Difference Between Ocean Surface Wind and Stress Variability

Wind is air in motion, which is part of the atmospheric circulation. Stress is the turbulent transfer of momentum between the ocean and the atmosphere, which is produced by wind shear and buoyance. There were almost no in situ stress measurements except in dedicated field campaigns and the stress estimates we used were almost entirely derived from wind measurements through a drag coefficient. Wind variation has been taken as stress variation. There are spaceborne active and passive microwave sensors that provide surface wind data over global ocean, but the backscatter and emissivity they observe imply that the variability of surface properties attributed to stress are different to those of wind above the surface. Two examples are given, one under the strong wind of hurricanes, where shear production of turbulence dominates, and another over temperature fronts, where buoyance production dominates.

Winds retrieved from scatterometers show saturation at wind speed above 25 m/s under hurricanes; the increase in backscatter with wind speed shows sharp drop. There is large uncertainty in the drag coefficient under hurricanes because there are little credible stress measurements. We developed a stress retrieval algorithm, and the large amount of stress derived from the scatterometer show that the drag coefficient decreases with increasing wind speed at a much steeper rate than previous formulation, at wind speeds between 25-40 m/s. The stress retrieved from the scatterometer implies that the ocean applies less drag to inhibit storm intensification and the storms cause less ocean mixing and surface cooling than using stress computed from wind by conventional methods.

High sea surface temperature increases turbulence production by buoyance, and should collocate with high stress and heat flux. The spatial coherence between spacebased measurements of wind and sea surface temperature has been observed over many locations and under various atmospheric conditions, e.g., tropical instability waves, extensions of western boundary currents, East China Sea during winter cold air outbreak, warm and cold ocean eddies, and typhoon wake. High satellite winds are collocated with warm centers and low magnitude at cool centers. High divergence is located at the high temperature gradient between warm and cool centers. There are disagreement in the explanations of the coherence through boundary dynamics and numerical models; boundary layer winds change with prevailing atmospheric flow. We demonstrate that the observed near ubiquitous coherence is between SST and surface stress. The effects boundary layer depth, pressure gradient force, Coriolis force, baroclinicity, and heat advection diminish as the surface is approached. The locations of divergence and vorticity centers of stress, as distinguished from those of winds, are important to study ocean mixing and other processes.