An <i>s</i> coordinate density evolving model of the northwest European continental shelf: 2. Seasonal currents and tides

Abstract

The barotropic tides and the residual currents from a year long run of a three‐dimensional baroclinic model (the Proudman Oceanographic Laboratory Three‐Dimensional Baroclinic B grid model) applied to the northwest European shelf are investigated. M 2 currents and elevations differ from observations by typically 0.114 m s −1 and 0.143 m; these errors are dominated by a number of large values at the coast. The density component of the residual currents is derived from the difference between the full model and wind and tide only model runs. This demonstrates the importance of density‐driven currents in a number of regions, particularly in the Norwegian Trench, around the St. George's Channel, to the east of the Shetland Isles, and in the river plumes. However, volume fluxes show the mass budget of the North Sea is primarily determined by the wind‐driven circulation. Calculations of the dynamic balance in the model show the significance of friction and/or advective effects, on a timescale of days, at a number of locations (e.g., the Dover Strait and in the central North Sea). However, the monthly mean currents are in geostrophic balance over virtually the whole domain; ageostrophic components adjust the surface and isopycnal slopes to restore this balance on comparatively short timescales. Particular attention is given to the western boundary, and we demonstrate that variations in the imposed temperature and salinity here have only a limited effect on the shelf. Current meters on the Hebrides shelf show that the model produces reasonably accurate residual currents but also that this agreement is degraded by the boundary condition on Porcupine Bank.