Effect of waves on the magnitude and direction of wind stress over the ocean

Wind Stress plays a vital role in the sea-atmosphere interaction process, which affects climate, weather, and oceanic circulation models. Wind drag coefficient parameterizations are usually used to estimate wind stress; these relationships tend to overestimate or underestimate the momentum transfer, especially in weak to moderate wind regimes during swell conditions. Also, it is commonly presumed that the wind stress is always aligned with the wind, but this is only sometimes the case. The wind stress and its turbulent and wave-coherent components were estimated through measurements of the free surface level and the wind speed using a spar buoy with a sonic anemometer and an array of six wave staffs. The state of the sea was also characterized by obtaining the directional spectrum of the waves. Continuous measurements were made for at least four months at three different sites (two in the Gulf of Mexico and one in the northern Mexican Pacific) with sampling rates of 10 Hz for free surface level and 100 Hz for wind speed. Multiple swell systems’ influence on the wave boundary layer is avoided by only analyzing events with a single dominant wave system.  It was observed that during swell conditions with wind traveling in the same direction, the wave-coherent wind stress component has an opposite direction to the wind, which dampens the total wind stress magnitude. During counter-directional wind relative to swell traveling direction, the wave boundary layer is modified; it appears that swell accelerates wind near the surface without changing its direction, resulting in a wind stress magnitude more significant than expected. Also, during the wind stress analysis, a reference frame oriented to the propagation direction of the primary wave system was found to be better for asses wind waves’ influence on the wind stress magnitude and direction. Using this frame of reference makes it possible to isolate wind wave influence on wind stress in its directional component aligned with the wave direction. The perpendicular directional component relative to the wave’s direction is primarily turbulent.