Measurements of the slope current, tidal characteristics and variability west of Vestfjorden, Norway

Observations of mean flow, tidal characteristics and variability, from the slope west of Vestfjorden, Norway (approximately 68.5°N) — obtained during the NEAT GIN 1989 experiment — are described. Current meter measurements confirm the presence of a slope current with mean values in the range 0.2–0.4 m s-1 and directed towards the NE, approximately parallel to the isobaths, in broad agreement with drifting buoy results (e.g. Booth and Meldrum, 1987, Poulain et al., 1996). Peak mean velocities of 0.7 m s-1 were observed during the period when current meters were deployed. Tidal analysis of the current meter records shows that the M2 component is always largest varying in amplitude from about 0.05 m s-1 at the top of the slope in about 200 m of water to about 0.03 m s-1 in the deeper waters on the slope. Examination of the coherence of scales in the current measurements suggests that most of the energy occurs in a slab like flow down to about 300 m. Over the deeper parts of the slope the bulk of the energy (some 80%) was contained in sub-tidal motions with periods of order 2 days or greater, while in shallower water at the top of the slope (and in deep water below the slab layer) the 80 percentile occurred at 0.5 days (i.e. tidal frequencies). This suggests that the bulk of the energy from baroclinic instability and eddy-like motions is contained in a ‘belt’ of activity confined to the upper stratified part of the water column and extending along the shelf edge. In this region, motions were observed to be coherent over scales of about 10 km which is comparable with the internal Rossby radius and stratification and eddy effects. Calculations of heat flux, on the other hand, suggest that coherence in the temperature fields may be on a smaller scale and of order 5 km or less. Thermistor chain measurements failed to identify a coherent internal tide signal in this region