Properties of vertical current shear across stratification in the North Sea

Observations from the central North Sea show that, as soon as thermal strat ification becomes established by solar insolation in the spring, the vertic al smoothly varying horizontal current structure observed in winter becomes distorted, with strongest vertical shear coincident with the strongest buo yancy gradients (thermoclines). This shear is predominant at the local iner tial frequency following strong wind-forcing or when the thermocline thickn ess is relatively large, and the semidiurnal tidal frequency otherwise. Alt hough the currents at these frequencies have a completely different charact er, being circularly polarized and mode-1 at the inertial frequency and alm ost rectilinear and barotropic at the tidal frequency, their shear vectors are both anticyclonically polarized. While this is understood for near-iner tial motions, it is less obvious for vertically varying tidal currents, in the absence of internal tides. Viscous flows are distinguished from those governed by inviscid physics by inspection of their vertical current structures. It is demonstrated that th e tidal frictional bottom boundary layer not only determines the depth and 'thickness' of the thermocline in shelf seas, but also the fate of shear ac ross the stratification. This shear is dominated by the change in phase of the anticyclonic rotary current component. The circular polarization of the shear vector implies that the shear magnitude varies much slower with time than its components, providing justification for the use of slowly varying exchange parameters in models. As stratification also varies with time muc h slower than the inertial period, a 'constant' eddy diffusivity is rendere d through a marginal stability equilibrium relating shear and stratificatio n and turbulent diapycnal exchange, irrespective of the generating frequenc y.