Intraseasonal northward evolution of the extreme autumn rainfall event in West China in 2021

Abstract

An excessive “autumn rain of West China” (ARWC) event, concentrated between August 23 and October 7, 2021, produced the greatest amount of precipitation recorded during 1961–2021. This event exhibited notable intraseasonal northward movement of rainfall from the first phase (P1; August 23–September 7) to the second phase (P2; September 17–October 7). Analysis links this event to two key atmospheric factors: the western North Pacific anticyclone (WNPAC) and the East Asian subtropical westerly jet (EAJ). Persistent and robust contrast in convection between the Maritime Continent (MC) and the central tropical Pacific strengthened the WNPAC, transporting moisture to West China. Simultaneously, the EAJ, positioned northward of West China, enhanced upward motion contributing to the excessive ARWC. During P1, anomalous cyclone prevails over the regions from northern China to Japan, due to the combined effects of tropical northward wave trains and quasi-barotropic wave trains from the North Pacific. This cyclonic circulation not only induces a southward shift in the EAJ but also restricts the northward expansion of the WNPAC, favoring its zonal development. In P2, the weakening of convective activity in the MC, coupled with changes in basic flow, leads to the attenuation of WNPAC. Additionally, influenced by the North Atlantic transient wave activity and wave trains originating from North Pacific, there is a transition in the mid-high latitudes wave trains, resulting in the presence of an anomalous anticyclone over Northeast Asia. Which drives the EAJ northward and facilitates the northeastward development of WNPAC. These factors contribute to a significant intraseasonal northward shift of ARWC in 2021. Such underlying mechanisms were also verified by using the dry Linear Baroclinic Model.