The distribution of large-scale drought/flood of summer in China under different configurations of monsoon and polar vortex

Abstract

From the view of warm and cold system configuration, an East Asian summer monsoon (EASM) index and the Asian polar vortex area (APVA) index in July were selected, and their climatic characteristics and relationships with the distribution of summer rainfall over China were analyzed. On this basis, the relationships between the EASM and APVA in July from 1951 to 2010 were divided into four types: type A, stronger monsoon and bigger polar vortex area; type B, stronger monsoon and smaller polar vortex; type C, weaker monsoon and bigger polar vortex; type D, weaker monsoon and smaller polar vortex. Based on the method of composite analysis, we found that on the seasonal scale, large-scale distributions of summer droughts and floods over China showed multi-faceted characteristics in different configurations of the EASM and the APVA in July. In A type years, the summer rainfall of China was overall less than normal, the region between eastern parts of Northwest and the western parts of North China, and the eastern regions of Northeast suffered from flood, while Yangtze River valley suffered from drought; in B type years, it had flood in South and drought in North, and the precipitation in the south/north of northern regions in the Yangtze River valley was more/less than normal; in C type years, North China was flooded and South of China suffered from drought, while the precipitation in the northern parts of the Huaihe River valley was more than normal; and in the middle and lower reaches of Yangtze River and to its south the precipitation was less than normal; in D type years, the overall summer rainfall was more than normal, with the precipitation core lying in the middle and lower reaches of Yangtze River. Thus, it can be seen that the distribution of summer rainfall is not only related to the EASM, but also to the PAVA in July. Under different configurations of the both, the precipitation and its distribution display obvious difference and regularity, which, to some extent, have effects on predicting the total trend of summer rainfall. Besides, by discussing the anomaly characteristics of atmospheric circulation due to different configurations and by analyzing the influences of different meteorological factors on the relationship between the monsoon and the polar vortex, we can conclude that the difference and regularity of summer rainfall directly depend on the configurations of circulation. Among them, the western Pacific subtropical high and the blocking situation in the mid and high latitudes are the overriding factors.