Impact of Atmospheric Stability on Swell-Induced Perturbations in Wave Boundary Layer

Abstract

Wind stress deviates both in magnitude and direction under light wind conditions due to the modulation effect of swell waves. Eddy covariance flux measurements were conducted from an offshore wind tower in the northern South China Sea to investigate the influence of swell-induced perturbations on the evolution of wind velocity fluctuations and wind stress reductions. Direct evidence of the wind field being affected by dominant swell waves was observed at an observational height of 20 m, indicating that swell-induced perturbations can penetrate the wave boundary layer to at least this height. Time series analysis implies that a stable atmosphere helps preserve the observed prominent peak in wind velocity spectra, extending it to a higher range under large wave age. Therefore, swell-induced perturbations are more easily observed at night. Additionally, our results show that swell wave modulation on wind stress is not merely restricted to moments with pronounced peaks in velocity spectra, indicating that wind stress reduction is not an exception. The most significant reduction occurs at wind speeds around 2–4 m/s, leading to negative wind stress, which implies momentum transfer from the ocean to the atmosphere. Our conclusion illustrates that the deviation of wind stress magnitude increases under stable atmospheric conditions; however, with a simple correction model, the modified wind stress is comparable to the observed values.