关键词: 夏季降水, 长江中下游, 大气环流, 全球变暖

Abstract: There were a series of severe floods along the middle to lower valley of Changjiang River of China during the 1990s. The extensive summer （June, July and August） precipitation is mostly responsible for the flooding. The summer rainfall in the 1980s and the 1990s is much higher than that in the prior 3 decades. The means for 1990～1999 is +87 62 mm above normal, marked the 1990s the wettest decade since 1951. Six stations are selected to establish century long rainfall series. These two series correlate at 0 92 for the period 1951～1999. It is found that the 1990s is also the wettest decade during the last 120 years. Four of the all 12 abnormal wet years, in which more than +40% of the rainfall over the normal, has occurred in the 1990s. Composite analysis for the six samples since 1951 presented that the atmospheric circulation anomalies associated with the surplus rainfall along the middle to lower valley of Changjiang River show positive southern wind velocity and moisture flux anomalies in southern China and negative values in northern China in the lower troposphere. There are strong northern winds during the 1990s over the northern China, which may be responsible for the simultaneous summer rainfall along the Changjiang River valley. Although there are some uncertainties in the climate models, it is strongly suggested that the summer rainfall would increase under the global warming as the IPCC modeling results demonstrated. Frequencies of summer rainfall types during the Medieval Warm Period and the Little Ice Age were compared to the normal （AD950 to 1999）. Contrast of greater frequency of types 2 and 1b in MWP and of types 4 and 3 in LIA is exciting. It proves again the statement that warm/dry （or cold/wet） climate was predominated in north and warm/wet （or cold/dry） climate in southeast China.

Key words: summer rainfall, Changjiang River, atmospheric circulation, global warming