Submesoscale Vertical Velocities Enhance Tracer Subduction in an Idealized Antarctic Circumpolar Current

Upper-ocean submesoscale fronts, with their associated strong vertical velocities, are often claimed to play a significant role in subducting tracers into the interior. The role of these submesoscale processes in restratifying the mixed layer is now well recognized, but whether they simultaneously flux tracers through the base of the boundary layer remains an open question. We vary the resolution in a semirealistic channel model to control turbulent processes at various scales and study their influence on tracers. It is found that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations: The 1-km simulation takes up 50% more tracer compared to the 20-km simulation, despite the increased restratifying influence of the resolved submesoscale processes. A full frequency-wave number cross-spectra of the vertical velocity and vertical tracer flux show that the high-frequency inertia-gravity waves that appear in the highest-resolution simulation play no role in irreversible downward tracer transport. Plain Language Summary The oceanic uptake and storage of anthropogenic carbon plays a central role in the global carbon budget, and a significant fraction of this uptake occurs in the Southern Ocean. It is well established that the eddies and fronts, turbulent fluctuations of the flow, are important for this uptake, but the relative influence of different scales is less understood. The eddies and fronts associated with the submesoscale (1-50km) have been recognized to play a leading role in shallowing the surface boundary layer depths, but the role of the strong vertical velocities, which are generally associated with these features, in transporting fluid below the mixed layer remains unknown. Here we investigate these questions using an idealized Southern Ocean model with an imposed tracer source at the surface. The model resolution is varied as a means to include or omit turbulent processes at various scales. We find that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations, despite the reduction in the depths of the vigorously mixed boundary layers. We also found that inertia-gravity waves, which are ubiquitous in the ocean and are generally associated with very strong vertical velocities, had no impact on the net tracer flux.