THE ESTABLISHMENT OF THE SOUTH CHINA SEA SUMMER MONSOON IN 1998:THE NORTHWARD JUMP OF SOUTH ASIA ANTICYCLONE AND ITS EFFECT ON THE ONSET OF SOUTH CHINA SEA SUMMER MONSOON

By using the NCEP/NCAR reanalysis data,the activities of the 200 hPa South Asia Anticyclone (SAA) and its effect on the onset of South China Sea (SCS) summer monsoon in May 1998 are studied.The results show that:(1)During the pre-onset period the reversal of the mid- upper troposphere meridional horizontal temperature gradient over South Asia to the east of 90°E was caused mainly by warm horizonal thermal advections over this area.About 36 h after the reversal,the SCS summer monsoon bursts.(2)The variation of thermal wind circulation caused by the reversal of temperature gradient gave rise to the 200 hPa center of the SAA to jump from 15°N to 21°N within 12 hours.(3)The northward jump of the SAA and the intrusion eastward of the SAA under the effect of the warm horizonal thermal advections resulted in the rapid development of a trough over the South China coast.This induced the West Pacific Subtropical Anticyclone ridge,which originally controlled the South China Sea,to weaken rapidly and retreat eastward.Subsequently,the SCS summer monsoon built.(4)The superimposition in phase of the anticyclone ridge moving eastward over the middle-latitudes of China with the SAA has a rather important influence on the development of the trough over the South China coast.