A Stratifying Submesoscale Mixed Layer Front: Observations, Dynamics and Implications

Lateral slumping of submesoscale mixed layer fronts stratify the upper ocean by converting the horizontal density gradients into vertical ones. The prevalence of submesoscale fronts in the upper ocean suggests that lateral slumping could play a significant role in setting large-scale stratification, with additional impacts that include modulation of large-scale air-sea interaction and primary productivity. Direct observations of frontal slumping are rare due to the small spatial and fast time scales associated with this process. This study utilizes observations that follow a single sharp density gradient as the nearly vertical isopycnals tilt and thereby stratify the mixed layer. A neutrally buoyant, subsurface Lagrangian float was deployed in a submesoscale front within the California Current System as part of the Office of Naval Research 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 undulating profiler. This provided high-resolution, 4-D sampling of the velocity and scalar fields surrounding the float. Down-front winds initially maintain a sharp density gradient confined above the pycnocline, in the upper 30m of the water column, characterized by Rossby number O(1). As wind strength weakens, isopycnals immediately begin to tilt and the mixed layer stratifies significantly in less than one day. The rate of stratification is an order of magnitude larger than predicted by geostrophic adjustment alone. We discuss the role of instabilities, wind and advection responsible for the frontolysis and the implications of the slumping on upper ocean stratification and properties of tracers.