A Preliminary Analysis On A Sharp Turn From Drought To Flood In The Middle And Lower Reaches Of The Yangtze River In2011

In this paper, global reanalysis data were provided by NCEP/NCAR, precipitation data at740observational stations of China were provided by the National Climate Center of the China Meteorological Administration, and grid data of precipitation in2011were provided by National Meteorological Information Center. Using above data, we analyzed the phenomenon (the sharp turn from drought to flood in the middle and lower reaches of Yangtze River) in early June2011from the standpoint of precipitation characteristics, water vapor transfer anomaly, atmospheric circulation anomaly before and after the sharp turn, and sea surface temperature anomaly. Furthermore, we also discussed the mechanism of this phenomenon from the aspects of ocean and atmosphere response caused by ocean. In the end, summarizing the result of above, we established a synoptic concept model.Firstly, the precipitation was less than corresponding climate condition from January to May in2011in the middle and lower reaches of the Yangtze River. But it suddenly surged in June after the sharp turn and leading to the flood in the middle and lower reaches of the Yangtze River. Further analysis show that this sharp turn occurred in the area of27-32°N and110-120°N (the middle and lower reaches of the Yangtze River) was the most serious event of sharp turn from drought to flood since1951, and its kick point was at the1st pentad in JuneSecondly, analysis by the first two modes of EOF analysis on water vapor transfer indicated that, compared with corresponding climate state, the total water vapor in South China was less and its strength was weaker from January to May in2011. However, it changed greatly in the period of sharp turn. Using Hysplit backward mode, we could observe the change of water vapor directly. Before the sharp turn, influenced by the abnormal East Asia Deep Trough the air point in120°E、30°N moved from The Korean peninsula toward southwest, which shows that it was hard to transfer water vapor to the middle and lower reaches of the Yangtze River. But, after the sharp turn, it changed.On the basis of above, we analyzed the atmospheric circulation before and after the sharp turn. It indicated that before the sharp turn, the pattern that summer monsoon in southern hemisphere was weak, and winter monsoon in northern hemisphere was strong was steady and which lead the delay of monsoon transform. Thus, less rainfall influenced by all of that factors occurred in south China eventually. In the period of sharp turn, the circulation adjusted quickly, especially the western Pacific subtropical high which jump to north and extended to west abruptly. At the same time, the strong East Asian Trough maintained in the west and blocking high in the Okhotsk Sea became weaken. All of that leaded the convergence of cold and warm air in the middle and lower reaches of the Yangtze River and leaded the continuation precipitation. Thus, the sharp turn from drought to flood occurred.Besides, SVD analysis between precipitation and sea surface temperature and anomaly analysis on SST shows that the early signals of this event exist in Indian Ocean and the equatorial Pacific Middle East. And it belong to a abnormal mode with smaller variance contribution.In the end, combined the analysis of above and Hadley circulation, aiming at this sharp turn from drought to flood, we eventually established a synoptic concept model to discuss the mechanism of the phenomenon in this paper.Before the sharp the sea surface temperature in the equatorial Pacific Middle East was negative anomaly, and it was a stronger La Nina. At the same time, it was controlled by negative SST anomaly in equatorial Indian Ocean on a large scale. Influenced by the both, Stretched ridge west point of the western Pacific subtropical high located on the east. Hadley circulation over the Indian Ocean was weaker and Walker circulation over the Pacific Ocean was stronger than climate state. Under the function of that, it leaded to a sustained and serious drought in the middle and lower reaches of the Yangtze River. Along with the change of sst anomaly in the both ocean, the atmospheric circulation adjusted. It presented that it extended toward west suddenly of the western Pacific subtropical high, became steady. Hadley circulation over the Indian Ocean changed to be stronger, and the Walker circulation over the Pacific Ocean became weak. The stable circulation in middle and high latitude areas also contributed to the convergence of the cold and warm air, leading to vast precipitation over the middle and lower reaches of the Yangtze River. Finally, the sharp turn occurred.