Leah Johnson (University of Washington)
Fronts and Stratification in the Upper Ocean
Ageostrophic motions at submesoscale fronts are thought to be leading order importance for stratifying the upper ocean by slumping horizontal density gradients to produce vertical stratification. The prevalence of submesoscale fronts suggests these motions have significant influence on large-scale patterns of upper ocean stratification. The first part of this talk takes a global perspective to assess the importance of fronts on upper ocean stratification. Enhanced stratification from frontal tilting occurs in regions of strong horizontal density gradients (e.g. mid-latitude subtropical gyres), with a small fraction in regions of deep mixed layers (e.g. high latitudes). These patterns are discussed in context of instabilities and frictional effects in order to understand the large-scale implications of these small-scale dynamics. The second portion of this talk zooms into the evolution of a single submesoscale front. The frontal evolution can be divided into two stages. Initially, downfront winds incite mixing and the float repeatedly traverses the 15m deep boundary layer. 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 as the front stratifies the boundary layer. 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.