Nature of Asian monsoon precipitation: Intraseasonal to interannual time-scales

Mean precipitation patterns show complex spatial patterns with maxima being located mainly over the ocean regions adjacent rather than over the land areas. The Southeast Asian Monsoon shows strong temporal variability in time scales ranging from weeks to decades with the rainfall structure within the wet season presenting a complex spatial and temporal structure. Local and regional effects of orography, propagation characteristics of the monsoon intraseasonal oscillations (MISOs), propagation of biweekly signals from the South China Sea and land-atmosphere feedbacks, modulate the precipitation patterns. Understanding the processes that contribute to the mean precipitation patterns is an important prerequisite to predicting rainfall variability.

Spectral analysis of most monsoon indices, including the All-India Rainfall and the Webster and Yang indices show a strong and broad peak between 2 and 8 years although with a magnitude that changes decadally. Whether the link between ENSO and the summer monsoon is the main mechanism that results in the broad spectral interannual peak is still in question. The biennial character of the monsoon rainfall also contributes to this peak, and it is very difficult to isolate from the observations the variability associated with a biennial mechanism and that induced by ENSO. In addition, it is probable that not all the interannual variability is externally induced with a fraction being internal to the monsoon system and hence essentially independent of the boundary conditions outside the Indian Ocean. The internal variability is essentially the result of the cumulative effect of rainfall variability in scales shorter than the wet season. In the summer monsoon case, the most important time scale correspond to the MISOs, with a temporal scale between 30-60 days of northward propagating positive anomalies of convection from the equatorial Indian Ocean. This scale acts as the main envelope of convection in the region, comprising almost all the synoptic activity.

While the driving mechanism of the monsoon annual cycle is the temperature gradient among the South-East Asian continent and the Indian, it does not explain the observed JJAS distribution with the maximum rainfall occurring over the east side of the Bay of Bengal (BoB). The nature of the BoB precipitation maximum, whether it is the result of MISO activity or due to orography, is a central issue in the study. Detailed diagnostics show that the northward propagation of the low pressure system generates cyclonic circulation that tends to drive moist air towards the Burma mountain range considerably enhancing rainfall in the northeast corner of the Bay explaining much of the observed seasonal maximum parallel to the mountains. In this sense, while the mountains do not seem to play a defining role in the ISO evolution and northward propagation, their existence directly influences the cumulative ISO associated rainfall, determining the observed mean seasonal pattern. This is an important conclusion since it suggests that in order for the climate models to reproduce the observed monsoon rainfall structure, they need to simulate better the ISO activity observed in nature.

This research is part of an attempt to provide a better insight in the overall interannual variability of the monsoon by considering the effects of the internal variability, focusing on the study the nature of the summer monsoon rainfall variability within the season to determine whether year-to-year differences in the intraseasonal activity over the Indian monsoon domain have an impact in the interannual variability of seasonal monsoon rainfall.