Some observations of nonlinearly modified internal wave spectra

Abstract

Four examples are discussed of different internal wave band kinetic energy spectra from moored instruments at middle and high latitudes (38° < ϕ < 61°) in the eastern Atlantic Ocean and the western Mediterranean Sea. It is observed that for N ≫ f, where N is the buoyancy frequency and f is the inertial frequency, the spectral falloff rate with frequency σ for internal waves tends to σ−3. Either spectra are dominated at tidal and/or inertial nonlinear higher harmonics superposed on a nearly white (noise) spectral continuum, the latter extending to supertidal frequencies up to N (it is hypothesized that such a spectrum is found near the source of nonlinear interactions), or the spectrum is smooth, showing a σ−3 falloff rate for most of the internal wave band continuum outside an inertial peak. Such a spectrum may be typical for regions well away from the sources for interaction, although governed by the same nonlinear advection dynamics. The present observations contrast with a recently proposed model for the internal wave spectrum, similar to the canonical Garrett and Munk [1972] model, showing a σ−2 falloff rate, except for a nearly white continuum in the range between f and the solar semidiurnal tidal frequency (S2) for latitudes ∣ϕ∣ < 20°.