Current spectra under varying stratification conditions in the central North Sea

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)∼Cσ−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.