Linking the tropical Indian Ocean basin mode to the central-Pacific type of ENSO: Observations and CMIP5 reproduction

Previous studies have demonstrated that the tropical Indian Ocean basin mode (IOB) favors the phase transition of ENSO; however, they have not differentiated the two types of ENSO. This study reports that the boreal winter-spring IOB appears more likely to induce the central-Pacific (CP) ENSO than the eastern-Pacific ENSO one year later based on observation data and CMIP5 simulations. We find a strong asymmetry in the forcing of the IOB on the CP ENSO. The impact of IOB warming on CP La Nina events is more significant than the impact of IOB cooling on CP El Nino events. IOB warming during late winter to early spring produces easterly wind anomalies over the western equatorial Pacific and an anomalous descending motion of the Walker circulation near the dateline, inducing cooling in the central Pacific during the subsequent summer to winter. This suggests that interbasin interactions with the tropical Indian Ocean, in addition to the northern tropical Atlantic Ocean, are capable of generating CP La Nina events. Most CMIP5 models generally capture the negative IOB-CP ENSO lagged correlation. However, the cooling center and sinking branch of the anomalous Walker circulation in the Pacific Ocean forced by the IOB warming are weaker and shifted eastward in the multimodel ensemble than in the observations. Additionally, the observed asymmetry of the IOB impact on the CP ENSO is not captured by the CMIP5 models. This study indicates that improving the simulation of the IOB as a contributor to ENSO diversity remains a challenge.