Submesoscale Frontogenesis Drives Strong Finescale Tracer Subduction

Abstract

Vertical exchange between the surface mixed layer (ML) and the interior is important for biogeochemistry and air–sea interaction. Focused observations have consistently shown that submesoscale processes (horizontal scales of 0.1–10 km) generate substantial exchange of heat, freshwater, and biogeochemical tracers across the ML base, particularly near fronts. However, the role of finescale turbulence in this exchange remains poorly understood. Here we investigate this multiscale problem by analyzing the transport of a surface‐fluxed passive tracer in large‐eddy simulations of baroclinic mixed‐layer instabilities (MLIs) under surface cooling. Budgets for the tracer below the evolving ML show that finescale turbulence contributes to tracer subduction only at fronts sharpened to a few hundred meters wide by eddy straining and horizontal convergence. Although submesoscale vertical flux dominates the exchange across the ML base, finescale turbulent flux is comparable to the indirect contribution from MLI‐induced restratification. Analysis of the finescale energetics shows that the turbulence at sharp fronts is primarily driven by shear production associated with frontogenesis, with large contributions from horizontal shear, rather than by convective forcing. In contrast, outside the frontal zone turbulence is driven solely by surface forcing, and the tracer is largely confined to the ML. This suggests a multiscale picture of vertical exchange at submesoscale fronts that is not represented in existing parameterizations. Submesoscale‐permitting ocean models may partially capture the frontogenetic process, but the associated forward energy flux is likely dissipated implicitly or explicitly, rather than contributing to vertical mixing and tracer transport.