Basin-wide Warming in the Equatorial Indian Ocean Associated with El Niño

Abstract

A physical process of basin-wide warming in the equatorial Indian Ocean (IO) is examined in view of the seasonally different coupling processes between the monsoon circulation and the modulated Walker circulation associated with major El Niño. In the composite analysis of six warm episodes, the reversed Walker circulation along the equator appears over the tropical IO through the Pacific Ocean, resulting in anomalous surface easterlies over the equatorial IO prior to the mature phase of the El Niño. From boreal fall to winter, the divergent easterly wind anomalies prevent the development of the climatological equatorial westerly. Using a reduced gravity ocean model, it is shown that the modulated surface flow over the equatorial IO has a potential to contribute to the basin-wide warming through the heat exchange of the ocean surface. These results imply that the strong seasonality involved in monsoon is important to understand the seasonal phase-locked feature of interannual SST anomalies.