Features Of The East Asian Tropical And Subtropical Summer Monsoons And Their Relations To The Thermal Forcing

Based on the NCEP/NCAR reanalysis data,1998 South China Sea Monsoon Experiment (SCSMEX) data and the TBB data with high resolution,features of convective activities over Asian-Australian "land bridge" areas and its importance,comparison of the SCS tropical summer trough and subtropical summer monsoon trough over East Asia and the relationship between the position variation of the west Pacific subtropical high and the apparent heating are analyzed by dynamical diagnosis,theoretical analysis.Main results are as follows:(1)It is true that convection first flares up in Indo-China Peninsula (ICP),which is closely associated with the seasonal migration of tropical convection in Sumatra along Asian-Australian "land bridge".The SCS tropical summer monsoon onset bears a close relation to the systematic migration of the convection center over Sumatra along "land bridge" and ICP from winter to summer.The sudden acceleration of convection shift affects directly the subsequent enhancement of the convection over ICP areas,which will excite a cyclonic circulation over the west part of the continent by releasing latent heat of condensation,in favor of the convection building over the east part of Bay of Bengal (BOB) and subtropical high belts rupturing over this region and withdrawing eastward,leading to SCS summer monsoon establishment.(2)The East Asian subtropical summer monsoon is established in mid-April,which is earlier than the SCS summer monsoon onset.The subtropical summer monsoon trough (subtropical-trough) has clearly frontal property,yet the SCS summer monsoon trough (SCS trough) has not.The SCS trough onsets early and abruptly with the low level positive relative vorticity appearing suddenly,and retreats slowly,but the subtropical-trough establishes step by step with the positive relative vorticity initially over the Yunnan-Guizhou Plateau and Guangxi areas spreading gradually northeastward,and withdraws,rapidly.The onset of SCS trough is obviously indicated by the reverse of the easterly,but the establishment of the subtropical-trough is characteristic of the westerly enhancement. (3)The subtropical non-adiabatic heating plays an essential role in the location changes of the west Pacific subtropical high (WPSH).When the north convection is stronger than the south, WPSH shifts southward. Along with the northward movement of WPSH,the convective latent heating extends northwards from its south area,which is in favor of the northward movement of WPSH.Each of the westward extension of WPSH corresponds to the significantly enhanced heating in the west of WPSH and the latter is prior to the former.(4)The exceptional heating source over the east coast of Arabian Sea and the Bay of Bengal is favorable to the heavy rain forming and maintaining in South China areas.The exceptional heating source forces an exceptional anticyclonic circulation over its northwest high level leading to the South Asian High enhancing and westward of mean.The exceptional westerlies controlled the south of the mainland,which goes against the South Asian High moving northward,but favored maintaining over the south of Tibetan Plateau and South China,so South China areas just located the updraft areas which was in the north of South Asian. High and the south of the high-level jet, making for heavy rain and severe flood.