A numerical study of the slope current northwest of the British Isles

A one active layer, eddy resolving (grid size ∼14 of the baroclinic Rossby radius) reduced gravity numerical model, forced by winds and by imposed boundary flows, is applied to the ocean area northwest of the British Isles in order to study the continental slope current and its variability during winter. It is found that local wind forcing alone is insufficient to reproduce the observed mean transports, and that an external transport from the south must be added to match observations. The required external transport is of the same magnitude (1–2 Sv) as required by the JEBAR component (combined effect of bottom relief and stratification). Experiments driven by observed winter wind fields produce transport variability on time scales of days to weeks which is in line with observational evidence, suggesting that wind forcing is an important mechanism for the variability of the slope current transport. This supports the view of the slope current being driven by a combination of barotropic and baroclinic processes. A study of the model results in the remaining part of the computational domain reveals that the one active layer model is partly unable to reproduce many of the known current features in the area. This result is used to point to additional physical processes which should be included in a more complete model of the upper ocean circulation in the area. These are (1) barotropicity, (2) vertical resolution (more vertical layers) and variable upper layer temperature and salinity, to provide for baroclinic and frontal instability processes, (3) bottom relief acting through the barotropic mode, (4) imposed inflow along the slope in the south, and (5) a large scale flux of water and vorticity from the North Atlantic.