Impacts of the Indian Ocean on the Indian Summer Monsoon–ENSO Relationship

Abstract

This study investigates the impacts of the Indian Ocean on the relationship between the Indian summer monsoon and the El Nino-Southern Oscillation (ENSO) through numerical simulations with a coupled atmosphere-ocean general circulation model and atmospheric general circulation model (AGCM) experiments with specified sea surface temperature (SST) and surface heat flux (SHF) forcing. Previous studies have shown that this particular coupled model captures many aspects of the observed Indian summer monsoon-ENSO relationship. However, it is found that, when the Indian Ocean is decoupled from the atmosphere, the Indian monsoon-ENSO relationship reverses. This change is linked to the relationships between surface evaporation, surface wind, and SST in the north Indian Ocean. In the coupled case, surface evaporation anomalies are positively correlated with surface wind anomalies during April-June and are of the same sign as SST anomalies during July-September. In the decoupled Indian Ocean case, surface evaporation anomalies are of the same sign as surface wind anomalies during the entire April-September period. Numerical experiments with an AGCM were performed with SST or SHF anomalies specified in the tropical Indian-Pacific Ocean, tropical Pacific Ocean only, and tropical Indian Ocean only. These experiments confirm the importance of local coupled air-sea feedback in the Indian Ocean for a proper simulation of the Indian monsoon-ENSO relationship.