A composite vertical current spectrum for strongly and weakly stratified seas and oceans
van Haren, H. (2015). A composite vertical current spectrum for strongly and weakly stratified seas and oceans. J. Mar. Res. 73(1-2): 33-48. dx.doi.org/10.1357/002224015815782670
In: Journal of Marine Research. Sears Foundation for Marine Research, Yale University: New Haven, Conn.. ISSN 0022-2402; e-ISSN 1543-9542, more
An overall frequency (s) spectrum f w (s, 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 f w that is found in semiequilibrium under the degradation of the lowest internal wave mode. This f w is not constant with frequency; however, its variance increases between s = f and its peak near s = (0.5 – 0.7) N >> f, where f denotes the local inertial frequency. In contrast, the f 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 s–1 for 1.1 f < s < 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 s ˜ f. This f 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 f w for N/fh =
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