Stratification at Ocean Fronts

Abstract

The large-scale changes of temperature and salinity in the ocean are not smooth and continuous, but comprise many smaller, sharper gradients. In regions of strong lateral density contrasts, sharp density fronts can slump, transforming potential energy into kinetic energy, and converting horizontal buoyancy gradients into vertical ones. The surface ocean is populated with such fronts, suggesting their cumulative impact is a leading order contribution to the upper ocean energy and buoyancy budgets. Chapter 1 takes a global approach to assess the importance of frontal slumping on springtime restratification. Observations from the global Argo database are contrasted with predictions from a 1D mixed layer model to assess where lateral processes influence mixed layer evolution. Enhanced stratification from frontal tilting occur in regions of strong horizontal density gradients, with a small fraction in regions of deep mixed layers. These patterns are discussed in the context of instabilities and frictional effects to understand the large-scale implications of these small-scale dynamics. Chapters 2 and 3 focus on a highly detailed process study of one surface intensified submesoscale front. A Lagrangian float was deployed in a small mixed layer front within the California Current System as part of the Assessing the Effects of Submesoscale Ocean Parameterizations (AESOP) program. Its trajectory was tracked acoustically, allowing the region surrounding the drifting float to be surveyed intensely by a ship towing a Triaxus profiler. Initially, downfront winds incite mixing and the float repeatedly traverses the boundary layer. As winds relax and vigorous mixing subsides, the the system enters a different dynamical regime as the front develops an overturning circulation associated with large vertical velocities that ultimately tilt isopycnals over and stratify the upper ocean within a day. Chapter 2 details the observations and the kinematics of the system to reveal the importance of the submesoscale in transferring energy to smaller scales. Chapter 3 combines the observations with idealized models to evaluate the importance of wind forcing and turbulent adjustment on the evolution of stratification in the mixed layer.