Ocean Surface Stress Under Tropical Cyclones Observed from Space

Spaceborne scatterometers, which measure backscatter from the ocean surface, have been providing ocean surface winds, with magnitude and direction for decades. Ocean surface stress, the turbulent transport of momentum, is largely derived from wind through a drag coefficient. In tropical cyclones (TC), scatterometers have difficulty in measuring strong wind and there is large uncertainty in the drag coefficient because there is a lack of stress measurements. We postulate that the microwave backscatter from ocean surface roughness, which is in equilibrium with local stress, does not distinguish weather systems. Under this assumption, we applied a stress retrieval algorithm for the scatterometer, developed over a moderate wind range, to retrieve stress under the strong winds of TCs. Using almost a million coincident stress and wind pairs, we showed that the drag coefficient decreases with wind speed at a much steeper rate than previously revealed, for wind speeds over 25 m/s. The result implies that the ocean applies less drag to inhibit TC intensification and the TC causes less ocean mixing and surface cooling than previous studies indicated. Under the strong wind regime, turbulence is generated mainly by wind shear, which is the vector difference between wind and current. Ocean surface stress derived from spaceborne scatterometers was put into a TC-centric coordinate and composites of vorticity and divergence of stress are compared with those of the wind field. The asymmetry of stress (tangential and radial components) is characterized in relation to other parameters, such as, translation velocity and surface temperature, with the perspective of TC intensification.