Opposing trends of the Subantarctic Mode Water thickness in the South Indian Ocean driven by atmospheric forcing and interior mixing

Abstract Formation and subduction of Subantarctic Mode Water (SAMW) contributes to the upper cell of the Southern Ocean overturning circulation and transports anthropogenic heat and carbon into the ocean interior. Understanding of the processes driving change in SAMW is therefore needed to assess the ocean’s capacity to store heat and carbon. An analysis of Argo data reveals that the SAMW thickness increased in the eastern subduction area of the South Indian Ocean (SIO) during 2005−2020, and decreased in the central SIO outside the subduction area. The increasing and decreasing trends of SAMW thickness are driven by enhanced subduction and by erosion through mixing with warmer overlying waters, respectively. The changes in SAMW subduction reflect changes in air-sea exchange. Trends in winter sea level pressure associated with an amplification of the winter atmospheric zonal wave 3 pattern caused increased equatorward flow and marine cold air outbreaks in the eastern SIO, driving larger ocean heat loss, deeper mixed layers, and stronger subduction by lateral induction. SAMW subducts beneath lighter waters that warmed during the 2005−2020 period. Mixing with the warmer overlying water eroded the upper part of the SAMW layer, causing deepening of the SAMW low potential vorticity core, and thinning of the SAMW in the central SIO. These results demonstrate that both anomalous meridional atmospheric circulation and erosion by interior mixing influence the thickness, and hence heat and carbon storage capacity, of the SAMW.