STUDY ON MODAL VARIATION OF SUBTROPICAL HIGH AND ITS MECHANISM DURING SEASONAL TRANSITION PART Ⅰ:CLIMATOLOGICAL FEATURES OF SUBTROPICAL HIGH STRUCTURE

Climatological characteristics of subtropical high structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data.The ridge the surface of subtropical anticyclone is defined by the boundary surface between westerly to the north and easterly to the south(WEB in brief).Under the constraint of thermal wind relation,the WEB is always tilted toward warmer region in the vertical.In Afro-Asian monsoon area,the subtropical anticyclone beltwhose ridgelines are consecutive in wintertime takes on relatively symmetrical and zonal structure,the WEB tilts southward with increasing height;In summer,the subtropical ant icyclone belt is discontinuous at lower level and continuous at upper level,the WEB tilts northward from the bottom up.The structure and properties of subtropical anticyclone during seasonal transition are dif ferent from area to area.May is the period when subtropical high modality most significantly varies,its ridgelines at middle and low levels start to break.Based on the formation and variation of the vertical axis of subtropical anticyclone,a new concept "seasonal transition axis" is proposed.The summer pattern of subtropical high firstly appearsover eastern Bay of Bengal in the beginning of May,then stablizes over eastern Bay of Bengal,Indochina,and western South China Sea on the 3rd pent ad of May.It exists over the South China Sea in the 4th-5th pentad of May and establishes over central India in the 1st-2nd pentad of June.The three consequential stages when summer modal subtropcal high occurs correspond to that of A sian summer monsoon onset,respectively.The onset of the Asian summer monsoon circulation is closely associated with such "seasonal transition axis".To a great extent,the monsoon onset over the Bay of Bengal depends on the reversal of meridional temperature gradient in vicinity of subtropical high ridge in upper troposphere.The meridional temperature gradient at middle and upper levels in t roposphere can be used as good index for defining Asian monsoon onset.