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

The dynamical pathways of subduction, by which water from the oceanic surface mixed layer makes its way into the pycnocline, are influenced by both mesoscale (geostrophic) frontogenesis and submesoscale (ageostrophic and vertical) frontogenesis in the mixed layer. In frontal zones, subducted water masses that are tens of kilometers in extent can be identified in the pycnocline for days to months. Here, we explore the pathways and mechanisms for subduction with only weak surface forcing using a submesoscale-resolving numerical model of a mesoscale front. We use particle tracking to identify Lagrangian trajectories that exit the mixed layer. By identifying the subducting water parcels, we study the evolution of their dynamical properties from a statistical standpoint. The velocity and buoyancy gradients increase as water parcels experience frontogenesis and subduct beneath the mixed layer into the stratified pycnocline. We find that water parcels subduct within coherent regions along the front. These coherent subduction regions set the length scales of the subducted features. As a result, the vertical transport rate of a tracer has a spectrum that is flatter than the spectrum of vertical velocity. An examination of specific subduction events reveals a range of submesoscale features and frontogenesis processes that support subduction. Contrary to the forced submesoscale processes that sequester low PV anomalies in the interior, we find that PV can be elevated in subducting water masses. The rate of subduction that we estimate is of similar magnitude to previous studies (~100 m per year), but the pathways that are unraveled in this study along with the Lagrangian evolution of properties on water parcels, emphasize the role of submesoscale dynamics coupled with mesoscale frontogenesis.