Abstract: Mixing Mechanisms at the Sill of the Norwegian Sogne-Fjord



Authors: V. Fiekas(1), L. Arneborg(2), I. Fer(3), M. Knoll(1), H. Prandke(4)

(1)    Research Department for Underwater Acoustics and Marine Geophysics (FWG), Klausdorfer Weg 2- 24, D-24148 Kiel, Germany
(2)    Department of Earth Sciences, University of Gothenburg, S-40530 Göteborg, Sweden
(3)    Geophysical Institute, University of Bergen, Allegaten 70, N-5007 Bergen, Norway
(4)    ISW Wassermesstechnik Dr. Hartmut Prandke, Gartenweg 1, D - 17213 Fünfseen, Germany

In February 2007 extensive turbulence measurements were carried out during a cruise in the Norwegian Sogne-Fjord. At the entrance of the fjord a 170 m deep sill separates the inner fjord with water depths of more than 1000 m from a 300-500 m deep basin, the Sogne-Sjöen that is detached from the open ocean by islands and shallow areas crossed by a few channels of more than 200 m depth. The general flow over the sill consists of a mean relatively fresh outflow at the surface and a saline inflow close to  the bottom. The tidal flow over the sill, which may exceed 0.5 m/s, causes turbulent mixing especially during flood tide on and east of the sill inside the fjord. The dissipation rate of turbulent kinetic energy west of the sill towards the open ocean is relatively small. In the deeper areas east of the sill two separate layers of high turbulence occur, the upper one in 100-170 m depth and the lower one close to the bottom. Different mixing mechanisms like hydraulic jumps, breaking internal waves or bottom friction are investigated.