Impact of local atmospheric forcing on the evolution of the South Indian Ocean heat content during and after the 2014-2016 El Niño event

The South Indian Ocean (SIO) has been characterized as one of the major heat accumulators among the oceanic basins due to its remarkable warming during the last two decades. The interannual-to-decadal variability of heat content and sea level in the SIO is strongly influenced by its connection with the Pacific and large-scale climatic forcing in the Indo-Pacific region primarily associated with El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). The La Niña-like conditions that persisted in the Pacific for about a decade ended with the very strong 2014-2016 El Niño event. The superposition of the 2014-2016 El Niño and a negative IOD event in 2016 led to a 40% decrease of the ITF volume and heat transports. The decade-long accumulation of the upper 2000-m heat content in the SIO subtropical gyre, as measured by Argo floats in 2004-2013, also ended with a dramatic drop in 2014-2016, returning to the values observed in 2004. Interestingly, heat content in the SIO nearly recovered during the next two years, following a switch from the strong 2014-2016 El Niño to the weak 2017-2018 La Niña. Such a rapid response is compelling and, as we show, it cannot be accounted for by the westward propagation of warming anomalies originated in the Pacific. Using observations and an ocean model, we investigate the fate of the 2014-2016 cooling in the SIO subtropical gyre and elucidate the mechanisms of the observed heat content recovery in 2017-2019. We show that this cooling as well as the following recovery occurred largely due to anomalous Ekman pumping in the western SIO not directly linked to the ENSO signal in the Pacific. In 2014-2016, a cyclonic wind stress curl anomaly led to the shoaling of isotherms in the SIO subtropical gyre. In addition, the associated southward Sverdrup transport anomaly advected colder mid-depth water from the northern periphery of the SIO subtropical gyre. In 2017-2018, while El Niño-forced negative heat content anomaly was still present in the eastern SIO, an anticyclonic wind stress curl anomaly led to the deepening of isotherms in the western SIO that can explain the observed heat content recovery. This study demonstrates that local wind forcing over the SIO can often suppress or reinforce signals propagating from the eastern boundary and, thus, affect the interannual changes of the SIO heat content.