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We assessed the performance of state-of-the-art coupled models in reproducing the equatorial undercurrent (EUC) in the Indian Ocean based on the outputs of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models and compared with the Phase 5 (CMIP5) models. Our results showed that the CMIP6 models reproduced the boreal winter–spring Indian Ocean EUC more realistically than the CMIP5 models, although both generations of models underestimated the strength of the Indian Ocean EUC compared with the observations. This underestimation of the Indian Ocean EUC can be attributed to the excessively strong and westward-extended cold tongue in the equatorial Pacific. In the CMIP models, a stronger winter-mean cold tongue favors a stronger zonal sea surface temperature gradient, which forces a strong easterly wind bias over the equatorial western Pacific. This, in turn, contributes to an acceleration of the Walker circulation. This enhanced Walker circulation over the Indo-Pacific Ocean directly causes a lower level westerly wind bias over the equatorial Indian Ocean and drives a shallow west–deep east thermocline tilt bias, ultimately leading to an excessively weak EUC in the Indian Ocean via wind-induced thermocline processes. Compared with the CMIP5 models, the overall improvement in the strength of the winter–spring Indian Ocean EUC in the CMIP6 models can be traced back to the improvement in the degree of the strong and westward-extended cold tongue bias. Our results suggest that efforts should be made to reduce the bias in the mean-state equatorial Pacific sea surface temperature to further improve the simulation and projection of the atmospheric and oceanic circulations in the Indian Ocean.

There is evidence that the interannual relationship between El Niño events and the following Indian summer monsoon rainfall (ISMR) has weakened with the more frequent occurrence of central Pacific (CP) El Niño events. We revisited the following ISMR responses to the two different types of El Niño events using observations and reanalysis datasets. Our results show that the ISMR anomalies associated with eastern Pacific (EP) and CP El Niño events are different, with decreased (increased) rainfall in early summer (June–July) following EP (CP) El Niño events. This is primarily attributed to the different responses to anomalous warming of the sea surface temperature (SST) in the northern Indian Ocean (NIO), which is characterized by double peaks in the warming SST during EP El Niño events, but only one peak during CP El Niño events. For EP El Niño events, the second SST warming peak in early summer contributes to the lower level antisymmetric wind pattern over the tropical Indian Ocean (TIO), which delays the onset of the Indian summer monsoon (ISM) and decreases the supply of moisture to India, implying a decrease in the ISMR. By contrast, for CP El Niño events, the cooling SST over the western TIO directly induces a significantly positive meridional SST gradient and drives the lower level southwesterly wind anomalies, resulting in an eastward shift in the decreased antisymmetric winds over TIO and the early onset of ISM. These circulation features are associated with anomalous upper-level divergence over TIO and sinking over India, jointly leading to the excess ISMR in early summer. These results suggest that, in addition to the key role of the warming of the NIO SST, cooling of the SST over the western TIO during CP El Niño events should be considered carefully in understanding the El Niño–ISMR relationship.