Effects of Submesoscale Eddies and Small-Scale Langmuir Turbulence on Multi-Scale Fluxes, Flow Instabilities, and Spectra in the Oceanic Mixed Layer

Interactions between three-dimensional Langmuir turbulence and quasi two-dimensional submesoscale processes are examined in the oceanic mixed layer using large eddy simulations of the spin-down of a mesoscale temperature front. The simulations solve the Boussinesq equations both with and without Stokes drift wave forcing in a computational domain of size 20km x 20km x -160m with 5m horizontal and 1.25m vertical resolution. The enormous scale range in the simulations allows interactions between kilometer-scale submesoscale eddies and small-scale Langmuir turbulence to be studied, and also reveals the presence of a “double” spectral energy cascade consisting of a two-dimensional cascade at large scales and a three-dimensional cascade at small scales. The effects of submesoscale eddies and Langmuir turbulence on spatial and spectral properties of velocity, buoyancy, and biochemically-relevant tracers are described, and multiscale fluxes are used to show that Langmuir turbulence counters the restratifying effects of submesoscale eddies, resulting in substantially greater vertical mixing of buoyancy and tracers as well as reduced prevalence of symmetric instabilities associated with submesoscale processes. Conversely, counter-gradient tracer transport occurs co-located with regions of negative potential vorticity, suggesting that symmetric instabilities or other submesoscale phenomenon may themselves act to oppose small-scale turbulent mixing. The implications of these results for parameterizations of buoyancy, momentum, and tracer transport in the oceanic mixed layer are briefly discussed.