A Preliminary Research on the Reason of a Sharp Turn from Drought to Flood in the Middle and Lower Reaches of the Yangtze River in Late Spring and Early Summer of 2011
2011年春末夏初长江中下游地区旱涝急转成因初探

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 (27oN-32oN, 110oE-120oE), 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 Ni?a 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.