Abstract

Observations from the central North Sea show that, as soon as thermal stratification becomes established by solar insolation in the spring, the vertical smoothly varying horizontal current structure observed in winter becomes distorted, with strongest vertical shear coincident with the strongest buoyancy gradients (thermoclines). This shear is predominant at the local inertial frequency following strong wind-forcing or when the thermocline thickness is relatively large, and the semidiurnal tidal frequency otherwise. Although the currents at these frequencies have a completely different character, being circularly polarized and mode-1 at the inertial frequency and almost rectilinear and barotropic at the tidal frequency, their shear vectors are both anticyclonically polarized. While this is understood for near-inertial 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 the tidal frictional bottom boundary layer not only determines the depth and 'thickness' of the thermocline in shelf seas, but also the fate of shear across 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 much slower than the inertial period, a 'constant' eddy diffusivity is rendered through a marginal stability equilibrium relating shear and stratification and turbulent diapycnal exchange, irrespective of the generating frequency.

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