Interannual Variation of the Indian Monsoon Simulated by the NCAR Community Climate Model: Effect of the Tropical Pacific SST

Abstract

Previous studies have shown diagnostically and statistically that the interannual variation of the Indian monsoon is closely correlated with the tropical Pacific sea surface temperature (SST). It seems likely that the interannual variation of the Indian monsoon results from the response of this monsoon system to the interannual variations of the Pacific SST. This hypothesis has not been substantiated in the past. In order to test it, Version 1 of the National Center for Atmospheric Research Community Climate Model (CCM) was used to perform two parallel climate simulations: a control run using the 12 calendar month climatological SST and a run using real-time Pacific SST. The SST data used in this study are derived from the Comprehensive 0cean-Atrnosphere Data set. Significant interannual variations of the Indian monsoon circulation are generated by the real-time Pacific SST experiment, but not the clmatological SST control experiment in real-time Pacific SST simulation weakened during the 1982 and 1987 summers and intensified in the 1984 and 1988 summers. The interannual variation of the model monsoon circulation resembles the observed in many ways. According to the linear theory of Matsuno and Gill, summertime stationary eddies are generated by steady tropical forcing. Because the Indian monsoon is a part of summertime stationary eddies, interannual variation of steady tropical heating induced by interannual Pacific SST anomalies results in interannual variations of summertime stationary eddies and the associated monsooon. Various diagnostic analyses are thus engaged to illustrate this explanation of the monsoon interannual variation.