Diapycnal mixing in the Faroe-Shetland Channel from density overturns and current shear

The Faroe-Shetland Channel is an important passageway for watermass exchange between the North Atlantic and the Arctic Oceans, being the primary route of overflow waters from the Nordic Seas entering the Iceland Basin. The hydrography of the Channel is dominated by a permanent pycnocline that separates high salinity North Atlantic Water, flowing northward against the West Shetland slope in a barotropic slope current, and typically southward flowing intermediate and deep waters that originate in the Arctic basin. Mixing in the pycnocline may be enhanced by turbulent instabilities resulting from the shear flow or by internal waves generated along the West Shetland slope and over the Wyville-Thompson Ridge. During F. S. Poseidon cruise 328 in September 2005, three CTD/LADCP sections across the Faroe-Shetland Channel were completed and a station over the West Shetland slope occupied for 24 hours with a cast every hour. Two methods of calculating diapycnal eddy diffusivity (κρ) from the cast data are used, scaling of density overturns [e.g., Thorpe, 1977], and comparison of the vertical shear spectrum with the Garrett and Munk model of the background internal wave field [Garrett and Munk, 1975] following the method of Naveira Garabato et al. [2004]. In the permanent pycnocline κρ is found to be ≥ 10-3 m2 s-1, an order of magnitude larger than in the regions of weak stratification. Over the Faroe slope, mixing is enhanced through out the watercolumn with κρ approaching 10-2 m2 s-1. Mixing is also enhanced where the pycnocline meets the West Shetland slope with a semi-diurnal cycle apparent in both the depth of the pycnocline and the strength of the mixing. In the slope current at the shelf edge κρ is less, of the order 10-3.5 m2 s-1. There is good agreement between the methods in the regions of stratification, although where N2 < 10-5 s-2 Thorpe scaling estimates κρ an order of magnitude larger than the vertical shear spectrum method