Regional and Global Satellite Observations of Surface Wind Modification by Sea Surface Temperature Fronts

Recent satellite observations have revealed the remarkably strong influence of sea surface temperature (SST) gradients on surface winds. As winds blow across SST fronts, modification of the marine atmospheric boundary layer (MABL) by spatially-varying surface forcing results in perturbations in surface wind speed and direction. Satellite measurements have shown that the mesoscale coupling between SST fronts and surface winds observed in regional, in situ studies occurs worldwide wherever large SST gradients exist. In this study, the geographical and seasonal characteristics of this coupling are described globally from observations of surface wind stress by the SeaWinds scatterometer on the QuikSCAT satellite and sea surface temperature (SST) by the Advanced Microwave Scanning Radiometer (AMSR) on the EOS-Aqua satellite.

SST gradients influence surface winds by modifying the stability of the MABL through changes in air-sea heat flux and through the development of secondary circulations. Over colder water, decreased surface heat fluxes stabilize the MABL, inhibiting the vertical turbulent mixing of momentum from aloft to the surface and decelerating the surface winds. Over warmer water, increased surface heat fluxes destabilize and deepen the MABL, enhancing the vertical turbulent mixing of momentum within the MABL, which accelerates the surface winds. These short-scale perturbations of the surface wind field are clearly manifest in the wind stress curl and divergence fields, which are linearly related to short-scale perturbations in the crosswind and downwind components of the SST gradient field, respectively.

Global fields of short-scale wind stress curl and divergence perturbations vary seasonally and geographically, with the strongest perturbations occurring in the winter hemisphere and the weakest in the summer hemisphere. The effect is especially clear along the Agulhas Return Current south of Africa, where the magnitudes of the short-scale curl and divergence responses to crosswind and downwind SST gradients are nearly twice as strong during the winter than during the summer along a zonal band between 40°-50°S. These seasonal variations closely follow temporal and geographical variability of the large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. An overall decrease in large-scale MABL static stability in the winter allows deeper turbulent and convective mixing of momentum and would thus be expected to increase the curl and divergence response to a given SST gradient.

From a preliminary analysis of the spatial variability of surface wind direction along SST fronts, it is evident that the surface winds tend to rotate towards warmer water over regions of strong SST gradients. The observed changes in wind direction as air blows obliquely to SST fronts may be due to the addition of enhanced cross-frontal surface flow driven by cross-frontal pressure gradients.