Evolution of the Subantarctic Mode Water in the Southern Ocean

Powerful westerlies in the Southern Ocean drive the Antarctic Circumpolar Current (ACC), the northward Ekman flow, and the associate upwelling of the Circumpolar Deep Water (CDW). The upwelled CDW is transported northward by the Ekman flow. Upon reaching the north side of the Subantarctic Front (SAF), these waters undergo intensive vertical mixing driven by cooling of the atmosphere in winter, forming the Subantarctic Mode Water (SAMW) with vertically homogenous properties. The SAMW is then transported eastward with the ACC and northward with the subtropical gyre, completing the ventilation of the Southern Hemisphere oceans. As a part of the upper limb of the Southern Ocean overturning circulation, the formation and transport of the SAMW play essential roles in the heat, freshwater, carbon, oxygen, and nutrient budgets both regionally and globally. Changes in its physical properties also provide good indications of global climate change. The SAMW has significant natural variability on different time scales, mainly regulated by factors such as sea surface buoyancy flux, the Ekman transport and pumping, and eddies. Under global warming, research has presented different conclusions regarding changes in the volume and properties of the SAMW, hindering our further understanding of its climate impacts.

By analyzing gridded Argo observations in the past decades and future warming model simulation from the Coupled Model Intercomparison Project (CMIP), we found that the volume of the SAMW is generally decreasing. This volume decreasing is mainly determined by the change in surface buoyancy flux. The volume of the SAMW slowly increases after the radiative forcing stabilized in the future warming simulation. We also found there is an opposite change in volume between different density layers, representing changes in properties of the SAMW. The opposite volume change is mainly determined by the change in the depth and position of the winter deep mixed layer. Meanwhile, the observed average temperature and salinity of the SAMW in the South Indian Ocean are increasing. But the freshening in the formation area and the southward shift of the isopycnal surfaces weaken the trend of the average temperature and salinity increase of the SAMW. In future warming simulations, the cooling and freshening on the isopycnal surfaces cause the minimum warming and strong freshening in the depth of SAMW. These conclusions deepen our understanding of the evolution of the SAMW in the Southern Ocean and its underlying physical mechanisms, providing a new perspective on the climate response and impact of water masses in the Southern Ocean.