Delayed withdrawal of the autumn rainy season over central Vietnam in recent decades

Abstract

ABSTRACTA significant delay in the autumn rainy season withdrawal (ARSW) along the central coast of Vietnam (CCV) is detected since 1993, and the associated dynamics are discussed. During 1979–1992, the mean ARSW date was in early December, which is three pentads earlier than that during 1993–2006. Since rainfall over the CCV is primarily produced by cold surge vortices formed by the interaction of easterly waves with the cold surge flow, the ARSW is characterized by a gradual equatorward propagation (retreat) of the northeasterly winter monsoon (tropical easterlies). Therefore, the relatively late ARSW in the recent epoch is determined by the intensification and delayed withdrawal of the tropical easterlies during December, which should cause an increase in the easterly wave activity over the Philippine Sea and South China Sea (SCS), in response to the significant warm sea surface temperature (SST) in the western Pacific and cold SST in the central‐eastern Pacific. Correspondingly, since 1993, remarkable increases of convection and water vapour flux convergence around the CCV have occurred in December. The decadal environmental circulation changes also favour an enhancement of the intraseasonal oscillation (ISO) activities on 30–60‐, 12–24‐, and 5‐day scales that maintained rainfall along the CCV. Another potential factor is a distinct increase in the number of tropical cyclones (TCs) passing through the southern SCS during 1993–2006 compared with those occurring during 1979–1992, which is related to a strengthening and more westward‐extended sub‐tropical high and a significant SST warming in the tropical western Pacific. This enhanced ISO and TC activity, and the markedly delayed ARSW, originated from the decadal circulation changes, may be attributed to a mean state change in the Pacific basin since the mid‐to‐late 1990s characterized by a grand La Niña‐like pattern, which also results in the simultaneous advance of the Asian summer monsoon onset.