Rapid 21st century warming in the Southern Indian Ocean driven by altered inter-basin connections

The Indian Ocean has been warming rapidly over the last few decades. However, this warming has not been uniform; with the subtropical and tropical Southern Indian Ocean (SIO, 5°S3-5°S) cooling until the late 20th Century and then warming abruptly after 2000. Increased Indonesian throughflow (ITF) into the Indian Ocean during the recent climate hiatus was considered to be the primary reason for this recent warming. Our analysis shows that though the ITF inflow decreased rapidly by ~10% after 2010, the upper ocean SIO continued to gain heat at the rate of 0.56 (±0.02) ×10 ²² J/decade. We argue that the increased ITF during the hiatus in global surface warming (1998-2010) initiated the SIO warming, resulting in a weaker Mascarene High and its decoupling from Southern Ocean atmospheric variability. This caused disparity in the poleward shift of the subtropical gyres between the Pacific and the Indian Oceans, resulted in a three-fold increase in the Tasman inflow to the Indian Ocean, offsetting the weakened influence of the ITF. This accumulation of heat further weakened Masacarene High and reduced the Agulhas outflow by ~20% from its peak, causing a positive feedback that continued to warm the upper ocean of SIO.