Mixing and dissipation of internal wave energy on a slope shelf

On the shelf slope north of Scotland the rate of turbulent kinetic energy (TKE) dissipation,calculated from density overturns, is 6±1×10-7 Wkg-1. Using a standard mixing efficiency, this dissipation rate is equivalent to a diapycnal diffusivity of 10-3 m2s-1, two orders of magnitude larger than typical open ocean background levels. The rate of TKE dissipation cannot be fully explained by local shear and strain measurements so we invoke a far-field source of energy, supplied in the form of internal waves, to drive mixing over the slope. The across-slope internal wave energy flux is 130 Wm-1 at semi-diurnal frequencies with 103±20 Wm-1 contained in the pycnocline. This can be reconciled with the TKE dissipation rate by assuming all the internal wave energy is dissipated in the ~4.5 km that the pycnocline intersects the slope. The rate of TKE dissipation inferred from this assumption is between 3 and 6×10-7 Wkg-1, comparable to the dissipation rate calculated from density overturns, and emphasizes the role of internal waves in the transfer of tidal energy, not just into the abyssal ocean, but around complex topography.