Surface boundary layer evolution and near-inertial wind power input

Abstract

Deep weakly stratified surface layers in the Southern Ocean complicate the identification of the mixed-layer base, which is critical in estimating the wind power input through the ocean surface. Typically used mixed-layer depth criteria often ignore weak stratification, which traps momentum near the surface and significantly enhances the near-inertial-band wind power input. The thickness of the active mixing-layer, the turbulent layer in contact with wind stress, is needed to accurately estimate wind power input. A fine-density-threshold criterion of 0.005 kg m-3, just above the noise floor of most autonomous instruments, was applied to observed profiles of potential density to estimate the thickness of the actively mixing-layer. Vertical shear, Langmuir cells, and buoyant convection are investigated as possible mechanisms maintaining turbulence within the mixing-layer. Over 90% of the observed variance of the mixing-layer thickness is explained by either shear-driven entrainment, which is simulated using the Price-Weller-Pinkel model, or by a parameterization of downwelling plumes due to Langmuir cell convergence. In general, surface buoyancy fluxes are too weak to drive mixed-layer turbulence. Comparison of National Oceanographic Data Center (NODC) climatological mixed-layer thickness to those determined using the 0.005 kg m-3 density threshold suggests a multiplicative seasonally varying correction of 1.5-3.5 should be applied to wind work estimates made using the NODC climatological mixed-layer thickness in the Southern Ocean.