THE LARGE AREA FLOOD AND DROUGHT OVER YANGTZE RIVER VALLEY AND ITS RELATION TO THE SOUTH ASIA HIGH

The temporal and spatial distributions of the summertime precipitation over Yangtze River Valley are examined in this paper by employing monthly mean observation data from 1958 to 1999. There is a distinct interdecadal trend in precipitation and the transition occurs in late 1970s. The significant contrast can he found in the persistent drought during 1960s and frequent occurrence of flood during 1990s. According to the standard deviation of the precipitation, the 6 severe flood years and 5 severe drought years are identified during the past 42 years. The obvious differences between flood year and drought year are found in the troposphere and stratosphere circulations as well as in the global SSTA.The subtropical high at both upper and lower troposphere shifts southward and becomes strengthened during the flood year, meanwhile the global SST exhibits large area positive anomalies,and vice versa during the drought year, The simultaneous correlation shows the significant positive correlation between SAH intensity index and precipitation anomalies in the Yangtze River Valley.When the SAH changes from a relative weak period to a strong period at the end of 1970s, the Yangtze River valley precipitation changs from a relative drought period to a flood period: whereas the relationship between the Yangtze River Valley precipitation and Equatorial Eastern Pacific SSTA is uncertain. The lead/llag correlation also shows the significant positive correlation between the intensity of winter/spring 100 hPa subtropical high and summertime Yangtze River Valley precipitation.The results of the paper suggest that due to the special geographic and land-sea distribution of China, the anomaly of the SAH has more direct impact on regional climate of China than that of ENSO. Thus we propose that the SAH should be considered as a strong anomalows signal and be paid more attention to.