A unique open-ocean upwelling exists in the tropical South Indian Ocean (SIO), a result of the negative wind curl between the southeasterly trades and equatorial westerlies, raising the thermocline in the west. Analysis of in situ measurements and a model-assimilated dataset reveals a strong influence of subsurface thermocline variability on sea surface temperature (SST) in this upwelling zone. El Niño–Southern Oscillation (ENSO) is found to be the dominant forcing for the SIO thermocline variability, with SST variability off Sumatra, Indonesia, also making a significant contribution. When either an El Niño or Sumatra cooling event takes place, anomalous easterlies appear in the equatorial Indian Ocean, forcing a westward-propagating downwelling Rossby wave in the SIO. In phase with this dynamic Rossby wave, there is a pronounced copropagation of SST. Moreover, a positive precipitation anomaly is found over, or just to the south of, the Rossby wave–induced positive SST anomaly, resulting in a cyclonic circulation in the surface wind field that appears to feedback onto the SST anomaly. Finally, this downwelling Rossby wave also increases tropical cyclone activity in the SIO through its SST effect.
This coupled Rossby wave thus offers potential predictability for SST and tropical cyclones in the western SIO. These results suggest that models that allow for the existence of upwelling and Rossby wave dynamics will have better seasonal forecasts than ones that use a slab ocean mixed layer. The lagged-correlation analysis shows that SST anomalies off Java, Indonesia, tend to precede those off Sumatra by a season, a time lead that may further increase the Indian Ocean predictability.