Influence of the Monthly Variation of the East Asia Westerly Jet on Summer Rainfall in China

The East Asian westerly jet (EAWJ) is one of the most important circulation systems in East Asia, linking the high and low latitude circulations of East Asia. Based on NCEP/DOE reanalysis data and monthly rainfall data at 160 meteorological stations in China during 1981-2010, the monthly relationship between the position and intensity variation of the EAWJ and summer rainfall in China was analyzed by using the statistical diagnosis method. It was found that a southward-moving EAWJ favors Jiangnan-Huanan rainfall in June, the Yangtze River-Huaihe River valley rainfall in July, and rainfall in the middle-upper reaches of the Yangtze River valley in August. Analyses of the high-low latitude circulation characteristics associated with the abnormal EAWJ of June 2010, July 2007, and August 2006 as well as the physical process of the influences of the EAWJ on rainfall were emphasized in this study. The results showed that in June 2010, the EAWJ moved southward to 35°N where an anomalous upper-level divergence (convergence) and a low-level convergence (divergence) occurred south (north) of the EAWJ, and which induced significant ascending (descending) motion. The anomalous vertical motion together with west vertically integrated water vapor transportation offered favorable dynamic and vapor conditions for persistent precipitation over the Jiangnan-Huanan region. During July 2007, the EAWJ was weaker and located south of its mean climatic position. In addition, the perturbation of the EAWJ was stronger than normal. In this case, the stationary rainy band over the Huaihe River valley was mainly caused by the strong cold air activities in the middle-high latitudes of East Asia which were due to the strong perturbation of the EAWJ. However, during August 2006 the EAWJ was stronger and located abnormally northward, favoring the westward and northward motions of the western Pacific subtropical high. In this case, weather conditions in the Sichuan-Chongqing region of southwestern China were controlled by subtropical high pressure, resulting in severe drought and heat waves.