The Structure of the Asian Monsoon Surface Wind Field over the Ocean

Abstract

This paper examines the structure of the summer monsoon over the Indian and western Pacific Oceans by analyzing the surface wind data from marine data sets for the period of 1860 to 1970. It is found that the southern edge of the huge monsoon low has three low-pressure troughs over the eastern edges of Africa, India, and the Indo-China peninsula. To the east of each trough is a broad region of intensified wind flow, with three branches in the western Arabian Sea, the Bay of Bengal, and the South China Sea, respectively. We call this the ?branching phenomenon of the SW-monsoon.? Each of these branches has its own characteristics, and these influence the rainfall pattern in South Asia. This is verified by the observed distribution of precipitation, which has maximum precipitation over the west coast of each peninsula. The cross-equatorial flow also shows the branching phenomenon, with three maxima at the same longitudes as the three branches of the SW-monsoon. A discussion of the relationships between the branching phenomenon and sea-land distribution suggests that the structure of this SW-monsoon wave is very different from that of waves in the westerlies in middle latitudes. The three monsoon troughs correspond to perturbations of the temperature field in the lower troposphere, reflecting the sea-land thermal contrast in this area, and correspond to the longitudes of strong cross-equatorial flows enhancing the monsoonal character of the surface wind field.