Understanding the Impacts of the Indian Ocean on ENSO Variability in a Coupled GCM

This study investigates the impacts of the Indian Ocean on El Niño–Southern Oscillation (ENSO) variability through numerical simulations with a coupled atmosphere–ocean general circulation model, composite analyses with the coupled model output, and simple atmosphere model experiments with specified sea surface temperature (SST) forcing. It is found that, when the Indian Ocean is decoupled from the atmosphere, the ENSO variability in the coupled model is significantly reduced. Conditional SST distributions indicate that the warm (cold) ENSO state is stronger and occurs more frequently when the Indian Ocean SST in summer is relatively cold (warm), whereas it is weaker and occurs less frequently when the Indian Ocean is relatively warm (cold). The impacts of the Indian Ocean are suggested by a comparison of SST composites under warm, normal, and cold Indian Ocean SST conditions in the developing stage of ENSO. It is demonstrated that the Indian Ocean affects the ENSO variability through modulating convective heating over the Indian Ocean and the Walker circulation over the tropical Indian and Pacific Oceans. Warmer (colder) Indian Ocean SST induces easterly (westerly) surface wind anomalies over the eastern Indian Ocean and western-central equatorial Pacific. Numerical experiments of a simple atmosphere model with specified SST forcing support the roles of imposed Indian Ocean SST anomalies. The applicability of the model results to nature is discussed. It is shown that the observed SST anomalies in the Indian Ocean were out of phase with those in the Pacific Ocean in some ENSO developing years. As such, the Indian Ocean SST anomalies could contribute to the intensity of ENSO. This impact is significant for the cold ENSO events, and there is some evidence for this impact during some warm ENSO events.