Effect of baroclinicity below the mixed layer on buoyancy and tracer fluxes

Recent sub-mesoscale resolving frontal experiments have often confined the frontal gradients to the upper ocean mixed layer. If, however, the sub-mesoscale meanders are occurring at the edge of mesoscale eddies with deeper baroclinic signatures, the depth of frontal baroclinicity is expected to play an important role in sub-mesoscale dynamics as well as biogeochemical tracer exchange between the mixed layer and the interior. To investigate this phenomenon, we perform three-dimensional, nonhydrostatic simulations of a mixed-layer front forced by downfront winds, with and without baroclinicity below the mixed layer (ML). We introduce a passive tracer whose initial profile increases linearly from bottom to top with no lateral gradients. Compared to the simulations where the front is confined to the mixed layer, those with deep baroclinicity yield significantly enhanced buoyancy and tracer fluxes both in the ML and below it over time scales associated with the mesoscale evolution of the interior flow. The enhanced buoyancy fluxes result from the release of available potential energy in the interior. They are oriented primarily along isopycnals and hence, parameterizable by a skew flux prescribed within the residual mean framework. We explore potential couplings between the interior and the ML by the enhanced buoyancy fluxes for different peak stratification values in the transition layer. Such coupling has important implications for ML restratification beyond that due to ML instabilities alone. The tracer fluxes are nearly isopycnal in the interior but contain significant down-gradient and rotational components with a negligible skew component. Thus, they are described less effectively by the residual mean framework. A second difference between the tracer and buoyancy fluxes is the presence of a significant diapycnal component of tracer flux at the ML base. The enhanced tracer fluxes below the ML lead to a significant increase in the tracer fluxes within the ML over time scales associated with the mesoscale evolution of the interior. This finding has important implications for the rate of nutrient influx into the upper ocean in the presence of deep baroclinicity. We provide estimates for the different eddy-diffusivity components assuming a linear flux-gradient relationship between the zonally averaged tracer fluxes and the tracer mean-gradients. We contrast the eddy-diffusivity values obtained with and without deep baroclinicity, and test scalings for the same derived in recent literature.