On Air–Sea Heat Flux, Subduction, and Ageostrophic Divergence in an Eddying Southern Ocean

Abstract Subduction hotspots in the Southern Ocean transfer heat, carbon, and oxygen from the ocean’s mixed layer into the upper thermocline. In this study, we use an eddy-resolving global ocean–sea ice model, forced by an atmospheric reanalysis, to estimate the subduction due to the mean flow and also, for the first time, directly compute the horizontal and vertical eddy contributions to subduction in the Southern Ocean. This is in contrast to the earlier studies that parameterized the eddy-induced term. Our results also show the dominance of the mean flow on subduction because the eddy-induced lateral induction is anticorrelated with the vertical eddy bolus velocity, which reduces their combined effect on subduction. Meridional displacements of the Antarctic Circumpolar Current (ACC) modulate the subduction rate because the mean lateral injection of fluid is a dominant term. Moreover, the eddy-induced lateral induction is smaller than the mean lateral induction across most of the Southern Ocean. In addition, a comparison between subsurface vertical velocity and vertical Ekman velocity shows that the Ekman velocity is an order of magnitude smaller in energetic regions with strong ageostrophic divergence. These results suggest a possible mechanism in ACC meanders to explain the observed ventilation of Antarctic Intermediate Water in regions of steep bathymetry and high eddy kinetic energy. Finally, we note that the asymmetry in air–sea heat flux in the Southern Ocean contributes to the distribution of the subduction zones of mode and intermediate waters. Significance Statement This study uncovers crucial processes in how surface waters in the Southern Ocean penetrate into the interior ocean. This process is called subduction, and it is enhanced at subduction hotspots. These hotspots play a vital role in transferring heat, carbon, and oxygen from the surface to the interior ocean. Using a sophisticated ocean–sea ice model, our research calculates the contributions of both the time-averaged flow and eddies to this subduction, providing clearer insights into the role of eddies than in previous studies. The findings reveal that the meridional movement of the Antarctic Circumpolar Current significantly influences subduction rates, which are essential for understanding ocean ventilation and climate dynamics. This work is important because it enhances our understanding of oceanic processes that affect global climate, highlighting areas for further exploration of these mechanisms to inform climate models and policy efforts aimed at mitigating climate change.