Contrast in the East Asian subtropical westerly jet and its association with precipitation in China between early summer and midsummer

Abstract

The interannual variations of the East Asian subtropical westerly jet (EASWJ) and its related precipitation and atmospheric circulations during early summer and midsummer were investigated. The possible causes for the contrast between early summer and midsummer were also investigated. The results indicate that, in early summer, the EASWJ is influenced by a teleconnection pattern spanning the middle to high latitude region from the North Atlantic to the East Asian coast. Associated with the eastward pointing wave activity flux (WAF) along this zonal propagated wave train, the height anomaly in the middle to high latitude of East Asia exhibits alternating negative−positive−negative distributions in abnormal southward EASWJ years, resulting in anomalous convergence and increased precipitation in regions south of the Yangtze River (SYR). However, in midsummer, the EASWJ is more closely related to lower latitude circulations. A south-to-north propagated WAF, which originates from the South China Sea and tropical western Pacific, is responsible for the maintenance of the anomalous EASWJ and western Pacific subtropical high. Associated with the stronger and westward shifted western Pacific subtropical high and abnormal southward EASWJ, precipitation increases in the Yangtze-Huaihe River Valley and decreases SYR, and vice versa. Contrast in the EASWJ-related preceding sea surface temperature (SST) between early summer and midsummer were also investigated. In early summer, the meridional displacement of the EASWJ was significantly influenced by the North Atlantic SST in the preceding March–April period, whereas in midsummer the EASWJ displacement was strongly connected to the tropical Pacific SST in the preceding April–June period.