Rapid Downwelling of Tracer Particles Across the Boundary Layer and Into the Pycnocline at Submesoscale Ocean Fronts

A neutrally buoyant float deployed in an atmospherically driven turbulent ocean boundary layer on the dense side of a submesoscale front was repeatedly carried across the boundary layer by the turbulence and then trapped beneath the slumping front. Lagrangian particles in a large-eddy simulation of a similar baroclinically unstable front forced by surface cooling move along convergent surface filaments toward filament junctions. They are also caught by convective plumes that downwell them at speeds similar to those of the float. Subsequently, some are trapped in the pycnocline by frontal slumping due to ageostrophic secondary frontal circulations. In both observations and simulations, boundary layer turbulence and frontal circulations work together to trap and subduct particles from the mixed layer. The small-scale boundary layer motions move them vertically within the boundary layer and larger, submesoscale frontal circulations move them laterally out of the boundary layer and under the slumping fronts. The present study provides evidence for enhanced tracer transport at ocean fronts in both observation and numerical simulation. The finding suggests a pathway for pollutants (e.g., microplastics) to accumulate in the ocean interior. The transport pathway is driven by both large-scale eddy dynamics and finescale turbulence, of which the latter plays a larger role in downwelling speed while the spatial sites of subduction are determined by the former. Lagrangian float observations reveal downwelling across and trapping beneath the turbulent mixed layer at a submesoscale front Similar particle trajectories in convectively forced LES result from joint effects of boundary layer turbulence and submesoscale dynamics Downwelling is due to finescale convective plumes while trapping and restratificationare due to the submesoscale circulation of the front