Effects of the sea state in the momentum flux over the ocean atmosphere interface

The sea surface wind stress is relevant in processes of different scales of space and time such as the exchange of gases and heat, the surface currents, the depth of the mixed layer, the turbulence injection into the ocean. The wind waves are the key component in the coupling of the lower layer of the the atmosphere and the surface layer of the ocean, and various studies have shown the direct and indirect effects on the surface wind stress. In the present study, we present the measurements of the momentum flux and the results meteorological variables at the interface between the ocean and the atmosphere, by using and Oceanographic and Marine Meteorology Buoy (BOMM1) between November 2017 and February 2018. The analysis of the results during moderate wind conditions (U_10N > 8 ms^-1) in which mixed sea state conditions occur (swell that interacts with locally generated wind waves) we found a decrease of the roughness length (z₀), related to developing waves with higher steepness (ak), the data suggest that the presence of swell alters the wind sea part of the spectrum, which leads a reduction of the energy level of the wind-generated waves, hence reducing the wind sea associated roughness. For well developed waves conditions, the roughness length is greater than the parametrization proposed by Drennan al., (2003) for pure wind sea conditions, the data suggest that this is due direct interaction of the wind airflow and swell with higher steepness.  The data of this work suggests that during these conditions (U_10N > 8 ms^-1) , the mechanism of reduction of the drag of the wind sea due to the presence of swell, and the increase of the wind stress by direct interaction of swell with the airflow causes the net effect of wave field to behave as expected under pure wind sea conditions, and there seems to be no swell effect.