Deviation of Wind Stress From Wind Direction Under Low Wind Conditions

The deviation of the wind stress vector from the wind direction at the air-sea interface under low wind conditions was investigated based on direct eddy covariance flux measurements taken at a coastal tower in the northern South China Sea. The wind stress deviates significantly from the mean wind direction under low wind conditions, with the deviation angle sometimes exceeding 90 degrees, indicating upward momentum transfer from the ocean to the atmosphere. Negative downwind drag coefficient values begin to occur at a wind speed of approximately 4m/s. Our results show that ocean swells and nonstationary airflow play critical roles in wind stress. Prominent peaks at the dominant swell frequency in the vertical velocity spectra are observed at a height of 17m over the mean sea surface, implying that swell-induced perturbations can reach a height of at least 17m, and the wave boundary layer can extend more than 10m above the sea surface. The results of our analysis indicate that at the observation height, the influence of nonstationarity in the wind field is more significant than that of swell-induced motions on the deviation of wind stress. After the removal of nonstationary motions, the deviation angles of the wind stress from the wind direction are generally reduced and vary substantially at low wind speeds. Plain Language Summary Under calm weather conditions, the wind and stress vectors are not aligned. Therefore, the deviation of the wind stress from the wind direction at the air-sea interface under low wind conditions was studied. At low wind speeds, the wind stress vector significantly deviated from the wind direction, with deviations that sometimes exceeded 90 degrees, indicating upward momentum transfer from the ocean to the atmosphere, and the wave boundary layer can extend more than 17m above the sea surface. Ocean swells and nonstationary motions in the wind field may cause the above phenomena. Understanding wind stress characteristics is important for developing ocean, wave, and climate models to improve ocean and climate predictions.