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 process: an equatorial eastward propagation of a coupled Kelvin-Rossby wave packet, an emanation of moist Rossyby 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 mechsnism 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.
Tropical intraseasonal oscillations (ISOs) display considerable seasonal variations in their intensity, frequency, and movement. The Madden-Julian oscillation (MJO) originally described by Madden and Julian (1971, 1972) is an equatorial eastward propagating mode, predominantly wavenumber one in zonal winds. Madden (1986) found that the zonal wind anomaly on the 40-50 day time-scale exceeds that in adjacent lower and higher frequency bands by the largest amount during boreal winter. The oscillation period was found to change from 50 days in boreal winter to about 35 days in boreal summer over the Indian Ocean region (Hartmann et al. 1992). The MJO mode dominates the intraseasonal variability in boreal winter from November to April, yet it is notably weaker in boreal summer from May to October during which events of northward and westward propagations occur more frequently (Wang and Rui 1990a, Hendon and Salby 1994). Pronounced seasonality is one of the fundamental features of the tropical intraseasonal oscillation.
The boreal summer ISO is considerably more complicated than its winter counterpart. The complexity lies in the co-existence of three types of propagating low-frequency modes: the MJO mode, the northward propagating mode over the Indian and western North Pacific monsoon regions (e.g., Yasunari 1979, 1981, Krishnamurti and Sabrahmanyam 1982, Chen and Murakami 1988), and the off-equatorial westward propagating mode (Murakami 1980). In addition, the boreal summer ISO exhibits a significant standing component between the equatorial Indian Ocean and the tropical western North Pacific (Zhu and Wang 1993). The different modes appear to interact with each other. Statistically, the equatorial Indian Ocean stands out as a preferred inception and intensification region for the intraseasonal convective anomalies; the western North Pacific is another active area of boreal summer intraseasonal variability; in contrast, the maritime continent and equatorial eastern-central Pacific are regions of dissipation (Wang and Rui 1990a).
The westward propagating mode has received less attention in the literature and deserves more discussion here. Murakami (1980) and Murakami et al. (1984) presented evidences of 20-30 day perturbations propagating westward along $10-20^\circ$N west of the dateline, using both OLR and wind data. Wang and Rui (1990a), based on an analysis of 10 years of pentad mean OLR anomaly maps, identified the primary tracks of the westward-moving intraseasonal convective anomalies. Typical westward phase speed is 5 \ms and the zonal wavelength is about 4,000 to 6,000 km. As shown in Fig. 1, on 20-72 day time scale, the OLR anomalies exhibit prevailing westward propagations along $15^\circ$N from the western Pacific to Bay of Bengal during July, August and September. Although the westward propagation is sometimes episodic and has prominent year-to-year variations, it is, nevertheless, a predominant mode in the off-equatorial monsoon regions.
It is interesting that the westward propagating disturbances were found to originate often from the equatorial western Pacific. Using satellite-observed high cloud amount data, Nitta (1987) found that the cloud anomalies move eastward from the equatorial Indian Ocean to western Pacific, and then turn sharply northwestward and move westward afterwards along $10-20^\circ$N (Fig. 11 of Nitta). The behavior of the westward propagating vorticity waves with a wavelength of 4000 km and a speed of about 5 m/s was documented in detail by Lau and Lau (1990). Their teleconnectivity map of the bandpass-filtered 850 hPa relative vorticity field (Fig. 5 of Lau and Lau 1990) show a clear propagation track which starts from the equatorial western Pacific and turns toward South Asian summer monsoon regions. Figure 16 of Murakami et al. (1984) presented a good example of the horizontal structure and westward propagation of this type of disturbances. Although individual waves have a synoptic-scale life-span, the wave activities are regulated on a 30-40 day time scale.
A number of theoretical and numerical studies have attempted to explain aspects of the boreal summer ISO. Most of them focus on the cause of the northward propagation on a time scale of 20-40 days in the Indian monsoon region. Webster (1983) emphasized the key role of the land surface heat fluxes into the boundary layer that destabilizes the atmosphere ahead of the ascending zone and causes northward shift of the convective zones. On the other hand, Goswami and Shukla (1984) stressed the role of convection-thermal relaxation feedback in the northward propagation: Convective activity results in an increase of static stability which depresses convection itself; meanwhile dynamic and radiative relaxation decreases moist static stability and bring the atmosphere to a new convectively unstable state. Lau and Peng (1990), based on their numerical experiments, suggested that the interaction of equatorial Kelvin waves with large-scale monsoon mean flows can generate unstable quasigeostrophic baroclinic waves with period of 5-6 days over the Indian monsoon region along $15-20^\circ$N; meanwhile the equatorial disturbances weaken.
