Impact of Surface Waves on Wind Stress under Low to Moderate Wind Conditions

Abstract

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.

Directional Characteristic of Wind Stress Vector Under Swell‐Dominated Conditions

“…The along-wind and cross-wind components can be positive or negative: τ x > 0 (τ x < 0), implies that the along-wind stress faces or is opposite to the wind direction, and τ y > 0 (τ y < 0), implies that the cross-wind stress is oriented to the right (left) of the wind vector. Notably, regardless of the cross-wind stress is positive or negative, once τ x < 0, the upward momentum will transfer from the ocean to the atmosphere, as proven by Grachev and Fairall (2001) through observations and by Chen et al (2019) via numerical modeling.…”

“…At this tower site, the eddy covariance system was mounted at a height of 17 m above the sea surface, and an acoustic wave and current (AWAC) buoy was deployed at the seabed with a water depth of approximately 16 m for the simultaneous measurement of air-sea momentum flux and surface waves. Detailed descriptions of the observational tower, the air-sea flux, and the surface wave measuring system can be found in Chen et al (2018Chen et al ( , 2019.…”

“…Parametrization of the wind stress is essential for ocean general circulation models and climate models. Although wind stress over the ocean surface has been studied for more than half a century, there are still high uncertainties associated with the determination of wind stress (Babanin et al, 2018;Chen et al, 2019;Jones & Toba, 2001).…”

“…Unlike the land surface, the ocean surface is mobile; therefore, surface waves, have a crucial impact on the stress vector (Chen et al, 2019;Pan et al, 2005;Zou et al, 2018Zou et al, , 2019. Giovanangeli et al (1994) showed that the deviation of wind stress from the wind direction depends on the swell direction and steepness based on the wind-wave tunnel measurements.…”

“…Swells are common in the global ocean and generally propagate in a direction different from the local wind direction (Chen et al, 2019;Semedo et al, 2011). Therefore, the misalignment between the swell direction and the wind direction provides a possibility to investigate the influence of the swell off-wind angle on the stress vector.…”