Differences in rainfall variability in the South and Southeast Asian summer monsoons

Abstract

Abstract Long‐term rainfall variability in the South and Southeast Asian summer monsoons is investigated using observational data for a 50 year period. Linear trends indicate a general level of summer rainfall decrease over northeast India that has intensified during recent decades. In contrast to the South Asian Monsoon (SAM), there is no significant rainfall trend in the Southeast Asian Monsoon (SEAM). The result of the empirical orthogonal functions (EOFs) analysis has shown that the leading EOF modes explain about 25% of total rainfall variability in both regions. Low correlation between respective principal components implies that rainfall variations in the two monsoon subsystems are not related, suggesting that different mechanisms are responsible for the interannual rainfall variability in the SAM and the SEAM. The interannual variability of the SAM rainfall is linked to sea‐surface temperature (SST) fluctuations in the equatorial eastern Pacific during the monsoon season and the following autumn. As revealed by lead–lag correlations, the warming of the Arabian Sea and a region located northwest of Australia during the boreal winter may lead to a strong SAM. The interannual behaviour of the SAM rainfall shows a feature of the quasi‐biennial oscillation with the reversal of correlations between months preceding the monsoon and the monsoon–post‐monsoon months. In comparison with the SAM, the interannual variability of the SEAM rainfall is associated with distinctly different SST patterns. Enhanced SEAM rainfall is preceded by a high SST in the northern Indian Ocean and the South China Sea in the previous January and February. Significant correlations between the SEAM rainfall and SST, which form a tripole pattern in the North Pacific Ocean, are detected during the preceding winter and early spring, suggesting that wintertime tropics–extratropics interactions may play a role in the SEAM rainfall variability. Copyright © 2004 Royal Meteorological Society