Whereas observations have documented the complicated behavior of the boreal summer ISO, the physical processes associated with the westward propagation and the preferred development of low-frequency disturbances over the equatorial Indian Ocean and western North Pacific remain unexplained. Given the fact that the low-frequency wave activity in boreal summer strongly confines to the NH tropical monsoon domain, we postulate that the large-scale monsoon mean flows and the moisture distribution might have fundamental impacts on the structure, propagation, and development of low-frequency disturbances and the maintenance of the tropical ISO.
The present study is aimed at testing this idea and developing our understanding of the dynamics of boreal summer ISO, in particular, the effects of spatial variations of planetary-scale monsoonal flows on the behavior of low-frequency equatorial waves. Specific questions to be addressed include: (1) Why is the western North Pacific a preferred region for generation of westward propagating disturbances and the equatorial Indian Ocean a preferred region for recurrence or inception of the eastward-moving disturbances? (2) How are the westward propagating low-frequency waves initiated, intensified and dissipated? (3) What roles does the monsoon circulation play in sustaining the boreal summer ISO?
An intermediate tropical atmospheric model with a steady, three-dimensional basic state is used for modeling boreal summer ISO (section 2). Section 3 describes results of the control experiment which reveals how the realistic July mean circulation and moist static energy distribution regulate transient wave activities in the model. In section 4 and 5 we further investigate the effects of mean monsoonal flows and moist static energy distribution on equatorial waves and the primary processes that are responsible for the simulated life-cycle of boreal summer low-frequency wave activities. The last section presents a summary and discusses limitations of the model.
This paper presents a simple model for boreal summer intraseasonal oscillation (ISO). With realistic July mean basic flow and moist static energy distribution, the life cycle of low-frequency waves simulated in the model is highlighted by the schematic diagram in Fig. 9. The life-cycle consists of four distinguished processes. (1) An initial semi-geostrophic disturbance located at the east coast of Africa first moves eastward along the equator as a coupled Kelvin-Rossby wave packet from the equatorial Indian Ocean to the Pacific. (2) As the equatorial disturbances rapidly decay in the eastern-central Pacific, moist Rossby waves are excited in the western North Pacific and move northwestward toward southeast Asian monsoon region. This bears close similarities to the movement of the low-frequency cloud and vorticity anomalies observed by Murakami et al. (1984) and Nitta (1987), and Lau and Lau (1990). (3) The slowly westward propagating Rossby waves amplify while crossing southeast Asian monsoon region and exhibit strong asymmetry with respect to the equator due primarily to the effects of asymmetric distribution of monsoon easterly vertical shears. (4) As the strongly developed Rossby wave approaches the sinking dry airmass over Middle East and North Africa, it starts to stall and decay, meanwhile re-initiates an equatorial convective disturbance which propagates eastward and starts the next cycle of the model low-frequency disturbances.
The life-cycle of low-frequency disturbances provides a self-sustained oscillation mechanism for boreal summer ISO. The life-cycle is a result of the trapping of the low-frequency moist equatorial waves by NH summer monsoon mean flows and by the distribution of mean moist static energy. The oscillation period is determined by the sum of the time taken by the equatorial disturbance traveling from the Indian Ocean to the dateline, the time taken by the westward propagating disturbances traveling from the western North Pacific to the Arabian Sea, and the time needed for emanation of Rossby waves in the western North Pacific and the reinitiation of equatorial disturbances over the Indian Ocean. With the Doppler shift effect of the mean flows, the time for low-frequency waves to complete the life-cycle is on an order of one month. The actual intraseasonal oscillation has a broad-band periodicity. The irregularity and regional difference may well be associated with episodic development, change of wave propagation speed due to local conditions, and modifications by other processes.
