Nature of Asian monsoon precipitation

The structure of the mean precipitation of the South Asian monsoon is spatially complex. Embedded in a broad precipitation maximum extending eastward from 70°E out into the North west tropical Pacific Ocean are strong local maxima to the west of the Western Ghats, over Cambodia, in the eastern China sea, over the eastern tropical Indian Ocean and the Bay of Bengal. Maximum precipitation is seen over the oceans and not over the land regions. Distinct temporal variability also exists with time scales ranging from weeks to decades. Neither the spatial or temporal variability of the monsoon can be explained simply as the response to the cross-equatorial pressure gradient force between the continental regions of Asia and the oceans of the southern hemisphere as suggested in classical descriptions of the monsoon. Noting that an understanding of the processes that produce the spatial and temporal variability of monsoon rainfall will increase skill in prediction, we conducted a detailed diagnostic study.

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, are found to modulate and determine mean precipitation patterns. To explain the interannual variability of the monsoon, many studies have tried to establish a relationship between the seasonal monsoon rainfall and ENSO. However the correlations among these two phenomena have considerably changed over the decades and the response of regional rainfall to external forcing does not appear to be spatially coherent. In addition, a proportion of interannual modulation of the monsoon is found to be essentially independent to boundary conditions outside the Indian Ocean. This internal variability of the South Asian monsoon results from the cumulative effect of rainfall variability on intraseasonal time scales, specifically MISOs, which are shown to be the main modulator of weather events and enclose almost all synoptic activity.

Determining whether the precipitation maximum located on the eastern side of the BoB is the result of MISO activity or due to orographic uplift (or a combination of both) is a central research issue in this study. Detailed diagnostics show that the northward propagation of the low pressure system associated with the evolution of a MISO event generates a cyclonic circulation that tends to drive moist air towards the Burma mountain range and, in so doing, considerably enhancing rainfall in the northeast corner of the BoB, explaining much of the observed summer maximum parallel to the mountains. This result suggests that in order for the climate models to reproduce the observed seasonal monsoon rainfall structure, it is necessary to improve the simulation of MISO activity.