Frontal Subduction in an Increasingly Stratified Southern Ocean

Pycnocline stratification is increasing across multiple ocean basins due to a warming surface ocean and changes in wind forcing. Pycnocline stratification plays a leading order role in tracer transport, shaping capacity for heat and carbon uptake, making it a key parameter of interest on timescales ranging from paleoclimate to plankton blooms. Part of the challenge in assessing the role of pycnocline stratification in global models is the two-way connection between physical processes at the mesoscale and submesoscale and stratification, with important implications for the resulting tracer transport. Using idealized runs of MITgcm, we find that the strength of pycnocline stratification influences the formation and evolution of submesoscale structure. When a constant isopycnal slope is initialized, tracers get efficiently transferred across the base of the mixed layer and get trapped in anticyclonic submesoscale vortices below the mixed layer. This leads to tracer concentrations below the mixed layer and fluxes through it to be stronger under decreased stratification conditions. In contrast, when the frontal lateral buoyancy gradient is held fixed while stratification changes, the vertical flux of tracers and the concentrations at depth stay constant across all examined stratification conditions. Understanding the relationship between pycnocline stratification and fine-scale physical motions is necessary to diagnose and predict trends in carbon uptake and storage, particularly in the Southern Ocean.