2011年春夏季节长江中下游旱涝急转可能成因

利用1980-2011年观测的降水和美国NCEP/NCAR大气再分析等资料，采用经验正交分解（EOF）和线性回归等统计方法，讨论了2011年发生在长江中下游地区春夏旱涝急转的成因。结果表明：该年1-5月持续的La Nina事件导致西北太平洋副热带高压位置位于125°E以东，西北太平洋副热带高压西北侧的西南风伴随的水汽通量无法达到长江流域，从而导致了2011年长江流域1-5月份的持续性干旱现象。伴随La Nina的减弱，6月份的青藏高原的感热明显增强，诱发西北太平洋副热带高压自东向西北方向移动到110°E，引导西南风水汽向长江流域输送，而青藏高原对流的加强和向东移动与来自西北太平洋副热带高压西北侧的西南气流在长江中下游地区的汇合导致了6月份长江中下游降水的急剧增加，从而形成了2011年长江中下游地区春夏季节的旱涝急转。

Possible causes of abrupt turning from drought to flooding in the middle and lower reaches of Yangtze River valley during spring to summer of 2011

Based on observed precipitation and the NCEP/NCAR reanalyzed datasets from 1980 to 2011,possible causes of abrupt turning from drought to flooding in the middle and lower reaches of Yangtze River valley （YRV） region during spring to early summer of 20 1 1 were discussed using methods of an EOF and a linear regression a-nalysis.The results indicate that the persistent La Nina events during January to May of 2011 maintain the 125°E eastward shift of western North Pacific subtropical high （WNPSH）,thus the southwesterly moisture flux fails to reach the YRV region,which leads to the persistent drought events from January to May of 20 1 1 .However,sensi-tive heating flux enhances in the Tibetan Plateau （TP）with the decaying of La Nin？a in June,which induces a westward shift of WNPSH moving to 1 10°E from east to northwest and makes water vapor from southwesterly wind transporting to the YRV.Convection from the TP enhances and moves to east,and it meets southwest air flow from the northwest side of WNPSH in the middle and lower reaches of the YRV,which leads to increase precipita-tion sharply,and then it forms abrupt turning from drought to flooding in 2011 .