Wave-wind interactions: Exploring swell wave influence on wind direction

The exchange of momentum between the atmosphere and ocean depends on the angular orientation of the wind relative to ocean currents and waves. Winds are traditionally measured several meters above the ocean surface and calibrated to 10 m height, but their direct impact on the ocean occurs at the surface. Some analyses of buoy winds (typically measured around 3 m height) have suggested directional biases compared with scatterometer winds (which reflect wind direction inferred from capillary waves on the ocean surface). To evaluate the magnitude of this effect, we co-locate scatterometer winds with buoy measurements from the TOGA-TAO, RAMA, and PIRATA arrays in the tropical oceans and with the Stratus mooring off the Pacific coast of South America. Results show a small but statistically significant bias in some regions. Directional biases cannot be explained by the atmosphere’s Ekman spiral because the planetary boundary layer has too large a vertical extent to account for any measurable angular difference over a distance of just a few meters, nor can they be accounted for by surface current corrections, which are relatively modest. Wave-induced stress has the potential to substantially modify wind-stress at the ocean surface relative to wind-stress at 3-m height. If waves are locally generated, the wind-induced stress and wave-induced stress will be oriented in approximately the same direction, and we would expect no directional bias between buoy and scatterometer wind measurements. However, in many parts of the global ocean, the wave field is dominated by swell associated with remote wind forcing and is not necessarily aligned with the local winds. WaveWatch3 model directional wave fields suggest that buoy--scatterometer differences are dependent on wave direction relative to wind direction. The resulting wave-induced stress has the potential to account for the observed directional bias between buoy and scatterometer winds and to modulate air-sea momentum exchange.