Mixed Layer Formation and Restratification in the Presence of Mesoscale and Submesoscale Turbulence

Abstract:

Recent realistic high resolution modeling studies show a net increase of submesoscale activity in fall and winter when the Mixed Layer Depth (hereafter MLD) is at its maximum. This submesoscale activity is associated with the shallowing of the MLD when compared to similar model configurations at lower resolution. Both phenomena can be related to the development of Mixed Layer Instabilities (MLIs), which by slumping the horizontal density gradient in the mixed layer convert Available Potential Energy into submesoscale Eddy Kinetic Energy and contribute to a fast restratification. While parameterizations of the shallowing of the mixed layer by MLIs have been proposed they are based on idealized configurations and are not yet fully tested in realistically forced simulations. In the present work, the ML formation and restratification is studied by uniformly cooling a fully turbulent zonal jet in a periodic channel at different resolutions (eddy resolving (10km) to submesoscale permitting (2km)). The effect of horizontal resolution is quantified in terms of MLD, restratification rate, buoyancy fluxes, and conversion of Available Potential Energy in Eddy Kinetic Energy. At the highest resolution when submesoscale is active the MLD formed during the surface cooling is shallower by about 30% and the total restratification almost three times faster. Such differences between low and high resolution models are explained by the submesoscale vertical buoyancy flux which compensates the convection during the formation phase and accelerates the restratification once the surface cooling stops. These findings are robust and the uncertainties are quantified by ensemble experiments. The most common parameterization of MLIs has been implemented and tested in our coarse resolution configuration, leading as expected to a slightly faster restratification, but the shallowing of the MLD is not represented, suggesting a caveat in such parameterization in presence of strong convection.