Vertical Transport Between the Mixed Layer and the Thermocline at Submesoscale Fronts

Recent observations in the Western Alboran Gyre suggest subduction across the surface mixed layer (ML) and deep into the thermocline. It has been suggested that the subduction in the surface ML is induced by turbulent motions while the subduction in the thermocline is induced by the slumping of a submesoscale front. Large-eddy simulations are performed to investigate particle transport between the ML and the thermocline at upper-ocean fronts. The model consists of a double-front (warm filament) that is 100 m deep and 1.2 km wide at the edges. The warm filament extends vertically from the surface into the thermocline. Two flow configurations are simulated to compare the vertical transport between the ML and the thermocline. In the first configuration, the ML is modeled as background turbulence controlled by a constant cooling flux at the surface. In the second configuration, the convective turbulence is absent, enabling a comparison of vertical transport with the first configuration. Without convective turbulence, the ML flow evolves through symmetric instability (SI) followed by baroclinic instability (BI), while the thermocline experiences only BI. The evolution of BI leads to the formation of coherent structures consisting of vortex filaments and submesoscale eddies. The coherent structures span the entire depth of the fronts and provide direct pathways for exchanging fluid effectively between the ML and the thermocline. When the convective turbulence is included, the vertical transport in the ML is influenced by both the convective plumes and the coherent structures associated with the development of BI. Dispersion statistics of single-, two- and multi-particle arrangements are studied to characterize the submesoscale turbulence in both configurations.