Modeling the atmospheric boundary layer wind response to mesoscale sea surface temperature perturbations

Abstract

The wind speed response to mesoscale SST variability is investigated over the Agulhas Return Current region of the Southern Ocean using the Weather Research and Forecasting (WRF) Model and the U.S. Navy Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) atmospheric model. The SSTinduced wind response is assessed from eight simulations with different subgrid-scale vertical mixing parameterizations, validated using Quick Scatterometer (QuikSCAT) winds and satellite-based sea surface temperature (SST) observations on 0.25° grids. The satellite data produce a coupling coefficient of sU = 0.42ms-1 °C-1 for wind to mesoscale SST perturbations. The eight model configurations produce coupling coefficients varying from 0.31 to 0.56ms-1 °C-1. Most closely matching QuikSCAT are a WRF simulation with the Grenier-Bretherton-McCaa (GBM) boundary layer mixing scheme (sU = 0.40ms-1 °C-1), and a COAMPS simulation with a form of Mellor-Yamada parameterization (sU = 0.38ms-1 °C-1). Model rankings based on coupling coefficients for wind stress, or for curl and divergence of vector winds and wind stress, are similar to that based on sU. In all simulations, the atmospheric potential temperature response to local SST variations decreases gradually with height throughout the boundary layer (0-1.5 km). In contrast, the wind speed response to local SST perturbations decreases rapidly with height to near zero at 150-300 m. The simulated wind speed coupling coefficient is found to correlate well with the height-averaged turbulent eddy viscosity coefficient. The details of the vertical structure of the eddy viscosity depend on both the absolute magnitude of local SST perturbations, and the orientation of the surface wind to the SST gradient.