A Study of Southwest Monsoon Onset Date over the South China Sea and Its Impact Factors
Liang Jianyin and Wu Shangsen(Guangzhou Institute of Tropical and Oceanic Meteorology, Guangzhou 510080)
By using the 1950-1999 NCEP daily averaged reanalyzed data and based on the major atmosphere characteristics at 850 hPa during southwest monsoon onset in the South China Sea (SCS), it is suggested an objective atmospheric circulation way to define the onset dates of SCS southwest monsoon. After compared to the onset dates defined by different indexes. We believe that the onset dates defined by the atmospheric circulation way are reasonable. The onset dates of the SCS southwest monsoon from 1950~ 1999 are given. It is found by compositive analyses and correlation analyses that the prophase distribution of the tropical Pacific SST is an important impact factor for the SCS monsoon onset. The higher SST in the eastern ocean of the Philippine and lower SST in the eastern area of the central equator Pacific will lead to a weaker western Pacific high in the lower layer, deeper oceanic trough of the central and eastern Pacific in higher layer, stronger westerly in tropical Indian Ocean, stronger Walker circulation in the region of the Indian-Pacific Ocean. They give the suitable conditions for convection development in the Bay of Bengal and the South China Sea. Under the effect of the strong westerly in the Bay of Bengal, it is easier and earlier that the convection in the SCS develops and the weaker western subtopic high extracts from the SCS, resulting in the earlier onset of the SCS monsoon, and vice versa.