HIGHER-ORDER SPECTRAL-ANALYSIS OF NONLINEAR OCEAN SURFACE GRAVITY-WAVES

Bispectral and trispectral analyses are used to detect secondary and t ertiary wave components resulting from nonlinear interactions among la rge-amplitude ocean surface gravity waves in 8- and 13-m water depths. Bispectra of bottom-pressure measurements indicate forced secondary w aves at frequencies 2f(p) about twice the primary power spectral peak frequency f(p). However, the interpretation of the bispectrum at sum f requencies of approximately 3f(p) is ambiguous because contributions o f both secondary and tertiary forced waves may be significant. Trispec tral analysis confirms the presence of tertiary waves with frequency a pproximately 3f(p) In 8 m depth the tertiary bottom-pressure field is dominated by interactions between three colinearly propagating wind-wa ve components with frequencies close to f(p). In 13 m depth these rela tively short-wavelength forced waves are strongly attenuated at the se afloor and the tertiary wave field is driven by interactions between t he dominant waves at f(p) and obliquely propagating higher-frequency w ind waves. The phases of the higher-order spectra are consistent with weakly nonlinear wave theory (Hasselmann, 1962).