The Effects Of The Thermal Contrast Between Indo-China Peninsula And South China Sea On SCS Summer Monsoon Onset

The seasonal and interannual variation characteristics of thermal contrastbetween Indo-China Peninsula(ICP) and South China Sea(SCS) and the effects of thethermal contrast on SCS summer monsoon onset are analyzed by using theNCEP/NCAR reanalysis data from 1958 to 1998 and out-going longwaveradiation(OLR) data from 1975 to 1998, as well as the effects of anomalous snowcover over the Tibetan-Plateau on the thermal contrast by using the monthly meandays of snow cover over the Tibetan-Plateau data from 1973 to 1998. The results areas follows:There is clear seasonal variation in the thermal contrast between ICP and SCS.The surface temperature of ICP is higher than SCS from the 3rd to 25thpentads dueto the effects of sensible heating. After 25th pentad such a thermal gradient willreverse.Above reversion of the thermal contrast is a precondition for SCS summermonsoon onset. The thermal contrast will reverse prior to(or simultaneous with) SCSsummer monsoon onset. The local longitudinal sea-land thermal contrast makes goodcondition for north(south) winds in low(high) levels between ICP and SCS, whichpies upon large-scale sea-land latitudinal thermal contrast between Eurasia and thesouth side of ocean, provides a favorable configuration for the appearance ofcyclonic circumfluence, evacuation of the subtropical high from SCS, and onset ofSCS summer monsoon. Additionally, it is found that the convection and precipitationappear over ICP in the 22nd and 23rd, which reduces the surface temperature of ICP,inducing the reversion of the thermal contrast between ICP and SCS.The interarmual variation in the thermal contrast between ICP and SCS alsoplays an important part in the timing of SCS summer monsoon onset. There is anprominent positive correlation between the last days of the surface temperature ofICP higher than SCS and the timing of SCS summer monsoon onset, which meansthat the earlier the timing of the surface temperature of ICP becomes higher and thelater the timing of the one reverses lower than SCS, SCS summer monsoon will onsetlater.It is found that the snow cover in winter and spring over the Tibetan-Plateauplay some role in above longitudinal sea-land thermal contrast. The mechanism ofthe effect shows: In more snow years, the sensible heating over the Tibetan-Plateau isweaker, which leads to the lower temperature over the Tibetan-Plateau and the eastside of areas, the later seasonal reversion of Hadley circulation in the NorthernHemisphere, the abnormity of downdraught over ICP, making for that the subtropicalhigh over the Bay of Bengal splits later, the strong convection over ICP appears later, the declining timing of the surface temperature of ICP is later, the longitudinaltemperature gradient between ICP and SCS reverses later.