Wave-wind nonlinearity observed at the sea floor. Part I: forced- wave energy

Abstract

This is Part 1 of a study of nonlinear effects on natural wind waves. Array measurements of pressure at the sea floor and middepth, collected 30km offshore of Virginia in 13m depth, are compared to an existing theory for weakly nonlinear surface gravity waves. In 13m depth, wind-generated free waves and corresponding long- wavelength, high-frequency forced waves can be simultaneously observed on the sea floor, and the coupling between the two examined in some detail. Bottom-pressure spectra observed over a 4-day period show large (O(10 "SUP 2" )) fluctuations in high-frequency (0.35-0.6Hz) forced-wave energy levels at the sea floor occurring in only a few hours. Correspondingly rapid changes in estimates of the free-wave frequency-directional spectrum show that forced-wave energy levels are weak in unidirectional seas and increase dramatically in response to nearly opposing seas, consistent with the theoretical generation mechanism. On one occasion, directionally opposing seas, and a corresponding double-frequency forced-wave peak, followed a rapidly veering wind. However, comparable increases in forced-wave energy levels were observed in response to the arrival of nonlocally generated seas with directions much different than local winds and seas. Although the accuracy of theoretical forced-wave predictions is limited by the directional resolution of the small aperture (20m X20m) middepth array, predicted and observed forced-wave energy levels agree within about a factor of 2. The observed weak decay between middepth and sea-floor wave pressure at double sea frequencies is also consistent with theoretically expected long wavelengths. (from Author)