Monday, 12 January 2009
Determination of the vector wind stress response to small-scale SST variability
Hall 5 (Phoenix Convention Center)
A method for determining the vector wind stress response to SST variations with spatial scales of 100-1000 km is described and evaluated using satellite measurements of vector surface winds by the QuikSCAT scatterometer and SST by the Advanced Microwave Scanning Radiometer onboard the EOS-Aqua (AMSR-E) satellite. It is necessary to develop a technique to determine the SST-induced vector wind stress components since no consistent statistical relationship exists between the individual vector wind stress components and SST. This method relies on interrelationships between the surface vorticity and divergence and crosswind and downwind SST gradients developed statistically from satellite observations. The vorticity and divergence fields are related analytically to the wind stress curl and divergence from which the vector wind stress components are determined from. The method presented here explicitly accounts for the effects of SST on the surface wind speed and direction and is fully consistent with the known interrelationships between the wind stress curl and divergence and the crosswind and downwind SST gradients derived from satellite observations. Comparisons between the wind stress components "reconstructed" from SST and those measured by QuikSCAT agree very well, with correlation coefficients greater than 0.9 for both the zonal and meridional wind stress components; likewise, the wind stress curl and divergence fields agree just as well.
These statistical relations between surface wind stress and SST have been developed into an empirical formulation for coupled wind stress-SST interactions that can be used to dynamically prescribe small-scale wind stress variability in uncoupled ocean general circulation models. SST-induced wind stress variability is not accurately simulated in numerical weather prediction model wind stress fields used to force ocean circulation models. Higher resolution SST fields and better modeling of boundary layer dynamics are needed to resolve this issue. In the meantime, this method can provide an accurate means of prescribing small-scale wind stress variability coupled dynamically to SST.