This study investigates the impacts of the Indian Ocean on El Niņo-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 reduced significantly. Probability density function indicates that the warm (cold) ENSO is stronger and more frequent when the Indian Ocean SST in summer is lower (higher), whereas it is weaker and less frequent when the Indian Ocean is higher (lower). The impacts of the Indian Ocean are verified by a comparison of SST composites under high, normal, and low Indian Ocean SST conditions in the developing stage of ENSO.

It is demonstrated that the Indian Ocean affects the ENSO variability through modulating the Walker circulation over the tropical Indian and Pacific Oceans. Higher (lower) 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 the reality 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 warm ENSO events.