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An ocean downscaling model product, forced under the Representative Concentration Pathway 8.5 future climate change scenario, has been used to understand the ocean heat balance of the Indian Ocean in a warming climate. Toward the end of the 21th century, the model simulates a significant reduction of Indonesian throughflow (ITF) transport, which reduces the Pacific to Indian Ocean heat transport by 0.20 PW, whereas across 32 degrees S in the Southern Indian Ocean (SIO), the southward heat transport is reduced by 0.28 PW, mainly contributed from the weakening western boundary current, the Agulhas Current (0.21 PW). The projected weakening of the Agulhas Current is to compensate for the reduction of the ITF transport, with additional contribution from the spin-down of the SIO subtropical gyre. Thus, being amplified by the ocean circulation changes in the SIO, the projected Indian Ocean warming trend will be faster than the direct air-sea heat flux input.

Plain Language Summary During the global mean temperature warming "hiatus" period during 1998-2014, significant warming of the Southern Indian Ocean is closely related to the increased Indonesian throughflow heat transport. However, under enhanced greenhouse warming, both climate models and our ocean downscaling model project a remarkable weakening trend of the ITF. In this study, we have found that the projected Indian Ocean warming trend is faster than the input from direct air-sea heat flux, despite the reduction of the ITF heat transport. This can be explained by the more significant weakening of the Agulhas Current transport in a warming climate, which reduces the southward heat transport out of the Indian Ocean. The weakening of the Agulhas Current is due to the ITF transport reduction and the spin-down of the Southern Indian Ocean circulation.