An s coordinate density evolving model of the northwest European continental shelf - 2, Seasonal currents and tides

The barotropic tides and the residual currents from a year long run of a th ree-dimensional baroclinic model (the Proudman Oceanographic Laboratory Thr ee-Dimensional Baroclinic B grid model) applied to the northwest European s helf 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 n umber of large values at the coast. The density component of the residual c urrents is derived from the difference between the full model and wind and tide only model runs. This demonstrates the importance of density-driven cu rrents 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 ri ver plumes. However, volume fluxes show the mass budget of the North Sea is primarily determined by the wind-driven circulation. Calculations of the d ynamic balance in the model show the significance of friction and/or advect ive effects, on a timescale of days, at a number of locations (e.g., the Do ver Strait and in the central North Sea). However, the monthly mean current s 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 wes tern boundary, and we demonstrate that variations in the imposed temperatur e and salinity here have only a limited effect on the shelf. Current meters on the Hebrides shelf show that the model produces reasonably accurate res idual currents but also that this agreement is degraded by the boundary con dition on Porcupine Bank.