Observational study on the onset of the South China Sea southwest monsoon

Based on the long-term marine ship observation data, records of meteorological stations and High-Reflective Cloud(HRC) data by satellite remote sensing , this paper has studied the circulation patterns and variability in elements during onset and the established periods of the South China Sea(SCS) southwest(SW) monsoon. The averaged date of the onset SW monsoon in the SCS occurs in the middle of May climatologically. The corresponding date for the northern part is little earlier (May 12) and those for the southern parts are little later (May 20). The interannual range of the onset dates is about one month. Following the onset of the SW monsoon, the cloud amount and the precipitation increase while the convection activities enhance over the SCS. But there is a strong spatial heterogeneity within the domain. After onset of the SW monsoon the strong convective area moves northwards, while the SCS rain band moves to the center and north. Sea surface temperature(SST) increases rapidly before the onset and the leading time is about one month. The increment of SST supplies heat and vapor for the onset. From April to May the surface heat fluxes display obvious changes, e.g., latent heat exchange and evaporation enhancement. It is one of the reasons why the SW monsoon bursts firstly in the SCS.