Large signals of climatic variation over the ocean in the asian monsoon region

Abstract

The aim of this paper is to identify and delineate large signals of climatic variation in the Asian monsoon region and try to understand the nature of transformation from one climate regime to another.

It is found that the summer monsoon over the Indian and western Pacific oceans shows distinct climatic regimes with changes occurring in the years around 1875, 1900, 1940 and 1960. The change of about 1900 is the largest one, which occurs in step with the variation of global oceanic climate pointed out by Fletcher, et al. (1982).

The main characteristics of the transformation from one regime to another is an alternation of meridionality of monsoon current. The transformation occurs most strongly in the western Pacific convergence zone, where monsoon has strong interaction with the trade wind systems. The variability of monsoon rainfall over India and East China also exhibits some large signals which are synchronous with those of wind field over the ocean: the monsoon rainfall increases (decreases) during the “meridional monsoon period” (zonal monsoon period) over the ocean.

It should be noted that the apparent decreasing of plum rains in East China since 1958 which is well known in China would be linked mainly with the sudden increasing ofU-component of SW monsoon over the South China Sea.

Finally a kind of seesaw between Indian monsoon and East China monsoon with somewhat time-lag is discussed.