Abstract

An overall frequency (σ) spectrum φw(σ, N, fh) is composed for vertical current (w) observations from ocean areas with different stratifications represented by large, 100 m scale buoyancy frequency N. The horizontal Coriolis parameter is denoted by fh. In the open Atlantic Ocean, the relatively strong horizontal layering in density supports the φw that is found in semiequilibrium under the degradation of the lowest internal wave mode. This φw is not constant with frequency; however, its variance increases between σ = f and its peak near σ = (0.5 – 0.7) N >> f, where f denotes the local inertial frequency. In contrast, the φw spectra from the weakly stratified deep Mediterranean and from the shallow North Sea peak at ∼ 1.1 f, except for a tidal peak in North Sea data, and fall off at a rate of σ–1 for 1.1 f < σ < Nm = 4fh, where Nm denotes maximum small-scale N. Background conditions are characterized by vertical instead of horizontal layering in density. Such spectra are best described by a nonequilibrium evolution of small-scale internal waves under a weak mean shear, which transfers variance to σ ≈ f. This φw ∼ Np, p > 0, is contrary to the existing model of open-ocean equilibrium spectra that decrease with N (p < 0). A smooth transition between the two regimes is found to coincide with top-peak φw for N/fh = 4.

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