Studies On The Definition Of The South China Sea Summer Monsoon Onset And Its Influence On The Precipitation Of China

South China Sea Summer Monsoon (SCSSM) which marks the arrival of the East Asian summer monsoon plays an important role in the establishment of Asian Summer Monsoon (ASM). Because of the close relation with the drought and flood, the movement of rain belts in the eastern China and the SCSSM, the research on the SCSSM activity will help improve the flood forecasting and disaster mitigation and prevention, especially in the development of the national economy and agricultural production.This paper not only reveals the climate average atmospheric circulation features before and after the onset of the SCSSM, but also analyzes the major global monsoon mode by using the MV-EOF method. Different definitions of the SCSSM onset are statistically analyzed to find out the key area (105°E-112.5°E,7.5°N-12.5°N) of the SCSSM and defined a new SCSSM onset index. We make in-depth research on the atmospheric circulation of the early and late years of the SCSSM and find out the main factors that influence the onset of the SCSSM. Finally, the author analyzes the impact of the onset of SCSSM on the precipitation in China.1. There are significant changes in the geopotential height and wind field of high and low level after the onset of SCSSM. The main changes at 850hPa are as follows: retreating of the subtropical high from SCS region, deepening of the India and Burma trough, weakening of the cyclone circulation in the southwest China, disappearance of the anti-cyclone in Arabia Sea, establishment of the cross-equatorial flow in the Somalia,85°E and 125°E regions. The seasonal movement of the planet wind at 200hPa is the major feature of seasonal transition of atmospheric circulation.2. We demonstrate that the primary climatological features of the tropical precipitation and high-low-level circulation can be represented by a three-parameter metrics:the solstitial mode, the equinoctial asymmetric mode and the equinoctial symmetric mode. Together, the two major modes of annual cycle can reflect the close relationship between the establishment of the SCSSM and the solar radiation and terrain. The third mode of half annual cycle explains the characteristics of the rainy weather in the equatorial region.3. In this paper, we determined a key area (105°E-112.5°E,7.5°N-12.5°N) and defined a new SCSSM onset index:(a) in the onset pentad the average zonal wind is above 0; (b) in the subsequent five pentads (including the onset pentad) the average zonal wind must be positive in at least two pentads, negative for less than 2 pentads continuously and the accumulative three pentad mean zonal wind must be positive.4. According to the SCSSM onset index defined in this paper, we define the early years of SCSSM onset as the years that the SCSSM breakout before pentad 26 (including pentad 26), including 1994,1996,1999,2001,2005 and 2007; the late years of SCSSM onset are the years that the SCSSM breakout after pentad 30 (including pentad 30), including 1982,1985,1987,1991 and 1993.5. During the former winter of the early years, the winter monsoon and the ascending motion of the Hadley Cell and Walker Cell in Nanhai region is stronger than that in the late years. The SST field present a La Nina mode which is helpful to the upward motion in Nanhai region.6. At 850hPa, if the East Asian trough is deeper, the subtropical high is weaker, the cyclone in the northern Bay of Bengal is stronger, then the SCSSM will onset earlier, otherwise, the SCSSM will onset later. At 200hPa, if the height in the Central Asian region is stronger, the height in the east China region is weaker, then the SCSSM will onset earlier, otherwise, the SCSSM will onset later. If the southwesterly wind in the SCS is stronger, the Somali cross-equatorial flow is stronger, the westerly wind in the 80-90°E region is stronger and the southeasterly wind in the South China is weaker, then the SCSSM will onset earlier, otherwise, the SCSSM will onset later.7. The precipitation in China during the early years advances the late years. During the early years, the precipitation in Hubei region, the lower Yangtze valley and the Hunan region is heavier than that in the late years, in contrary, the precipitation of the late years in the Dongbei region is heavier than in early years.8. We analyze the regression field of the normalized principal components of the first multi-variable EOF mode and make the difference analysis. The results are as follows:During the early years, the precipitation in Huabei and Huanan region is heavier than that in the late years while in contrary, the precipitation of the late years in Dongbei, Sichuan and Yunnan region is heavier than in early years.