Wind stress in the presence of swell under moderate to strong wind conditions

Wind stress is a key parameter for oceanic and atmospheric modeling, forecasting, and hydrodynamic studies. It is generally accepted that wind stress depends on the sea state. In particular, it has been shown that the presence of swell can modify both magnitude and direction of the wind stress. The presence of swell enhances momentum flux when swell propagates opposite to the wind direction and reduces it when it travels along the wind direction. However, those conclusions are mainly based on data acquired in low wind speed conditions and it is not clear to what extent an effect of swell persists at higher winds. Here simultaneous measurements of wind stress and waves, carried out in an area characterized by the occurrence of strong offshore winds with counter long-period swell, are presented and analyzed. The observations indicate that swell causes substantial changes to the wind stress at all observed wind conditions, including wind speeds as high as 20 ms(-1). It is believed that in low wind conditions swell increases drag by directly interacting with the air flow, whereas at higher winds, swell reduces drag by modifying the wind-sea-associated roughness.