Three processes are critical for sustaining the model's northern summer ISO: the excitation of Rossby waves in the western North Pacific, the amplification of the Rossby waves over the Indian monsoon region, and the reinitiation of the equatorial disturbances Over the Indian Ocean.
Why is the western North Pacific a favorable location for the emanation of Rossby waves? This is primarily due to the reduced moisture availability along with mean sinking motion (downdraft branch of Walker circulation) in the central-eastern Pacific which severely suppresses the convection, leading to disintegration of the equatorial Kelvin-Rossby wave packet. The high moist static energy supply and mean upward motion in the western North Pacific further favor the emanation of the Rossby waves from the decaying equatorial wave packet. The simultaneous weakening of the equatorial eastward-moving coupled Kelvin-Rossby wave packet and the generation of off-equatorial Rossby waves imply an energy transfer from Kelvin wave to Rossby wave component.
What causes the development of the Rossby waves in the South Asian monsoon domain? The easterly vertical shear and seasonally enhanced moist static energy source play essential roles. Wang and Xie (1996) has shown that the meridional shear of zonal flow has a rather moderate effects on large-scale equatorial waves, whereas the vertical shear of zonal flow has remarkable effects on westward propagating Rossby waves and Yanai waves because the vertical shear couples the barotropic and baroclinic modes and can resonantly excite the barotropic motion for Rossby waves. They also found that an easterly (westerly) shear confines the Rossby waves to the lower (upper) troposphere. This is because the excitation of barotropic mode depends on the sign of the vertical shear: The excited barotropic geopotential is precisely $180^\circ$ out of phase with the geopotential thickness in an easterly shear, so that the superposition of the two vertical modes leads to a stronger (weaker) geopotential perturbation in the lower (upper) troposphere. Xie and Wang (1996) further demonstrated that in the presence of boundary layer, the easterly shear makes equatorial Rossby waves unstable because the enhanced lower-tropospheric perturbation motion induces stronger frictional moisture convergence and associated latent-heating which favors for instability. The localized easterly shear was shown to be capable of destabilizing and trapping the Rossby waves "in situ" and capable of changing the Rossby wave structure from symmetric to asymmetric with regard to the equator. The Rossby wave with a wavelength of 4000 km becomes most unstable with its constant phase lines tilting horizontally (eastward with latitude) and vertically (against the shear). These features compare favorably with those of the boreal summer vorticity waves documented by Murakami et al. (1984) and Lau and Lau (1990). The theory can also well explain the behavior of the Rossby waves simulated in the present model.
How are the Indian Ocean near-equatorial disturbances re-initiated? We have demonstrated that the mean vertical circulation (the Hadley and Walker circulation) plays a critical role. In addition, whether the equatorial disturbances can be triggered depends also on the intensity of the off-equatorial Rossby waves and their asymmetry. In this sense, the monsoon easterly vertical shear and seasonally enhanced moist static energy source in the monsoon regions are also important. The mean flow asymmetry makes the Rossby waves of the lowest meridional mode extremely asymmetric: The northern cell is much more intense than the southern cell, while the southern cell is much closer to the equator. The intensification of the northern cell reinforces the southern cell. When the northern cell decays over the Arabian Sea, the southern cell appears to gain energy and generate a new equatorial disturbance. We speculate that this process implies an energy transfer from the moist Rossby wave to equatorial Kelvin wave in the presence of the mean Hadley circulation and strong asymmetry in the Rossby wave structure.
Regardless of the notable success of the model in reproducing qualitatively observed features of the northern summer ISO, the model involves a number of crucial simplifications and approximations. Relatively heavy dissipation was used for achieving a slowly growing low-frequency waves over the warm oceans, so that long-time integration is possible. The model dissipation can be reduced to normal by adjusting other parameters that control wave instability. The results here do not specifically relies on the heavy damping. The model serves merely as a mechanistic tool for illustrating idealized intraseasonal oscillation scenario possibly occurring during the northern summer. The hypothesis raised in this paper needs to be further investigated by analyzing observations and through numerical experiments with more sophisticated general circulation models. The precise mechanisms responsible for the energy transfer processes between equatorial Kelvin and Rossby waves require further theoretical studies. It also calls for further study to explore possible interactions between the tropical intraseasonal disturbances and various other processes such as cloud-radiation feedback and the ocean mixed layer thermodynamics in the future.
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