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. Altho ugh pressure fluctuations observed at the seafloor in deep water are i ndeed relatively weak at wind-wave frequencies, the energy at double w ind-wave frequencies is frequently much higher than predicted by apply ing linear wave theory to near-surface measurements. These double-freq uency waves can in theory be excited by nonlinear interactions between two surface wave components of about equal frequency, traveling in ne arly opposing directions. Observations from a large aperture, 24-eleme nt array of pressure sensors deployed in 13-m depth are presented that quantitatively support this generation mechanism. As in previous stud ies, 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 obliqu ely to preexisting seas. The observed spectral levels and vector waven umbers 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 generate d by strong, steady or slowly rotating winds. Bispectral analysis sugg ests that these pressure fluctuations are generated by nonlinear mecha nisms similar to the veering wind cases.