The impact of ocean surface waves on wind stress at the air–sea interface under low to moderate wind conditions was systematically investigated based on a simple constant flux model and flux measurements obtained from two coastal towers in the East China Sea and South China Sea. It is first revealed that the swell-induced perturbations can reach a height of nearly 30 m above the mean sea surface, and these perturbations disturb the overlying airflow under low wind and strong swell conditions. The wind profiles severely depart from the classical logarithmic profiles, and the deviations increase with the peak wave phase speeds. At wind speeds of less than 4 m s−1, an upward momentum transfer from the wave to the atmosphere is predicted, which is consistent with previous studies. A comparison between the observations and model indicates that the wind stress calculated by the model is largely consistent with the observational wind stress when considering the effects of surface waves, which provides a solution for accurately calculating wind stress in ocean and climate models. Furthermore, the surface waves at the air–sea interface invalidate the traditional Monin–Obukhov similarity theory (MOST), and this invalidity decreases as observational height increases.

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°, indicating upward momentum transfer from the ocean to the atmosphere. Negative downwind drag coefficient values begin to occur at a wind speed of approximately 4 m/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 17 m over the mean sea surface, implying that swell‐induced perturbations can reach a height of at least 17 m, and the wave boundary layer can extend more than 10 m 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.