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 CQhina Sea (SCS) summer monsoon onset. Basin-wide warming in the tropical Indian Ocean (TIO) is found to occur in the spring following an El Niño event, and the opposite occurs for a La Niña event. Such changes of the Indian Ocean SSTA apparently prolong the El Niño-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 anti-cyclone 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 © 2008 Royal Meteorological Society.