Remote and Local SST Forcing in Shaping Asian-Australian Monsoon Anomalies

Abstract

The most striking feature of the Asian-Australian monsoon associated with the El Nino tele- connection is the evolution of anomalous anticyclones over the western North Pacific (WNP) and south- east Indian Ocean (SIO). In this study we investigated the relative role of remote and local SST forcing in shaping the monsoon anomalies with an atmospheric general circulation model (AGCM). Four ideal- ized AGCM experiments were designed to isolate the effect of anomalous SST forcing from the tropical eastern Pacific, tropical western Pacific and tropical Indian Ocean. In the first experiment observed SST is specified in the tropical eastern Pacific, while climatological monthly mean SST is specified elsewhere. In the second experiment the observed SST is specified in the tropical western Pacific only. In the third experiment realistic SST is specified in both the tropical Indian Ocean and eastern Pacific. In the fourth experiment the observed SST is specified across the tropical Indian and Pacific Oceans. Our numerical experiments indicate that the anomalous anticyclone in the WNP is initiated by local SST anomaly (SSTA) forcing in northern fall, and further maintained by both the remote (El Nino) and local SSTA forcing. The initiation of the anomalous anticyclone over the SIO is primarily attributed to the local SSTA, though the remote forcing from the eastern Pacific also plays a role, particularly in 1997. The numerical experiments reveal a seasonal-dependent character of inter-basin teleconnection between the tropical Pacific and Indian Oceans, this is, the Indian Ocean SSTA exerts a significant impact on the western Pacific wind in northern summer and fall of the El Nino developing year, whereas the eastern Pacific SSTA has a greater impact on the Indian Ocean wind during the mature phase of the El Nino (boreal winter), even though the central Pacific heating is stronger in boreal summer. A special feature for 1997-98 El Nino is that the meridional wind anomaly over the Indian Ocean in DJF is primarily driven by local SSTA forcing, while the zonal wind component is forced by the remote SSTA in the east- ern Pacific.