Long-term variability of Asian summer monsoon rainfall during 1946-1988

Abstract

An observational study has been made of the long-term variability of Asian summer monsoon rainfall for the period 1946-1988 and its relationship to global sea-surface temperatures (SSTs) and geopotential height fields. The trend analysis of June-September rainfall for the recent four decades shows a distinct long-term trend in rainfall over several Asian regions; increasing trends in southwest India, and decreasing trends in northwest and central India, on the south side of the Himalayas, and in the western Indochina peninsula facing the Bay of Bengal. A rotated empirical orthogonal function (R-EOF) analysis was applied to global SSTs for the above 43-year period (Shinoda and Kawamura 1994). The second R-EOF (R-EOF2) mode exhibits the warming trend of the Indian Ocean. The correlation analysis between the time coefficients of the R-EOF2 mode and monsoon rainfall shows that the trend components which are observed in both the R-EOF2 mode and rainfall for the above areas are responsible for the high correlation between them. The R-EOF2 mode is highly correlated not only with monsoon rainfall, but also with tropical 850 hPa geopotential heights. This correlation pattern exhibits an east-west contrast having positive correlations over Africa and the western Pacific Ocean and negative correlations over the northern Indian Ocean. The above positive and negative correlations coincide with the increasing and decreasing trends in 850 hPa heights. Over the Indian subcontinent, areas having negative and positive trends of 850 hPa heights coincide with those having increasing and decreasing trends in régional rainfall, respectively. On a monthly basis, for June and August, the Indian subcontinent is extensively covered with areas having increasing trends in rainfall and decreasing trends in 850 hPa heights, whereas the areas are limited to southwest India during July and September.