Analysis of characteristics of a sharp turn from drought to flood in the middle and lower reaches of the Yangtze River in spring and summer in 2011

Abstract

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.