia, and the westward propagation of the intraseasonal oscillation (ISO) originated in the South China Sea (SCS) and the western Pacific with a dominant timescale of 12-25 days.A close relationship between the interannual variations of the monsoon onset and El Nino/La Nina was identified. Years with warm (cold) sea surface temperature (SST) anomalies in the western Pacific and cold (warm) SST anomalies in the central-eastern Pacific in the preceding spring have an early (late) onset. For an early onset year, strong convective activities appear over the southern Indochina Peninsula and the southern SCS in the preceding winter and spring. Associated with the changes of the Walker circulation and the local Hadley circulation related to La Nina, strong convective activities were maintained by the convergence between the anomalous southwesterlies in the Indian Ocean and northeasterlies over the northern SCS. The anomalous southwesterlies in the Indian Ocean were induced by both the anomalous Walker circulation associated with La Nina and anomalous land-sea thermal contrast. The anomalous northeasterlies over the northern SCS were originated in northern winter due to the combined effects of the cold east China land and the warm Philippine Sea, and further maintained by a positive thermodynamic air-sea feedback mechanism related to La Nina. An opposite scenario is found for a late onset year with warm SST anomalies in the central-eastern Pacific (El Nino).
Zhi, X. (1994). "Low-frequency oscillations of the apparent heat source and moisture sink over northern summer monsoon areas." Journal of Nanjing Institute of Meteorology, Nanjing, China 17(1): 44-50.
The apparent heat source (Q sub(1) ) and apparent moisture sink (Q sub(2) ) over African monsoon area (AMA), Indian monsoon area (IMA), South China Sea monsoon area (SCSMA) and subtropical monsoon area (SMA) have been calculated, based on ECMWF/WMO data from May to September of 1986. The results show that quasi-40-day periods of Q sub(1) and Q sub(2) over both AMA and IMA, quasi biweekly over SCSMA and about 8-day of Q sub(1) and Q sub(2) over SMA, quasi-40-day period are also important signals over SCSMA and SMA. Quasi-40-day oscillations of Q sub(1) and Q sub(2) over IMA are much stronger than those over the others. The oscillation phase of Q sub(1) is ahead of that of Q sub(2) over AMA, but with a homophase for Q sub(1) and Q sub(2) over other monsoon areas. The dipole distribution of low frequency oscillations of Q sub(2) and zonal wind in the low troposphere is found over IMA and SCSMA. The low frequency component of Q sub(2) corresponds to latent heat source (sink), or apparent moisture sink (source) as the low frequency monsoon becomes active (inactive) over the monsoon areas.
Zhu, B. and B. Wang (1993). "The 30-60-day convection seesaw between the tropical Indian and western Pacific Oceans." Journal of the Atmospheric Sciences, Boston, MA 50(2): 184-199.
The tropical Indian and western Pacific oceans are two prominent action centers for tropical 30-60-day convective variability. When convection is enhanced over the equatorial Indian Ocean, the tropical western Pacific often experiences an abnormal dry condition (phase I), whereas the development of the convection over the tropical western Pacific tends to be accompanied by suppressed convection in the equatorial Indian Ocean (phase II). This convection seesaw is a fundamental characteristic of the tropical 30-60-day oscillation. The seesaw is intimately associated with the activity of propagating low-frequency convective systems (LFCSs). Its formation process is season dependent. Typical boreal summer seesaw results from a time-lagged development of two systems: a western system that originates in the equatorial Indian Ocean and moves eastward and/or northward and an eastern system that develops in the western Pacific monsoon region and moves westward and/or northward. The boreal winter seesaw, on the other hand, is caused by the longitudinal dependence of the evolution of eastward-moving LFCSs that strongly amplify in the equatorial Indian Ocean, weaken and/or split when rapidly passing over the maritime continent, and reintensify in the South Pacific convergence zone (SPCZ). There are two phases of the seesaw. During the first phase, the LFCSs interact with the Indian monsoon in boreal summer and Indonesian-Australian monsoon in boreal winter. Likewise, during the second phase, the LFCSs interplay with monsoon circulations over the western Pacific monsoon trough in boreal summer and over the SPCZ in boreal winter. The convection seesaw activity is closely tied to the corresponding active-break monsoon cycles over the two polar regions of the seesaw.
