A Physically Based Modification of the KPP Turbulence Closure Scheme for the Upper Ocean that Accounts for the Effects of Langmuir Turbulence

Abstract Nonbreaking surface gravity waves enhance turbulence in the ocean surface boundary layer (OSBL) due to the generation of the coherent structures known as Langmuir circulations by the Stokes drift. In this study, a representation of this effect inspired by surface layer theory and rapid distortion theory (RDT) is developed and subject to preliminary tests using the 1D General Ocean Turbulence Model (GOTM) and observations in the North Atlantic from the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) field campaign. The eddy viscosity and heat diffusivity coefficients used in the K -profile parameterization (KPP) of turbulence are modified according to scalings suggested by RDT and extrapolated from the surface layer to the whole mixed layer. RDT is used, in particular, to motivate the fact that the eddy diffusivity for heat is much less enhanced by waves than the eddy viscosity in convective conditions. This effect is implemented by setting the wave correction to eddy diffusivity for heat to zero when the heat flux out of the ocean exceeds a certain threshold. GOTM simulations of the upper ocean over the approximately 1-yr period of the OSMOSIS campaign, using a KPP corrected for wave effects, show substantial improvement relative to the KPP without wave effects during spring and summer (when the ocean is stable and wave effects dominate mixing), without degrading the performance of the model during autumn and winter (when the effect of convection becomes dominant, and the KPP overestimates mixing).