Satellite Measurements of Coupled Ocean-Atmosphere Interaction

Simultaneous satellite measurements of surface wind stress by the QuikSCAT scatterometer and sea-surface temperature (SST) by the Advanced Microwave Scanning Radiometer (AMSR) have revealed a remarkably strong influence of SST on the low-level wind field. In distinct contrast to the negative correlation observed on basin scales, SST and wind stress are positively correlated on scales of 100-3000 km. While in situ observations had hinted at the existence of this ocean-atmosphere interaction at a few locations around the World Ocean, the prevalence and intensity of this phenomenon did not become apparent until the availability of the global coverage afforded by the satellite data. Examples of this coupling between SST and wind stress will be shown for several regions of the World Ocean, including the Gulf Stream, Agulhas Return Current, Pacific equatorial cold tongue, and California Current regions. Although the air-sea interaction processes that are responsible for this observed correlation are not completely understood, it is believed that SST influence on boundary layer stability plays a preeminent role. Enhanced convective and turbulent mixing over warm water deepens the boundary layer and mixes momentum from aloft down to the sea surface, thus increasing the surface winds. Stabilization of the boundary layer over cold water decreases the boundary layer thickness and reduces turbulent mixing, thus decoupling the surface winds from the winds aloft and decreasing the surface winds. The SST-induced spatial variations in the wind stress field result in wind stress curl and divergence that are linearly related respectively to the crosswind and downwind components of the SST gradient. Because of the thermal wind effect of SST-induced temperature gradients in the atmospheric boundary layer, the divergence response is consistently stronger than that of the curl. The feedback effects of SST-induced perturbations of the wind stress curl field on the ocean circulation are only beginning to be explored.