A three-dimensional model of the principal tides on the European shelf

A three-dimensional hydrodynamic model of the continental shelf is used to examine the spatial distribution, both elevations and currents, of 28 of the major tides in the region. Tidal constituents in the diurnal and semi-diurnal band together with higher harmonics and long period tides are used to illustrate the magnitude and spatial Variability of the tides over the shelf. Comparisons are made between model computed profiles and harmonic analyses of 278 current time series, at a range of locations in varying water depths, with particular emphasis on locations where there are current measurements at a number of points in the vertical. Observed and computed tidal elevation amplitudes and phases are also compared at up to 257 shore-based and off-shore tide gauges. These detailed comparisons are used to determine any bias in the model and the accuracy of predicted elevations and currents derived with the model. Results of these comparisons show that although the diurnal and semi-diurnal tidal elevations and currents can be accurately reproduced using a 12 km grid, the error in the higher harmonics is significantly larger. The fact that the model contains all the non-linear terms (advection, quadratic friction, wave drift, time varying viscosity) necessary to generate these harmonics, together with a full set of tidal harmonics in order to have the correct level of friction, suggests that potentially it should reproduce these constituents. However, a rigorous comparison was found to be very difficult, because of the rapid variation of the higher harmonics in shallow water which cannot be adequately resolved in the model, and possible inaccuracy in the measurements of the small currents associated with the higher harmonics. (C) 1997 Elsevier Science Ltd. All rights reserved.