Directional Characteristic of Wind Stress Vector Under Swell-Dominated Conditions

The wind stress vector over the ocean surface is often misaligned with the wind direction; this misalignment in a swell-dominated regime is not well investigated. Directional characteristic of the wind stress vector under swell-dominated conditions is analyzed through direct flux observations taken over the coastal region in the northern South China Sea. Observational Ogive curves demonstrate that an additional velocity component induced by a strong swell at wind speeds below 4 m/s can be captured at a height of 17 m away from the sea surface. Our results show that swell can cause a deviation in the wind stress from the wind direction, and the stress vectors mostly reside between the wind direction and the opposite swell direction under low to moderate wind and strong background swell conditions. Further directional analysis indicates that the deviation in the stress vector from the wind vector is mainly governed by the speeds and directions of the wind and swell. With the increase in the wind speed and inverse wave age, this deviation decreases and becomes less scattered; additionally, it increases with the angle between the wind and swell directions, and a linear relation is proposed at wind speeds greater than 6 m/s.