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Remarkable warming of the Southern Indian Ocean during the recent two decades is assessed using a heat budget analysis based on the Estimating the Circulation and Climate of the Ocean version 4 release 3 model results. The annual mean temperature averaged in the upper-700m Southern Indian Ocean during 1998-2015 has experienced significant warming at a rate of 1.03x10(-2)degrees C/year. A heat budget analysis indicates that the increase is mostly driven by decreased cooling from net air-sea heat flux and increased warming from heat advection. Increased Indonesian Throughflow advection is the largest contributor to warming the upper 700m of the Southern Indian Ocean, while the reduction of surface turbulent heat flux is of secondary importance. These results expand our understanding of the decadal heat balance in the Indian Ocean and of Indo-Pacific decadal climate variability.

Plain Language Summary This study identified the important role of the strengthened Indonesian Throughflow volume and heat transports during the global surface warming hiatus period in warming up the Southern Indian Ocean in the past two decades. Roles of other processes, such as air-sea heat fluxes, cross-equatorial cells, and the Agulhas Current, have also been quantified using the model. It appears that the heat transport anomaly is the dominant factor that drives the Southern Indian Ocean warming. Quantifying the decadal ocean heat balance is important to understand the redistribution of the anthropogenic heat uptake in the oceans. The results are useful in assessing coupled model performance in simulating climate change impacts on the global oceans. The study may have broad implications in the climate change research field, as well as the study of broad-scale oceanography, ocean circulation, and the Indonesian Throughflow.