Surface boundary layer evolution and near‐inertial wind power input

Abstract

AbstractDeep 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.