Mixing within the interior of the Faeroe-Shetland Channel

Abstract

Observations of the distribution of the density field, the dissipation rate of turbulent kinetic energy, e, the vertical diffusivity, Kz, and optical backscatter over a repeated cross-section of the FaeroeShetland Channel (FSC) are presented within the context of ocean mixing. Turbulence in the permanent pycnocline occurs in discrete vertical patches of thickness 20‐50 m coincident with layers of elevated vertical current shear magnitude, S 10 2 s 1 . The layers of elevated shear result from high-wavenumber internal waves which may break through shear instabilities as indicated by critical Richardson numbers, Ri 1. The internal waves are ubiquitous features of strong boundary currents flowing along continental shelves and are likely generated here by geostrophic adjustment of the pycnocline in response to lateral excursions of the boundary current over the Shetland slope or by the scattering of internal tide energy. The pycnocline as a whole exhibits Kz 10 4.5 m 2 s 1 , a factor of 3 larger than typical open ocean thermoclines but still implying a weak exchange of water mass properties across the density interface despite the confined and isolated patches of enhanced turbulence which elevate Kz by an order of magnitude. Both the boundary current and the deep interior are typified by weak stratification and locally high Kz 10 3 m 2 s 1 , but low turbulent buoyancy fluxes. The strongest observed turbulence and mixing in the channel, e O(10 7 Wk g 1 ) and Kz 10 3 m 2 s 1 respectively, is observed in the near-bed region over the Shetland slope and results from solibore propagation up the slope and the asymmetric response of the bottom boundary layer to the tidal currents. The overall contribution of the FSC to oceanic mixing would appear to be about half of the required canonical value of Kz 10 4 m 2 s 1 given the observed short duration and/or infrequent occurrence of the processes generating turbulence in the near-bed region, the spatially confined sporadic mixing patches in the pycnocline, and the low turbulent buoyancy fluxes in the interior and slope current.