Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-range Weather Forecast analysis winds, we show that the Northern Hemisphere summer monsoon displays statistically significant Climatological Intra-Seasonal 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 northern Philippines during early summer (May to July), whereas westward along 15oN from 170oE to 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 which starts the monsoon over South China Sea and Philippines. Its dry phase in late May and early June brings the pre-monsoon 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 the 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.
The Northern Hemisphere (NH) summer monsoon is a primary component of the global climate system. In the literature, regional monsoon components in the NH have been named somewhat differently. To avoid confusions, we define NH summer monsoon consisting of three major regional components: the Indian Summer Monsoon (ISM), East Asian Subtropical Monsoon (EASM), and Western North Pacific Summer Monsoon (WNPSM). Previous studies have summarized major circulation systems associated with ISM (Krishnamurti 1985), EASM (Tao and Chen 1987, Ninomiya and Murakami 1987), and WNPSM (Murakami and Matsumoto 1994). The WNPSM and ISM are tropical monsoons characterized by (1) low-level monsoon troughs and associated southwesterlies equatorward side of the troughs and northward cross-equatorial flows; (2) upper-tropospheric easterlies to the south of the subtropical ridge. The vertical shear of zonal flows provides a meaningful measure of the strength and extent of the tropical monsoon (Webster and Yang 1992). The EASM, on the other hand, is characterized by (1) a low-level convergence zone between southerly from the tropics and weak northerlies from the midlatitudes and (2) upper tropospheric westerlies north of the subtropical ridge.
The Northern Hemisphere summer monsoon is well known for its prominent subseasonal variation---the active and break monsoons (Krishnamurti and Bhalme 1976, Murakami 1976)--- and the abrupt changes during its seasonal march (Yeh et al. 1959). The active/break monsoons and the alternation between sudden changes and steady evolution are directly associated with intraseasonal oscillations which are widely observed in the ISM (e.g., Yasunari 1979, 1980, Sikka and Gadgel 1980, Krishnamurti and Subrahmanyan 1982, and many others), the EASM (e.g., Chen and Jin 1984, Lau and Chan 1985), the WNPSM (e.g., Chen and Murakami 1988, Huang 1994), and the Australian summer monsoon (e. g., Murakami et al. 1986, Holland 1986, McBride 1987, Hendon and Liebmann 1990). The alternation between abrupt changes and steady evolution is particularly evident in the EASM. Abrupt northward advance of the Meiyu front occurs normally in mid-June and mid-July ( Tao and Chen 1987, Ding 1992, Tanaka 1992). Matsumoto (1992) used "discontinuities" (or "jumps') to characterize sub-seasonal variations of monsoon. Ueda et al. (1995) showed that the abrupt northward shift of convective anomalies over the western Pacific around 20oN, 150oE in late July is accompanied by a sudden appearance of large-scale cyclonic circulations and by changes in tropical cyclone tracks. The causes of the abrupt changes were speculated but not well understood (Ueda et al. 1995, Matsumoto and Murakami 1995).
Our knowledge on the interaction between tropical intraseasonal oscillation (ISO) and monsoons has expanded rapidly In the last decade or so (Wang and Ding 1992). On the one hand, transient ISOs are responsible for active/break monsoons. On the other hand, the annual cycle modulates ISO's intensity (e.g., Madden 1986), movement (Wang and Rui 1990, Zhu and Wang 1993), and frequency (Hartmann et al. 1992).
A prominent feature of monsoon (annual cycle) - ISO interaction is that the annual variation of general circulation tends to regulate phase propagation of tropical ISO. Figure 1 shows a set of longitude-time diagrams of OLR pentad mean anomalies (subjected to a 20-73 day filtering) along 15-20oN. Notable westward propagation episodes tend to repeat themselves during about the same period of the calendar year. The phase-locking of ISOs to the annual cycle results in a significant westward propagating Climatological Intra-Seasonal Oscillation (CISO), which is in turn responsible for subseasonal variation of summer monsoons.
The extremely wet and dry phases of CISO imply climatological active and break monsoons, respectively. We refer the peak active/break monsoons that occur on or near a fixed pentad with usual regularity to as summer monsoon singularities. For instance, at 140oE and 15-20oE (the core region of the WNPSM), dry anomalies occur at pentad 49 (August 29-September 2) on a regular basis, forming a climatological break monsoon singularity (Fig. 1). In this paper, we will show that the presence of climate singularities is one of the fundamental characteristics of the NH summer monsoon.
