Anomalous Midsummer Rainfall in Yangtze River-Huaihe River Valleys and Its Association with the East Asia Westerly Jet

In this study, the interannual and interdecadal relationship between midsummer Yangtze River-Huaihe River valley (YHRV) rainfall and the position of the East Asia westerly jet (EAWJ) were investigated. The midsummer YHRV rainfall was found to significantly increase after the 1980s. Moreover, the location of the EAWJ was found abnormally south of the climatic mean during 1980-2008 (ID2) compared to 1951-1979 (ID1). During ID2, associated with the southward movement of the EAWJ, an anomalous upper-level convergence occurred over middle-high latitudes (35 degrees-55 degrees N) and divergence occurred over lower latitudes (similar to 30 degrees N) of East Asia. Correspondingly, anomalous descending and ascending motion was observed in middle-high and lower latitudes along 90 degrees-130 degrees E, respectively, favoring more precipitation over YHRV. On an interannual time scale, the EAWJ and YHRV rainfall exhibited similar relationships during the two periods. When the EAWJ was centered abnormally southward, rainfall over YHRV tended to increase. However, EMU-related circulations were significantly different during the two periods. During ID1, the circulation of the southward-moving EAWJ exhibited alternating positive negative positive distributions from low to middle high latitudes along the East Asian coast; the most significant anomaly appeared west of the Okhotsk Sea. However, during ID2 the EAWJ was more closely correlated with the tropical and subtropical circulations. Significant differences between ID1 and ID2 were also recorded sea surface temperatures (SSTs). During ID1, the EAWJ was influenced by the extratropical SST over the northern Pacific; however, the EAWJ was more significantly affected by the SST of the tropical western Pacific during ID2.