The Vertical Structure of Meso- and Submeso-scale Vertical Transport

Submesoscale frontal instabilities are known to be surface-enhanced and can generate vertical velocities of 100s of meters/day in the mixed layer. However, the implications of these surface-enhanced vertical velocities for tracer exchange between the surface and interior ocean depends on the extent to which submesoscale vertical velocity modulates vertical transport in the ocean interior. We use high-resolution models with deep lateral density gradients to show that submesoscale surface features influence the rate and pathway of subduction in the ocean interior even on isopycnals that do not outcrop in the mixed layer. In a summer scenario where the mixed layer is shallow, both the vertical velocity and tracer subduction are related to a single mode of variability that is due to restratification of the mesoscale front. Long-term subduction is due to restratification of the front and to weak time-variability. Comparatively, in a winter scenario when the mixed layer is deep, vertical velocity below the mixed layer is influenced by surface submesoscale features. There is substantial along-front variability in vertical velocity that results in a variety of subduction pathways and more variability in the timescales of Lagrangian subduction. We identify and discuss cases of interaction between surface submesoscale features and trajectories well below the mixed layer that results in either rapid or long-term subduction. The model results are used to interpret summertime and wintertime observations of the vertical distributions of biogeochemical tracers in the Alboran Gyres.