Sea surface temperature anomaly of Southwest Pacific dipole in boreal Winter–Spring: a mechanism for the Indian Ocean Dipole

The formation mechanisms of the Indian Ocean Dipole (IOD) have been studied extensively. However, few studies have considered the influence of the South Pacific atmosphere and sea surface temperature (SST) variability on IOD formation. In this study, reanalysis data, CMIP6 model outputs, and linear baroclinic model experiments were used to investigate this influence. The results revealed a close connection between the preceding boreal winter–spring Southwest Pacific Dipole (SWPD) SST anomaly (SSTA) pattern and the subsequent IOD and associated physical mechanisms. Facilitated by the local air–sea positive feedback, the positive SWPD, coupled with low-level cyclonic circulations, persists from the preceding winter to early summer. During early summer, the below-normal precipitation and diabatic heating associated with the SWPD western pole cooling strengthens and induces an anticyclonic circulation on its western side as the climatological southeasterly winds encroach northward. This anticyclonic circulation extends from southern Australia into the tropical southeastern Indian Ocean, favoring enhanced southeasterly winds and cooling off Java-Sumatra. Subsequently, the IOD develops in summer–autumn with the aid of wind–evaporation–SST and wind–thermocline–SST positive feedback. The westward downwelling Rossby wave and eastward upwelling Kelvin wave associated with surface wind anomalies also contribute to the IOD western pole warming and eastern pole cooling, respectively. Furthermore, the IOD interacts with El Niño through Walker circulation and increases its amplitude. These findings are supported by the results obtained from the linear baroclinic model and CMIP6 historical simulations and can potentially improve the predictability of the IOD.