The submesoscale, the finescale and their interaction at a mixed layer front

Abstract: The spindown of a geostrophically balanced density front in an upper-ocean mixed layer is simulated with a large eddy simulation (LES) model that resolves O(1000) m down to O(1) m scale. Our goal is to examine the interaction between the submesoscale and the turbulent finescale, and better characterize vertical transport, frontogenesis and dissipative processes. The flow passes through symmetric and baroclinic instabilities, spawns vortex filaments of O(100) m thickness as well as larger eddies, and develops turbulence that is spatially localized and organized. A O(100) m physical-space filter is applied to the simulated flow to separate the coherent submesoscale from the finescale in a decomposition that preserves their spatial organization unlike the typical practice of along-front averaging. Analysis of the submesoscale vertical velocity (as large as 5 mm/s) reveals that downwelling is limited to the thin vortex filaments while upwelling occurs over spatially extensive regions in the eddies, resulting in an overall buoyancy flux that is restratifying. The kinetic energy (KE) transport equations are evaluated separately at both the scales to understand energy pathways in this problem. The buoyancy flux associated with coherent motions acts as the primary source of submesoscale KE which is then transported across the front with a fraction transferred to the finescale. The transfer, limited to thin regions of O(100) m horizontal width, is accomplished by primarily horizontal strain in the upper 10 m and by vertical shear in the rest of the 50 m deep mixed layer. Frontogenetic mechanisms are diagnosed through analysis of the transport equation for squared buoyancy gradient. Horizontal strain is the primary frontogenetic term and is counteracted primarily by horizontal diffusion in the top 10 m and by the horizontal gradient of vertical velocity further below.