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《Acta Meteorologica Sinica》 2007-04
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Liu Yanju Ding Yihui Laboratory for Climate Studies,National Climate Center,CMA,Beijing 100081  
A relatively systematic climatological research on onset of the Asian tropical summer monsoon(ATSM) was carried out.Based on a unified index for onset of the ATSM,the advancing of the whole ATSM was newly made and then further documented was the view that the ATSM breaks out firstly over the tropical eastern Indian Ocean and the middle and south part of the Indo-China Peninsula in the 26th pentad(about May 10),then successively over the South China Sea(SCS) in the 28th pentad(around May 20),and it seems that the onsets over the above two regions belong to the different stages of the same monsoon system.Then,the onset mechanism of ATSM was further investigated by the comprehensive analysis on the land-sea thermodynamic contrast,intra-seasonal oscillation and so on,and the several key factors climatologically influencing the ATSM onset are put forward.Based on these results,a possible climatological schematic map for the onset of ATSM firstly over the tropical eastern Indian Ocean-the Indo-China Peninsula-the SCS is also presented.Namely the seasonal evolution of the atmospheric circulation is the background of the monsoon onset;the enhancement and northward advance of convections and of the vertical transfer of sensible heat and latent heat over the Indo-China Peninsula and neighboring areas,the dramatic deepening of the India-Burma trough,and the westerly warm advection over the eastern Tibetan Plateau are the major driving forces for the summer monsoon onset,which make the longitudinal gradient of temperature reversed firstly and ascending motion vigorous over this region,then the tropical monsoon and precipitation rapidly enhanced;and the phase-lock between the 30-60 day and 10-20 day low frequency oscillations originated from different sources is a triggering factor for the summer monsoon onset.It is just the joint effect of these factors that induces the earliest onset of ATSM over this region.
【Fund】: 国家自然科学基金(40531006 40576012);; “973”计划(2006CB403604);; 攀登项目B“SCSMEX”
【CateGory Index】: P462.41
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