Seasonal And Interannual Variability Of The Pre-monsoon Circulation To The South Of Tibetan Plateau In Spring And Relation Between Pre-monsoon Circulation And East Asian Summer Monsoon

Based on the 1979～2007 NCEP/NCAR reanalysisⅡdata,Tibetan Plateau snow-deep and rainfall data of 160 stations in China,the seasonal and interannual variability of the pre-monsoon circulation to the south of Tibetan Plateau in spring are investigated.And also the relation between east Asian summer monsoon circulation system,summer rainfall in China and pre-monsoon circulation are discussed.The major results in this paper are described as follows:1.Pre-monsoon circulation is a transition situation between Hadley circulation in winter and monsoon circulation in summer.It occurs to the south of Tibetan Plateau and in the northwest of BOB from ground to the lower troposphere.In paper,building time,ending time and intensity indices of pre-monsoon circulation are defined.Based on the definitions,time serial data of interannual variation of pre-monsoon circulation are obtained.There are significant interannual variability in the building time,ending time and intensity of the pre-monsoon circulation.2.Because of the baroclinic structure of subtropical High from 700hPa to 1000hPa,north wind occurs in upper level of pre-monsoon circulation and south wind in lower.South wind turns to updraft when it is obstructed by Tibetan Plateau.Topography and structure of subtropical High are dynamic factors of pre-monsoon circulation.The sensible heat increases in the north of pre-monsoon circulation,and sensible and latent heat reduce in the south when pre-monsoon sets up.This is a important thermal factor.In more-snow years of Tibetan Plateau,and SST of Indian Ocean is higher,the land-sea thermal contrast is reduced,so Pre-monsoon sets up later and strength reduction.The both are important factors of interannual variation of pre-monsoon circulation.3.The relation between pre-monsoon circulation and east Asian summer monsoon circulation system and summer rainfall in China are investigated.When the pre-monsoon circulation sets up later,the summer West Pacific Subtropical High is stronger and controlling range westward.The South Asia High transfers from winter code to summer code later while the pre-monsoon is weak.The pre-flood season of Southern China and Meiyu are also relation to pre-monsoon circulation.When pre-mosoon circulation sets up later,rainy season of South China will be later and rainfall be weaker,subsequently,easily causes the flood of middle and lower Yangtze River Basin,and vice versa.The physical linking between the pre-monsoon circulation and monsoon circulation system and summer rainfall in China are the winter and spring snow-deep of Tibetan Plateau and SST of Indian Ocean..