Balanced and Unbalanced Motions at a Submesoscale Front

A wealth of research in the past decade has unveiled the importance of small fronts in the upper ocean buoyancy budget, yet process study type observations are still rare due to the challenges of resolving the relevant spatial and temporal scales. Here, a neutrally buoyant, subsurface Lagrangian float was deployed in a small mixed layer front within the California Current System as part of the Assessing the Effect of Submesoscale Ocean Parameterizations (AESOP) program. Its trajectory was acoustically tracked, allowing the region surrounding the drifting float to be intensely surveyed by a ship towing a Triaxus profiler. The survey lasted 30 hours as the float traveled approximately 50km along the front. This Lagrangian approach provides uniquely detailed measurements of the frontal structure and evolution within and below the boundary layer. The frontal evolution can be divided into two stages. Initially, downfront winds incite mixing and the float repeatedly traverses the 15m deep boundary layer. Directly below, but above the stratified pycnocline, lies a distinct 15m-thick layer of stably-stratified fluid with negative potential vorticity, indicative of symmetric instability. As winds relax and vigorous mixing subsides, the front continues to evolve and the system enters a different dynamical regime, revealing intrusive features that imply cross frontal exchange and subduction of water masses. Buoyancy flux estimates and inferred vertical velocity from the float are compared with theoretical scalings of submesoscale dynamics. Initially, buoyancy flux estimates are consistent with the expected response to downfront winds, but as the front evolves, observational estimates deviate from theoretical predictions. During this stage, tracer distribution and inferred circulation delineate the roles of both balanced and unbalanced motions. This thorough account and analysis affirm that several processes occur in concert to dictate the evolution of fronts in the upper ocean.