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

Abstract

AbstractA 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.