Modeling Wind Response to Fine-Scale SST Gradients

Satellite observations of vector wind and sea surface temperature (SST) at 25-km grid spacing positive correlation between small-scale SST and surface wind speed anomalies with a coupling coefficient of 0.43 m/s/°C. We perform atmospheric simulations with the Weather Research and Forecasting (WRF) model to characterize surface wind response to SST on fine spatial scales. Atmospheric simulations over the Agulhas Current region are integrated for one month. Boundary conditions are derived from satellite-based NOAA Reynolds high resolution daily SST analyses on a 0.25 degree grid. Realistic synoptic atmospheric boundary conditions are supplied laterally from the NOAA global forecast system (GFS) reanalysis.

The simulated surface winds using the Grenier-Bretherton vertical mixing parameterization yields a surface wind speed coupling coefficient to small-scale SST of 0.46 m/s/ºC, in agreement with QuikSCAT observations. The WRF implementation of the Janjic (MYJ) vertical mixing scheme gives a lower coupling coefficient (0.31 m/s/ºC) and the Nakanishi-Niino (MYNN) mixing scheme gives a higher coupling coefficient (0.55 m/s/ºC).

The response of winds to SST is shown at different elevations throughout the troposphere. Warm small-scale SST anomalies increase wind speed in the lower 100 m and slightly decrease wind speed in the cloud-topped boundary layer below 1.5 km.