Nonlocal Fluxes in Second Moment Closures of Langmuir Turbulence and Convection

The use of nonlocal fluxes in upper ocean vertical mixing schemes has been limited primarily to low-order turbulence closures based on the K-profile mixing parameterization. There, they represent the vertical redistribution of surface tracer fluxes across the ocean boundary layer through turbulent transport by convective plumes, large-eddy structures with dynamics not uniquely determined by the local mean shear or stratification. Here, nonlocal terms are incorporated consistently in both the algebraic and dynamic components of a two-equation ‘q2-q2l’ second moment closure for Langmuir turbulence, where fluxes are carried across the boundary layer by counter-rotating vortices generated by the near-surface interaction of turbulent momentum fluxes and the Stokes drift of surface waves. Nonlocal Reynolds stresses and fluxes are set in a unified framework applicable to both Langmuir and convective turbulence, so that profiles of these second moments account for both local production and nonlocal sources, as well as for the local modification of nonlocal sources. Predictions of vertical mixing are compared with large eddy simulations and observations, focusing on the formation of oceanic diurnal warm layers and jets, on the mean profiles of momentum in the lower mixed layer of the upper ocean, and on impacts for the mean vertical distribution and horizontal dispersion of neutral and buoyant tracers.