Mechanism of Thermal Features over the Indo-China Peninsula and Possible Effects on the Onset of the South China Sea Monsoon

The thermal characteristics during the South China Sea (SCS) summer monsoon onset period near theIndo-China Peninsula are analyzed by using the South China Sea Monsoon Experiment (SCSMEX)reanalysis data from I May to 31 August 1998 and the NCEP/NCAR pentad-mean reanalysis data fromJanuary 1980 to December 1995. The possible relationships between the anomaly of thermal features nearthe Indo-China Peninsula and the SCS monsoon onset are investigated, and the mechanism causing theSCS summer monsoon onset is also discussed. Results from the 1998 SCSMEX reanalysis data show thatthere exists a strong persistent surface sensible heating near the Indo-China Peninsula prior to the SCSmonsoon onset, which has apparent low frequency oscillation features. This sensible heating leads to awarmer center in the lower atmosphere near the Indo-China Peninsula and strong local horizontal tempera-ture and geopotential height gradients which are favorable to strengthening the southwest wind over theIndo-China Peninsula. It is also found that stronger convergent winds at lower levels and stronger divergentwinds at high levels appear, which provide a favorable configuration for the development of vertical motion,enhancement of precipitation, and onset of the SCS monsoon. These results can be verified by analysis of themulti-year mean data. Additionally, it is found that the temperature at 850 hPa increases more rapidly overthe Indo-China Peninsula than the South China Sea prior to the SCS monsoon onset, which leads to astrengthening of the temperature difference between the Indo-China Peninsula and the South China Sea.Moreover, results from the analysis of the Iongitudinal temperature and geopotential height differencesshow that the eastern retreat of the subtropical high over the Indo-China Peninsula during the period ofSCS monsoon onset is associated with the temperature increase over the Indo-China Peninsula and theeastern extension of low trough over the Bay of Bengal.