Effects of the Thermal Contrast Between Indo-China Peninsula and South China Sea on the SCS Monsoon Onset

The seasonal and interannual variations of the thermal contrast between Indo-China Peninsula (ICP)and South China Sea (SCS) were analyzed using the pentad mean NCEP/NCAR reanalysis data during 1958-1998 and the pentad mean outgoing long-wave radiation (OLR) data during 19751998, along with the effects of such a thermal contrast on the SCS monsoon onset (SCSMO). It is shown that there exists significant seasonal evolution for such a thermal contrast. The surface temperature of ICP is higher than that of SCS from pentad 3 to pentad 25 due to the sensible heating of the ICP. After pentad 25, such a thermal gradient reverses due to the temperature decrease resulted from the convection and rainfall over the ICP from pentad 22 to pentad 23. Furthermore, the above seasonal evolution of the discussed thermal contrast also demonstrates a remarkable interannual change which plays an important role in the SCSMO.On one hand, the reversion happens prior to (or simultaneously with) the SCSMO each year during 1958-1998, thus becoming a precondition for the SCSMO. On the other hand, the earlier (later) the date when the surface temperature of ICP becomes higher (lower) than that of the SCS, the later the SCSMO.