Zonal Extension and Retraction of the Subtropical Westerly Jet Stream and Evolution of Precipitation over East Asia and the Western Pacific

Abstract

Abstract The relationship between the zonal extension and retraction of the East Asian subtropical westerly jet stream (EASWJ) and the seasonal evolution of precipitation is investigated. The EASWJ exhibits significant zonal variations associated with its seasonal evolution, which affects both large-scale atmospheric circulation and precipitation over East Asia and the North Pacific. The midlatitude rain belts and the intertropical convergence zone are located more eastward and southward during jet extensions compared to jet retractions. The seasonal cycle of the zonal variations of EASWJ can be divided into four periods: December–February, March–May, June–July, and September–November. Particularly in the mei-yu period of June–July, jet retraction events are associated with a northward shift of rain belts over eastern China, which may play an important role in the northward movement of the mei-yu rain belt to the Huaihe River valley of eastern China after 1999. The zonal extension and retraction of the jet stream are more likely to occur in the ENSO decaying phase. A composite analysis of diabatic heating and outgoing longwave radiation shows a tripolar anomaly pattern that is strongest in El Niño winter and is important for the zonal extension of the jet stream over the Pacific. This spatial structure is conducive to a strengthening local Hadley cell over the Pacific and can also appear in weak El Niño periods. On the other hand, there is no tripolar anomaly pattern for zonally retracted jet streams.