Scale and Rossby Number Dependence of Observed Wind Responses to Ocean-Mesoscale Sea Surface Temperatures

Abstract

The horizontal scale dependences of in-phase and lagged imprints of ocean-mesoscale sea surface temperatures on surface winds are investigated using daily AMSR-E radiometer and QuikSCAT scatterometer observations in the Southern Ocean. Spectral transfer functions separate underlying processes dependent on large-scale winds, horizontal wavenumbers, and corresponding Rossby numbers. For Rossby numbers smaller than 1, winds reflect modulations of the Ekman layer by sea surface temperature–induced changes of hydrostatic pressure. Rossby numbers large compared to 1 suggest a balance of advection and modulations of vertical mixing. Impulse response functions reveal Southern Hemisphere, Doppler-shifted, near-inertial lee waves excited by warm ocean-mesoscale sea surface temperatures. On the right (left) flank of the downwind wake of warm air and low atmospheric pressure, winds are enhanced (diminished) due to constructive (destructive) interference of inertial turning, pressure gradient forces, and vertical mixing. Wind convergence over the warm wake is stronger compared to the upwind divergence. Time averaging smooths the response, and degrades the lee wave.