An investigation of wave-induced momentum flux through phase averaging of open ocean wind and wave fields

Abstract

This thesis presents an investigation of the influence of the dominant, wind-driven, surface waves on the vertical flux of horizontal momentum in the marine surface layer over open ocean conditions. Through a procedure which involves phase averaging the wind fields at the period of the dominant waves, the wave-induced component of the atmospheric fluctuations is isolated and vertical profiles of mean wave-induced momentum flux are computed. Previous investigators have used phase averaging to remove the turbulence from an oscillatory signal, but the absence of a monochromatic wave field in open ocean conditions complicates this approach. This difficulty is overcome by choosing only the time periods characterized by the most monochromatic-like waves present and filtering those sections of wind data with a narrow band-pass filter centered around the dominant wave frequency before beginning the phase averaging process. The analysis is carried a step further by investigating the dependence of wave-induced momentum flux on sea state through bin averaging according to wave age. This results in a set of profiles which express the ratio of wave-induced momentum flux to the total flux as a function of the wave age parameter c/U10. These profiles all tend to zero with height, and remain negligible at all heights over fully developed seas. Over younger seas, this ratio becomes increasingly more positive (corresponding to a positive value for - uw) with decreasing wave age; while over older seas, this ratio becomes increasingly more negative with increasing wave age. This provides strong quantitative evidence that there is a significant flux of momentum to the atmosphere from decaying waves, and a transfer of atmospheric momentum to developing waves.