ADVANCE AND RETREAT OF THE SUMMER MONSOON IN CHINA

By using the daily-14 year(1983-1996)NCEP/NCAR 2.5°×2.5° reanalysis data,wecarefully study in each pentad the advance and retreat of the summer monsoon in China and givento it a new definition.This definition considered the intensity of southwesterly winds at 850 h Patogether with its degree in temperature and moisture.The result revealed that:(1)The advance of the summer monsoon in China shows three abrupt northward shifts andfour relatively stationary stays.The four stable stages correspond to the peak of the pro-summerrainy period in South China,the"Meiyu"season in the Changjiang(Yangtze)-Huaihe RiverValleys.the rainy season in the downstream of the Huanghe (Yellow) Riyer Valleys and the rainyseason in northern China.The retreat of the summer monsoon is so fast that it totally retreatsfrom the mainland at about the mid-August.(2)The northward advance of summer monsoon in China is basically controlled by theseasonal variation latitudinally of the upper level planetary westerlies.It is in roughly accord withthe temporal variation in the position of 15 m s-1 isotaeh at 200 hPa.The fast retreat of thesummer monsoon is mainly due to the blocking effect of the Tibetan Plateau.(3)The advance of 500 hPa subtropical high of the western Pacific is also in aecordanee withthe advance of the summer monsoon in China.During the advancement of the summer monsoon,the eastward movement of the subtropical high shows great meaning that it creates the essentialcondition for the convergence of southward intrusion cold airs with the warm and humid southwesterly winds,which result in precipitation.There are three manifest eastward movementsof the subtropical high during its northward advancement.They coincide correspondingly to thebeginning of the peak of the pre-summer rainy period in South China,the"Meiyu"season in the Changjiang(Yangtze)-Huaihe River Valleys and finally the rainy season in northern China.Thewestern part of the subtropical high moves eastward to the region of Japan in late July and thebeginning of August.It then stays there for quite a long time which results in the straight movement of cold airs intruding from the north to the east of Tibetan Plateau,i.e.the easternregion of China.This provides good condition for the fast southward retreat of the summermonsoon.(4)The intensifieation and development of the Tibetan high at 200 hPa are closely related to the eastward movement of the subtropical high,they often occur simultaneously.