Reduction of wind stress due to swell at high wind conditions

Wind stress over the ocean depends on the sea surface roughness which is determined by the sea state. On one hand, underdeveloped wind seas, rougher than their fully developed counterpart, increase the drag. On the other hand, the presence of swell can modify wind stress by modifying the wind sea roughness. This latter mechanism is believed to have a great impact at high winds whenever underdeveloped local waves coexist with swell. Detailed measurements of wind stress and wavefield in fetch-limited growth conditions were made in an area subjected to strong and persistent winds. Through the analysis of wavefield observations, it is found that the presence of swell dampens the short wind waves. The observed attenuation is greater for younger wind seas and decreases as the wind waves become older. Results obtained from modeling the interaction of wind waves and the air flow above point out that the attenuation of short wind waves causes a reduction of the wave-supported stress, which in turn decreases the total wind stress.