Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea

摘要

The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea.This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall oversouthern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associatedwith the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWFdata for 1981-86 are used to study the tropical circulations associated with the development of low-levelwesterlies during both events.Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets andnine lune surges, respectively, indicate that both events occur with the approach of a midlatitude trough-frontsystem. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system mayexplain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring,this is the only Asian monsoon region where midlatitude fronts can move into the Tropics without having toovercome significant terrain barriers.Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres.During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extensionof a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over theAndaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convectionis indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddlingthe equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convectionand propagates westward to dominate the flow changes over the Bay of Bengal and the southern Indian Ocean.The steady 4-5 m S-I westward speed and near-perfect symmetry with respect to the equator indicate thepossibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northernSouth China Sea.