Minimum Warming in the South Indian Ocean Thermocline in a Warming Climate Linked to Freshening Processes

Abstract

Abstract The Southern Ocean (SO) is one of the key regions in absorbing and storing anthropogenic heat. An analysis of the CMIP6 models finds a distinct warming minimum/cooling and freshening in the subtropical ocean thermocline of the south Indian Ocean (SIO) under a medium-emission scenario (SSP245). The warming minimum/cooling has also been found in other warming scenarios in previous studies. However, the freshening here has received less attention. On account of increased precipitation in the models, the SO high latitudes get fresher in a warmer world. We show that this freshening anomaly is advected to the north of the deep mixed layer by the horizontal current and then subducts into the ocean interior in the SIO. As a result, the isopycnal surfaces become fresher, deeper, and cooler. This freshening and cooling signal then propagates to the north along isopycnals through the subtropical gyre and leads to freshening and cooling on the depth coordinates where the vertical movement of isopycnals (heaving) is insignificant. Lacking deep-enough mixed layers, the other two basins show smaller freshening and cooling signs in the models. Here the importance of freshening in temperature redistribution in the ocean interior in the SIO under extensive global warming is emphasized. The result helps interpret the future heat storage in the SO in a warmer world. Significance Statement The Southern Ocean (SO) is a key region in absorbing and storing anthropogenic heat. The observed past and simulated future warming in the SO show maximum values between 40° and 45°S and minimum values downward and northward of the deep mixed layers. CMIP6 models show the same pattern, and it is found that this pattern is most distinguished in the south Indian Ocean. The freshening anomaly advected to the deep mixed layers subducts into the ocean interior, leading to freshening and cooling signals on isopycnals. These signals spread with the subtropical gyre and induce warming minimum/cooling there. The result would help interpret the warming pattern in the SO in a warmer world.