|Current spectra under varying stratification conditions in the central North Sea|In: Journal of Sea Research. Elsevier/Netherlands Institute for Sea Research: Amsterdam; Den Burg. ISSN 1385-1101, more
Atmospheric forcing; Internal waves; Kinetic energy; Seasonal variations; Shear strength; Spectra; Stratification; Summer; Winter; ANE, North Sea [Marine Regions]; Marine
Given strongly different vertical stratification and not significantly different kinetic energy, `internal wave' band spectral properties were studied using two 8-day representative sets of observations from the central North Sea in winter and summer. The observed similar spectral shape of the internal wave band was due to a combination of deterministic narrow band and intermittent signals. In addition to dominant tidal harmonics, 25% of total kinetic energy was found at inertial and non-linear inertial-tidal interaction frequencies in summer and about the same amount was found in broad-band response to atmospheric forcing in winter. The energy at the (seasonal) non-linear interaction frequencies was proportional to stratification, specifically, to near-inertial shear magnitude. In summer, motions at frequencies (σ) between f<σ< 2.5 cpd (cycles per day) appeared in low vertical mode, whilst the power P(σ) of motions at σ> 4 ±1 cpd obeyed canonical internal wave scaling P(σ) ~ C-2, provided the shear magnitude [S] was used instead of the buoyancy frequency N for the factor C. Motions at 2.5 <σ< 4 ± 1 cpd appeared as transition between the two regimes. In winter no inertial motions were observed.