Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onset

This article explores the impacts of the Indian Ocean basin-scale sea surface temperature anomaly (SSTA) on the South China Sea (SCS) summer monsoon onset. Basin-wide warming in the tropical Indian Ocean (TIO) is found to occur in the spring following an El Nino event, and the opposite occurs for a La Nina event. Such changes of the Indian Ocean SSTA apparently prolong the El Nino-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the Philippine Sea anti-cyclone. Warming in the TIO induces an anomalous reversed Walker circulation over the tropical Indo-Pacific Ocean, which leads to descending motion, and hence suppressed convection in the western Pacific. The intensified Philippine Sea anticyclone in May and June advances more westward and prevents the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late SCS monsoon onset. The case is opposite for the TIO cooling such that the Philippine Sea anti-cyclone weakens and retreats eastward, thus favouring an early onset of the SCS monsoon. Copyright (C) 2008 Royal Meteorological Society