The Origin and Fate of Subantarctic Mode Water and Antarctic Intermediate Water in the Southern Ocean

Abstract

The global ocean plays a major role in moderating atmospheric temperature rise, thereby buffering climate change. Amongst the various oceanic regions undergoing warming, the Southern Ocean is a primary heat sink in the climate system. Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are the dominant water masses in the upper Southern Ocean, and play a fundamental role in ocean ventilation and the uptake of heat and carbon into the ocean interior. This thesis focuses on understanding the geographic and seasonal variability in the formation of SAMW and AAIW, as well as the role of SAMW, AAIW, and other mode and intermediate waters in recent global ocean warming, using observationally based hydrography and estimates of mixing strength. Firstly, the mechanisms controlling the volumetric change of SAMW within the mixed layer and in the ocean interior are investigated separately. We find that the seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August) and entrainment (35%), while the interior SAMW formation is controlled by subduction during August-October. The annual mean subduction estimate shows strong regional variability with hotspots of large SAMW subduction, consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Secondly, a volume budget analysis is performed to identify the mechanisms governing the spatial and seasonal variability of AAIW. Firstly, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter variety AAIW and SAMW. The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. Secondly, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing. Lastly, part of AAIW is exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the Antarctic Circumpolar Current (ACC) and northward into the subtropical gyres. Finally, the ocean absorbs >90% of anthropogenic heat in the Earth system. However, it remains unclear how this heat uptake is distributed across water masses. Here we show that ocean heat accumulation during 2010–2020 has more than doubled relative to 1990–2000. Of the total ocean heat uptake, 94% is found in global mode and intermediate water layers that have subsequently warmed and increased in volume. After factoring out volumetric changes, warming of mode and intermediate waters explains ~40% of net global ocean warming, despite occupying just ~16% of the total ocean volume. These water masses in the subtropical Pacific and Atlantic Oceans, as well as in the Southern Ocean, are responsible for a large fraction of total heat uptake, with important implications for ongoing ocean warming, sea-level rise, and climate impacts.