Questions arise here include: (1) To what extent are transient ISOs controlled by the annual cycle in NH summer monsoon domain? Is the phase-locking of ISOs to annual cycles and resulted monsoon singularities statistically significant and physically meaningful? (2) How are monsoon singularities distributed spatially and temporally? Are they related to each other? If so, how are they related? (3) What physical processes possibly determine the movement of CISO and the linkage among monsoon singularities over different regions? The present paper is aimed at addressing and discussing these questions.
We will describe the data and define CISO first in section 2 and then demonstrate, in section 3, the statistical significance of the NH summer monsoon singularities associated with the extreme phases of CISO. The spatial and temporal distributions of the summer monsoon singularities occurring in various monsoon regions are then conveniently studied by examining the behavior of CISO. In sections 4, 5, and 6, we analyze, respectively, the dynamic structure, the propagation, and the life cycle of CISO and the relationship among the singularities in the three major NH summer monsoon components. The possible causes for the CISO propagation and monsoon singularities are discussed in the last section.
We have presented evidences that the transient intraseasonal oscillations (ISOs) in the NH summer monsoon domain, especially in the western North Pacific, tend to be phase-locked to annual march of planetary-scale monsoon circulation. The phase-locking results in a Climatological Intraseasonal Oscillation (CISO) which appears to be significant at 95% confidence level in terms of three different types of statistical test. The extreme phases of CISO characterize summer monsoon singularities. In the context of weather and climate, singularity is an event that occurs on or near a fixed date with usual regularity.
The concept of summer monsoon singularity is important and useful for describing and understanding temporal structures of monsoon climate and variability. Whereas mosoon singularities depict those events that have remarkable regularity, the occasional break down or debase of the regularity implies a robust signal of interannual variation. More importantly, transitional phases of CISO, in contrast to extreme phases, indicate the periods during which summer monsoons experience largest year-to-year variability. Since a strong or weak monsoon season is often linked to changes in the duration and intensity of major active/break monsoons, study of monsoon singularity may provide useful information for understanding and predicting monsoon variability.
The CISO exhibits a coherent dynamic structure between convective and circulation anomalies (Fig. 9). Enhanced convection is located to the southeast of 850 hPa positive vorticity center and in phase with 200 hPa divergence anomalies. Both enhanced 850 hPa westerlies and 200 hPa northeasterlies are located to the southwest of the enhanced convection. The upper-level winds contain a significant divergent component which manifests itself as an exceptionally strong cross-equatorial northerly over the Maritime Continent. The low-level cross-equatorial southerlies are enhanced during the period of enhanced convection occurring in the western North Pacific but their strength is relatively weak.
The CISO propagates. Although the movement is two-dimensional, there exist season-dependent, preferred patterns. Northward propagation dominates during early summer (May-July), whereas westward propagation prevails in August and September (Fig. 10). Most significant northward propagation is found along Philippine maritime continent. The averaged northward propagation speed is about 1.8 m/s. Strongest signal of westward propagation occurs along 15oN-20oN and from 160oE to 110oE with a typical speed of about 4-5 m/s. The movement of CISO links characteristic evolution of the monsoons in one region with those of neighboring areas.
Extreme phases of CISO often benchmark summer monsoon singularities (Fig. 6). Four major CISO cycles are identified from May to October (see Table 1). The peak wet phase of the first cycle at P28 (May 16-20) marks the onset of South China Sea-Philippine summer monsoon (Fig. 11). It is followed by an extremely dry phase at P30-P31 (May 26-June 4) which corresponds to the pre-monsoon dry period over Indian Summer Monsoon (ISM), WNPSM, and Meiyu/Baiu regions. The wet phase of Cycle II peaking at P34 (June 15-19) represents the first wet spell after the grand onsets of ISM, WNPSM, and Meiyu/Baiu (Fig. 12). The crest of the dry phase of Cycle II at P39 (July 10-14) concurs with a grand break of ISM, WNPSM and the retreat of Meiyu/Baiu. The extremely wet phase of Cycle III at P46 (August 14-18) benchmarks the height of WNPSM (Fig. 13). The dry phase of Cycle III exhibits a prominent westward propagation from the dateline at P47 (August 19-23) all the way to Bay of Bengal at P53 (September 18-22) which is responsible for the second break of WNPSM and ISM. The wet phase of Cycle IV in mid-October brings the last active monsoon in WNPSM and terminates ISM (Fig. 14).
