Influence of Stratification upon Diurnal Tidal Currents in Shelf Edge Regions

Abstract

A three-dimensional stratified tidal model of the Hebrides shelf area off the west coast of Scotland is used to examine the spatial distribution of the M2, O1, and K1 tides in the region. The model has a finer horizontal finite-difference grid than previous models of the region, with a biharmonic form of horizontal diffusion, and the total variation diminishing scheme for density advection. By this means horizontal diffusion is kept to a minimum and internal tidal effects can be included. Preliminary calculations assuming a homogeneous sea region show that the model can accurately reproduce the M2, O1, and K1 tidal elevations in the region. An enhancement of the O1 and K1 tidal elevations and currents at the shelf edge to the northwest of the Hebrides is found in the model, in agreement with theory and observations. However the enhancement of the diurnal tides, in particular tidal currents along the shelf edge, is larger than that found in the observations, with some regions of local enhancement that are not supported by observations. Including stratification effects reduces the diurnal tidal currents along the shelf edge and removes the areas of spurious local intensification, giving a distribution in good agreement with observations. A reduction in the amplitude of the diurnal tidal currents at the shelf edge can also be achieved in the three-dimensional model by relating bed stress to depth-mean currents (in essence a two-dimensional model). Although this reduction improves the agreement with observations, regions of spurious intensification still remain, which can only be removed by including stratification effects.