A composite vertical current spectrum for strongly and weakly stratified seas and oceans
An overall frequency (σ) spectrum φ w (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 φ w that is found in semiequilibrium under the degradation of the lowest internal wave mode. This φ w 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 φ w 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 φ w ∼ 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 φ w for N/fh = 4.
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Document Type: Research Article
Publication date: January 1, 2015
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