The Rockall slope current and shelf-edge processes

Recent knowledge of physical-oceanographic processes is reviewed for the eastern shelf-edge boundary of the Rockall Channel. A year-round northward current flows along the steep continental slope. Mean currents vary from 3 to 30cm/s, generally increasing northwards, but estimates of transport inshore of the 2000 m depth contour are much more consistent, averaging about 1·5 × 106m3/s. The current is thought to be forced largely by a long-shelf pressure gradient, associated with large-scale N-S density variations in the upper ocean. Although cross-slope changes of temperature and salinity are much less than occur, for example, east of the U.S.A., cross-slope exchange velocities appear to be only around 2cm/s (1/5 of long-slope fluctuations). There is a sharp change between winter-cooled water on the shelf and adjacent slope water. Upwelling against the upper continental slope may occur following northerly long-slope winds. Tidal currents and surges depend strongly on continental shelf wave properties. At near-diurnal frequencies, and in response to winds of short scale (< 100 km), clockwise-rotating currents near the shelf break are expected, especially where the shelf is broadest. Tidal currents show this character. Internal tides are significant; any non-linearity is most likely when stratification is present but weak. Bottom stirring has been observed, and may be important at some depth (where the bottom slope becomes as steep as the semi-diurnal internal characteristics) almost everywhere around the Rockall Channel. The energetic internal waves should contribute significant internal mixing as they approach the shelf break, intensify and dissipate