2011年春夏季长江中下游地区旱涝急转特征分析

摘要

本文选用美国环境预报中心／国家大气研究中心提供的全球再分析资料，中国气象局国家气候中心提供的中国740站降水资料及中国气象信息中心提供的中国区域2011年降水格点资料，对2011年长江中下游地区6月初旱涝急转现象及环流背景特征进行简要分析，结论如下：1）2011年1-5月长江中下游地区降水较气候平均值偏少且降水变率较小，6月长江中下游地区降水量急剧增多，发生旱涝急转现象，其转折点为第31候（6月第1候）；2）旱涝急转前后，水汽通量及其经验正交函数分解的第1，2模态的时间和空间演变均表现出弱水汽输送向强水汽输送转变的特征；3）转折前后大气环流场差异显著．急转前，北半球冬季风偏强，南半球夏季风偏弱，在春季，东亚大槽偏强导致季风的转换延迟，不利于南方暖湿气流的北上，南方降水偏少，并发生大范围持续性的严重干旱．6月初（急转期）环流迅速调整，西太平洋副高突然西伸北跳，东亚大槽异常偏西偏强、鄂海阻高减弱，使冷暖空气在长江中下游地区汇合，有利于该地区的降水发生并持续，是长江中下游地区由严重干旱向洪涝急转的主要原因．%We use global reanalysis data probided by NCEP/NCAR, precipitation data at 740 observational stations of China provided by the National Climate Center of the China Meteorological Administration, and grid data of precipitation in 2011 provided by National Meteorological Information Center to analyze the phenomenon of a sharp turn from drought to flood in the middle and lower reacher of Yangtze River in early June 2011, and the characteristics of its circulation background and briefly conclude as follows： 1） the precipitation in the middle and lower reaches of the Yangtze River was less and its change rate was smaller than that of corresponding climatological normals from January to May in 2011, both surged suddenly in June, leading to the appearence of a sharp turn from drought to flood in June, and the kickpoint was at the 31st pentad （the 1st pentad in June）; 2） around the sharp turn, both flood water vapor flux and the space-time evolution characteristics of the first and the second modes of EOF analysis represented the transform of water vapor transport from a weaker state to a stronger one; 3） before and after the turn, atmospheric circulation fields were significantly different. Before the sharp turn, winter monsoon in northern hemisphere was strong, and summer monsoon in southern hemisphere was weak, leading to the delay of monsoon tranform, stronger East Asian Trough, which went against warm-moist air blowing to the north. All of that eventually led to less rainfall in south China and occurance of this sharp turn. In early June, the period of turining, the circulation was adjusted quickly, which presented that the western Pacific subtropical high extended to west and jump to north abruptly, East Asian Trough kept strong and was maintained in the west, and blocking high located in the Okhotsk Sea weakened. Thus, cold and warm air converged in the middle and lower reaches of the Yangtze River and contributed to the occurance and continuation of precipitation. It is the main reason of the sharp turn from drought to flood in the middle and lower reaches of the Yangtze River.