The relationship in subseasonal variability between ISM and WNPSM varies with seasonal march. During the onset cycle (Cycle II), the evolution of ISM and WNPSM tend to be in phase due to the northward migration of east-west elongated CISO anomalies. On the other hand, during the peak cycle (Cycle III), the evolution of ISM is nearly 180o out of phase with WNPSM due to slow westward propagation of an east-west dipole CISO anomaly. During the first and the last cycle, however, they tend to evolve independently (Fig. 10b). The intraseasonal monsoon variations of EASM link more intimately with WNPSM than with ISM. This is due to the coupling among the WNPSM trough, Western Pacific High, and the EASM trough. The coupling between WNPSM trough and Western Pacific High, however, is also season-dependent. During the first-half of summer (early June to mid-July), evolution of WNPSM and EASM (Meiyu/Baiu) are nearly in phase due to a strong coupling between WNPSM trough and Western Pacific High. In contrast, from late July to the end of August, the linkage between WNPSM and EASM is very weak due to (a) relatively independent evolution of Western Pacific High and WNPSM trough , and (b) dominant westward propagation of CISO anomalies during cycle III.
What physical processes cause the phase-locking of transient ISO to annual cycle? We speculate that regulation of planetary-scale circulation change on movements of ISOs may play critical roles. The following discussions are focused on the most prominent CISO movements which are associated with the onset and peak cycles (the cycle II and cycle III).
The onset is associated with the wet phase of Cycle II of CISO during which enhanced convection originates from Maritime Continent and moves northward to about 20oN. It is initially associated with westerly anomalies to the south of an equatorial anticyclone (Buffer Zone) which moves northward, causing sudden onset of WNPSM in middle June. The prevailing northward propagation of CISO during early summer is favored by the seasonal march of planetary-scale circulation systems, in particular, the South Asia High and Western Pacific High. The northward movement of OLR CISO displays a close association with the "step-wise" displacement of Western Pacific High. Note also that the northward propagation is most significant along Philippine maritime continent. The island surface sensible heat flux may contribute to local rising temperature and dropping pressure, thus favors northward shift of the equatorial convergence zone. Yanai and Li (1994) emphasized the importance of sensible heating over Tibetan Plateau in reversing meridional temperature gradient and the onset of ISM. The thermal effect of maritime continent over Philippines may function in a similar manner. The preferred northward propagation region is also a confluence zone between ISM southwesterly, cross equatorial southerly, and WNP southeasterly. The role of the establishment of background low-level southerlies in "steering' convection anomalies northward should not be rule out.
The WNPSM peak singularity is associated with the wet phase of CISO Cycle III during which convective anomalies are originated around 170oE at P44 (August 4-8) and predominantly propagate westward along 15- 20oN (Fig. 10b and Fig. 3a), resulting the height of WNPSM rainfall in middle August. The convective anomalies appear to be initiated in a region where the upper-level divergence associated with tropical upper tropospheric trough (TUTT) nearly coincides with low-level convergence at the eastern end of the western Pacific monsoon trough. Previous studies have noticed the association of the convection in tropical Pacific with the TUTT (Sadler et al. 1976, Liebmann and Hartmann 1984, Chen and Yen 1991) and the convergence zone between monsoon westerlies and trades (Holland 1995). In August, the easterly vertical shear extends from South Asia all the way eastward to 170oE. The area of warmest SSTs expands to its northeast-most location in the western Pacific. Wang and Xie (1996) has shown that easterly vertical shear tends to strongly confine Rossby waves to the lower troposphere, which leads to enhanced boundary layer frictional moisture convergence and convective heating feedback. In addition, the easterly shear also enhances baroclinic conversion of mean available potential energy to equatorial Rossby waves (Xie and Wang 1996). The combined heating and baroclinic processes can effectively destabilize moist equatorial Rossby waves which move westward slowly. During the mid-summer, the northmost position of Western Pacific High and WNPSM trough provide a favorable environment for the development of westward propagating Rossby waves. The phase-lock of Rossby wave activity with the peak of WNPSM circulation is very likely responsible for westward propagation of CISO during cycle III.
The largest amplitude of CISO is found over western North Pacific Summer Monsoon (WNPSM) region between mean summer monsoon trough and the ridge of Western Pacific High. The dominant period of CISO is about 20-30 days there. The WNPSM, therefore, features most pronounced CISO and monsoon singularities. This appears to be due to (1) the impacts of subseasonal variability of the Western Pacific High and (2) the annual regulation of transient activity of slowly westward-propagating wave disturbances in the western North Pacific as described by Murakami (1980), Chen and Xie (1988), and Wang and Rui (1990).
The strong activity of CISO and monsoon singularities over East Asia and western North Pacific is one of the prominent characteristics of the boreal summer general circulation. Similar phenomenon was found significant in Austral summer monsoons in Indonesia and northern Australia. It bears profound implications to seasonal climate predictions. The present study provides a new perspective and framework for study of summer monsoon singularities, yet much more works are required to further understand dynamics of CISO and monsoon singularities, in particular, the precise dynamic processes that control the phase-locking of ISO to seasonal march. This appears to be a result of complex nonlinear interaction between atmosphere, ocean, and land.
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