NONLINEAR-WAVE INTERACTIONS AND HIGH-FREQUENCY SEA-FLOOR PRESSURE

Linear wave theory predicts that pressure fluctuations induced by wind-generated surface gravity waves are maximum at the ocean surface and strongly attenuated at depths exceeding a horizontal wavelength. Although pressure fluctuations observed at the seafloor in deep water are indeed relatively weak at wind-wave frequencies, the energy at double wind-wave frequencies is frequently much higher than predicted by applying linear wave theory to near-surface measurements. These double-frequency waves can in theory be excited by nonlinear interactions between two surface wave components of about equal frequency, traveling in nearly opposing directions. Observations from a large aperture, 24-element array of pressure sensors deployed in 13-m depth are presented that quantitatively support this generation mechanism. As in previous studies, dramatic increases in the spectral levels of seafloor pressure at double wind-wave frequencies (0.3-0.7 Hz) frequently occurred after a sudden veering in wind direction resulted in waves propagating obliquely to preexisting seas. The observed spectral levels and vector wavenumbers of these double-frequency pressure fluctuations agree well with predictions obtained by applying second-order nonlinear, finite depth wave theory (Hasselmann, 1962) to the observed directionally bimodal seas. High-frequency seafloor pressure spectral levels also increased in response to directionally narrower but more energetic seas generated by strong, steady or slowly rotating winds. Bispectral analysis suggests that these pressure fluctuations are generated by nonlinear mechanisms similar to the veering wind cases.