PDF《东亚夏季风爆发和北进的年际变化特征及其 与热带西太平洋热...》

1 期2005 年1月大气科学Chinese Journal of Atmospheric Sciences Vol. 29,No.

1 Jan.

2005 western Pacific along 137?E and the NCEP/NCAR reanalysis data. The results show that when the tropical western Pacific is in a warming state in spring,convective activities are intensified around the Philippines. In this case,there is a cyclonic anomaly circulation in the lower troposphere over the South China Sea,and the western Pacific subtropical high shifts eastward,thus,the early onset of the South China Sea summer monsoon (SCSM)can be caused. Moreover,when the tropical western Pacific is also in a warming state,convective activities around the Philippines are also strong in summer,since the western Pacific subtropical high abruptly shifts northward in mid-June and early July during its northward advance in this case,the abrupt northward-shift of the East Asian summer monsoon rainband from South China to the Yangtze River and the Huaihe River valleys is obvious in mid-June and this monsoon rainband again jumps northward from the Yangtze River and the Huaihe River valleys to the Yellow River valley,North China and Northeast China in early July. This can cause that summer monsoon rainfall is below normal and drought may occur in the Yangtze River and the Huaihe River valleys and the middle and lower reaches of the Yangtze River,but summer rainfall is normal or above normal in the Yellow River valley,North China and Northeast China. On the other hand,when the tropical western Pacific is in a cooling state in spring,convective activities are weakened around the Philippines. In this case, there is an anticyclonic anomaly circulation in the lower troposphere over the South China Sea,and the western Pacific subtropical high shifts westward,thus,the late onset of the SCSM can be caused. Moreover,when the tropical western Pacific is also in a cooling state,convective activities around the Philippines are also weak in summer,since the abrupt northward-shift of the western Pacific subtropical high is not obvious in mid-June and early July and it gradually shifts northward during its northward advance in this case,the East Asian summer monsoon rainband can be maintained in the Yangtze River and the Huaihe River valleys. This can cause that the summer monsoon rainfall is above normal and flood may occur in these two valleys,but summer rainfall is below normal and drought may occur in the Yellow River valley, North China and Northeast China. It is also explained by using the dynamical theory of nonlinear multiple equilibrium that the strong or weak convective activities around the Philippines play an important role in the abrupt or gradual northward shift of the western Pacific subtropical high. Key words summer m........