Using some kinds of meteorological factors data provided by NCEP/NCAR NOAA and National Climate Center (NCC), and NOAA-Hysplit model to preliminarily analyze characteristics of precipitation and mechanism of a sharp turn from drought to flood in the middle and lower reaches of the Yangtze River in the early summer of 2011, the authors establish a synoptic concept model and conclude that: 1) The most serious sharp turn from drought to flood occurred in the middle and lower reaches of the Yangtze River, especially in the region of (27°N-32°N, 110°E-120°E), in the 1st pentad in June (around 3 June); 2) analysis of sea surface temperature (SST) anomaly and the singular value decomposition (SVD) between SST and precipitation shows that the variance contribution of this sharp turn is smaller and it is an anomalous event, whose early signals exist in the equatorial middle an eastern Pacific and the equatorial Indian Ocean; 3) analysis of vorticity low frequency oscillation and 10-30 d stable component indicates that dynamic mechanism across the Pacific Ocean has a great influence on precipitation in the middle and lower reaches of the Yangtze River in June, and the stable circulation in middle and high latitude areas provides a large scale circulation background field; 4) before the sharp turn, there existed a stronger La Nina in the equatorial middle an eastern Pacific, and the equatorial Indian Ocean was controlled by negative SST anomaly on a large scale, influenced by the both, the western Pacific subtropical high was abnormally eastward, the Walker circulation was stronger, and the Hadley circulation over the Indian Ocean was weaker. All of those leaded to the lack of water vapor transport from southwest and southeast, thus a serious and sustained drought occurred in the middle and lower reaches of the Yangtze River. As the SST anomaly in both the oceans weakened, the western Pacific subtropical high suddenly extended westward and became stable (in the 1st pentad in June), the Walker circulation became weak, and the Hadley circulation over the Indian Ocean became stronger. Matching with the stable circulation on a large scale in middle and high latitude areas, it resulted in continuous converging of cold and warm air and vast precipitation over the middle and lower reaches of the Yangtze River. Finally, the sharp turn happened.%选用NCEP/NCAR、NOAA、国家气候中心(NCC)提供的各要素资料及NOAA-Hysplit模型,对2011年春末夏初发生在中国长江中下游地区旱涝急转的降水异常事件及其影响机制进行初步分析,并建立天气学概念模型.结果表明:(1)长江中下游地区,尤其是(27°N～～32°N,l10°E～120°E)区域在6月第1候(6月3日左右)发生了1951年以来最严重的一次旱涝急转事件；(2)海温距平及SVD分析表明,此次事件属于方差贡献较小的异常模态,其前期信号存在于赤道中东太平洋及赤道印度洋；(3)涡度低频振荡分析表明太平洋上空动力机制对6月长江中下游降水有重要的作用,在10～30天稳定分量分析中发现中高纬环流形势持续稳定为此次事件提供大尺度环流背景场；(4)前期赤道中东太平洋为较强的拉尼娜事件,赤道印度洋为大范围的异常冷海温控制,受其影响,副热带高压异常偏东、Walker环流偏强、印度洋Hadley环流偏弱,中国西南、东南水汽输送不足,致使长江中下游地区急转前发生持续严重干旱.随着赤道中东太平洋及印度洋冷海温异常的减弱,副热带高压的突然异常西伸(6月第1候)且持续、Walker环流减弱、印度洋Hadley环流增强,匹配中高纬度大尺度稳定维持的气候背景,使冷暖空气在长江中下游地区持续交绥,该地区降水异常偏多并持续,导致6月长江中下游持续性洪涝,最终造成此次“旱涝急转”事件的发生.
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