Vertical Pathways Associated with the Evolution of a Mesoscale Front into Submesoscale Cyclonic Eddies

Mesoscale and submesoscale features play a critical role in transporting heat and biogeochemical tracers from the surface ocean to depths below the mixed layer, by driving vertical motions across density gradients. In the winter of 2022, strong mesoscale and submesoscale features were observed in the Western Mediterranean Sea during the ONR CALYPSO oceanographic campaign. This multidisciplinary experiment combined multiplatform in-situ observations with high-resolution numerical simulations to observe and predict small-scale ocean variability. In particular,  a mesoscale density front associated with a vortex dipole was observed using CALYPSO observations and satellite imagery. A 650m resolution model simulation is used here to understand the evolution of the front and the energy transfer to submesoscale cyclonic eddies. The simulation properly reproduces the intense, narrow, and elongated frontal convergence structure and a dense cyclonic ridge linked to the vortex dipole. The evolution of the front is characterized by: i) an intensification through frontogenesis, and ii) a decay due to favorable conditions for overturning instabilities during a down-front wind event. The frontogenesis and instabilities processes enhance vertical motion via an across-front ageostrophic secondary circulation and contribute to the restratifying effect. The front decayed within days, interacting with the mesoscale ridge to break down into smaller structures and generate submesoscale cyclonic eddies (SCEs) at its edges. The formation of SCEs is associated with the frontal decay, as well as centrifugal and gravitational instabilities, which transfer energy from the mesoscale front to the SCEs. The SCE structure reveals a 3D helical-spiral recirculation pattern that transports parcels vertically. Observations of oxygen and chlorophyll confirm the enhancement of the vertical transport of tracers from the surface to the ocean interior. Submesoscale eddy-induced frontogenesis mechanism and instability processes drove subduction along outcropping isopycnals at the periphery of the SCE.