The Anomalous Heat Content/Sea Level Changes in the South Indian Ocean During 1992-2018

The Indian Ocean is the warmest ocean on the Earth, and its southern part has been reported as a hot spot for heat accumulation and sea level rise over the last two decades. The amount of heat stored in the South Indian Ocean (SIO) experienced a significant decrease during the 2014-2016 El Nino as evidenced by both Argo and ocean model data. Satellite altimetry measurements also display a concurrent decrease of sea surface height (SSH), which is almost entirely due to the thermosteric component. Additionally, the noticeable change in both heat content and SSH was observed during the 1997-1998 El-Nino event. The present study aims to determine and quantify the underpinning mechanisms driving the observed anomalies. The variability of heat content in the SIO is affected by processes over the Indian Ocean (local effect) as well as by the processes over the Pacific Ocean (remote effect); Indonesian Througflow serves as a conduit for signal penetration into the Indian Ocean. These processes are strongly modulated by El-Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). In this study, we conduct two numerical simulations: in the first, we block the transport through Indonesian passages to effectively remove the influence of the Pacific Ocean, and in the second, the model is forced by the climatological wind forcing over the equatorial Indian Ocean to suppress the IOD-induced variability. Comparing the time series of simulated SSH averaged over different regions we find that the remote forcing has the most pronounced effect in the eastern part of the SIO. Additionally, the observed time lag between the time series in different regions signify the westward propagation of the anomaly associated with Rossby waves. The partial correlation maps of SSH with both Nino3 and IOD indices suggest that the variability in the SIO is primarily affected by ENSO, and as a consequence, the 2014-2016 negative anomaly in heat content/seal level can be viewed as a response to the strong 2014-2016 El-Nino event. The heat budget analysis shows that the sharp drop in heat content values is driven by advection rather than by surface heat fluxes. Furthermore, we show that the weakening of the South Equatorial and Leeuwin Currents in a simulation with blocked ITF transport reduces the magnitude of the 2014-2016 anomaly.