Zipser, E. J. and K. R. Lutz (1994). "The vertical profile of radar reflectivity of convective cells: a strong indicator of storm intensity and lightning probability?" Monthly Weather Review, Boston, MA 122(8): 1751-1759.
Reflectivity data from Doppler radars are used to construct vertical profiles of radar reflectivity (VPRR) of convective cells in mesoscale convective systems (MCSs) in three different environmental regimes. The National Center for Atmospheric Research CP-3 and CP-4 radars are used to calculate median VPRR for MCSs in the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central in 1985. The National Oceanic and Atmospheric Administration-Tropical Ocean Global Atmosphere radar in Darwin, Australia, is used to calculate VPRR for MCSs observed both in oceanic, monsoon regimes and in continental, break period regimes during the wet seasons of 1987/88 and 1988/89. The midlatitude and tropical continental VPRRs both exhibit maximum reflectivity somewhat above the surface and have a gradual decrease in reflectivity with height above the freezing level. In sharp contrast, the tropical oceanic profile has a maximum reflectivity at the lowest level and a very rapid decrease in reflectivity with height beginning just above the freezing level. The tropical oceanic profile in the Darwin area is almost the same shape as that for two other tropical oceanic regimes, leading to the conclusion that it is characteristic. The absolute values of reflectivity in the 0 degrees to -20 degrees C range are compared with values in the literature thought to represent a threshold for rapid storm electrification leading to lightning, about 40 dBZ at -10 degrees C. Most oceanic cells have reflectivities below the threshold; most midlatitude continental cells exceed the threshold, and the tropical continental cells are about equally divided above and below the threshold. The large negative vertical gradient of reflectivity in this temperature range for oceanic storms is hypothesized to be a direct result of the characteristically weaker vertical velocities observed in MCSs over tropical oceans. It is proposed, as a necessary condition for rapid electrification, that a convective cell must have its updraft speed exceed some threshold value. Based upon field program data, a tentative estimate for the magnitude of this threshold is 6-7 m s super(-) super(1) for mean speed and 10-12 m s super(-) super(1) for peak speed.
Zveryaev, II (2002). "Interdecadal changes in the zonal wind and the intensity of intraseasonal oscillations during boreal summer Asian monsoon." Tellus. Series A: Dynamic Meteorology and Oceanography 54A(3): 288-298.
Decadal-interdecadal changes in the intensity of intraseasonal oscillations (ISO) and in the summer mean wind fields in the Asian monsoon system are investigated using 51 yr of 850 hPa zonal wind data obtained from the National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. Decadal-interdecadal variations contribute significantly to the total variability of the summer mean 850 hPa zonal wind (30-45%) and the ISOs intensity (20-35%). These variations in the summer mean 850 hPa zonal wind and in the intensity of the 30-60 d ISO have a distinct zonal structure and are associated with the strength of low level westerlies and with meridional dynamics of the Tropical Convergence Zone (TCZ). Interdecadal changes in the intensity of the 10-20 d ISO are most pronounced over the eastern Indian Ocean, the South China Sea and the western tropical Pacific. Singular value decomposition (SVD) analysis revealed a strong correlation on an interdecadal timescale between the sea surface temperatures (SST) in the Indian Ocean and the summer mean 850 hPa zonal wind and the intensity of the ISOs in the Asian summer monsoon, whereas such correlations on a decadal timescale are weak. The temporal expansion coefficients of the first SVD mode show a climate regime shift in the mid-late 1970s. During the last few decades, SST in the Indian Ocean increased, resulting in a decreased land-sea heat contrast and weaker low-level westerlies over northern Indian Ocean, the Indian subcontinent, and Indochina. In response to sea surface warming and associated enhanced convection, the 30-60 d ISO became stronger over the equatorial central and western Indian Ocean and the South China Sea, and weaker over the Indian subcontinent, the northern Arabian Sea and the Bay of Bengal. Meanwhile a 10-20 d ISO intensified over the eastern Indian Ocean, the South China Sea, and the western tropical Pacific.