On the Process of Summer Monsoon Onset over East Asia
东亚地区夏季风爆发过程

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 and NCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997, the distri- bution of rainy season onset over Asian area from spring to summer is discussed The results show that there exists two stages of rainy season onset over East Asia region from spring to summer-rainy season onset accompanying subtropical monsoon and tropical monsoon respectively. The former started over north of South China and south of the Yangtze River basin in early April and them extended southward to South China coastal regions and westsouthward to the Indo-China Peninsula by the end of April. The corresponding rain belt is mainly formed by the convergence of cold air and the recurved southwesterly flow from the western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurred during winter to spring over South Asia. We usually call it as "starting period of pre-flood rainy season over South china". So it should belong to the onset of subtropical monsoon rainy season. The latter is formed in the process of subtropical monsoon rainy belt over inshore regions of South China Sea originally came from the Yangtze River basin advancing with northward shift of subtropical high after the onset of tropical monsoon over South China Sea. The pre-flood rainy season over South China region then came into mature period and the second peak of rainy season appeared, "plum season", rainy season over the Yangtze River basin-the Huihe River ba- sin and North China then formed consequently. Meanwhile the tropical monsoon rain belt has propa- gated westward to South Asia and formed tropical monsoon rainy season. The process of summer tropi- cal monsoon onset over South China Sea in 1998 is also discussed it indicated that the monsoon during summer tropical monsoon onset over South China Sea is the result of the westerly flow over middle of South China Sea from the new generated cyclone formed in north subtropical high entering into South China Sea converged with the tropical southwesterly flow recurved by the "intensified cross-equatorial flow".Therefore the monsoon onset over SCS is mainly caused by the seasonal variation of circulation system in East Asian monsoonal system but has no relationship with Indian monsoon. During monsoon onset over South China Sea, Arabian Sea is still controlled by subtropical anticyclone, South Asia is controlled by the recurved westerly flow of the south-moving northwesterly flow from north part of pre-mentioned subtropical anticyclone, and the Somali low-level jet has not outbroken yet. The equa- torial westerly over Southern Asia does not affect South China Sea.