Subduction by Submesoscales

Abstract

Several studies have demonstrated the relevance of submesoscales (SM) to the production of phytoplankton and the ocean absorption of anthropogenic CO2. One variable that thus far has not been fully quantified is the SM‐induced rates of subduction and obduction at the bottom of the mixed layer. In this work, we use an SM parameterization that had been previously assessed to estimate the rates of subduction‐obduction using a coarse resolution stand‐alone ocean code to compute the large‐scale fields. To separate the contributions of baroclinic instabilities from those due to wind stress, we derive an analytic expression for the SM subduction‐obduction rates. Since there are no numerical simulations and/or measurements to assess the results, we compare them to subduction rates due to mesoscales for which an eddy‐permitting southern ocean state estimate is available. An interesting result is that in the Antarctic Circumpolar Current the maximum SM subduction rates are not significantly smaller than those due to mesoscales, but the SM subduction rates exhibit a topology opposite to that of mesoscales: obduction is equatorward and subduction is poleward. Sensitivity studies are presented to explore the dependence of the results on the depth of the SM vertical extent and the SM kinetic energy. On the basis of these results, it is suggested that the SM‐induced subduction should not be neglected because topology and magnitude indicate that mesoscales and submesoscales may add to or subtract from one another.

Coherent pathways for subduction from the surface mixed layer at ocean fronts

“…The second process, which will be the focus of this study, involves movement along sloping density surfaces. If a density surface from the pycnocline outcrops into the mixed layer, then water parcels can move adiabatically along that surface from the mixed layer into the interior (Canuto et al, 2018;Gebbie, 2007;MacGilchrist et al, 2017;Nurser & Marshall, 1991;Stommel, 1979).…”

Vertical fluxes in the upper ocean

“…Eddy fluxes at the meso-and submeso-scale are increasingly recognized as important for the transport of water masses and biogeochemical tracers from the surface to the interior Balwada et al, 2018;Canuto et al, 2018;Resplandy et al, 2019). Submesoscale processes will be most important during times of the year and in locations with deep mixed layers, namely the winter and early spring but may only affect oxygen and carbon transport on annual timescales if the water parcels are transported below the deepest wintertime mixed layer.…”

“…The subduction rate is diagnosed using the mixed layer tracer and is consistent with past mesoscale and submesoscale resolving simulations. (Gebbie, 2007;Canuto et al, 2018).…”

“…If a density surface from the pycnocline outcrops into the mixed layer, then water parcels can move adiabatically along that surface from the mixed layer into the interior (Stommel, 1979;Nurser and Marshall, 1991;Gebbie, 2007;MacGilchrist et al, 2017;Canuto et al, 2018).…”

“…Advective biogeochemical fluxes in oligotrophic gyres are likely driven by eddy processes at the mesoscale and submesoscale. In the high latitudes, mixed layer eddy-driven subduction and restratification have been observed to export carbon from the surface ocean into the interior Dall'Olmo et al, 2016;Balwada et al, 2018;Canuto et al, 2018;Llort et al, 2018;Resplandy et al, 2019). Subducted mixed layer carbon can persist for months as coherent features that are detectable in vertical profiles (Llort et al, 2018;Johnson and Omand, 2021).…”