Abnormal drought and flood disasters in the area between the Yangtze River and Huaihe River in the summer in 1991 and 1994 were observed respectively. They are caused by anomalous changes of the main general circulation systems in the Northern Hemisphere,such as the intensity and position of the polar vortex, long-wave trough and ridge in the westerlies, the west Pacific subtropical high at 500hPa,and South Asia high at 100hPa,as well as that of SST in the tropical ocean.