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

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 ofthe 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/ U 10 • 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.

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