Abstract
A three-dimensional model of the Irish Sea, and the shelf and shelf-edge region off the west coast of Scotland is used to examine the interaction of wind induced surface currents and M2tidal currents which can lead to an increased near surface M4tidal current in the region. Calculations are performed using uniform southerly and westerly wind stresses, with vertical eddy viscosity computed using either a simple flow dependent form of viscosity, or one related to the flow and surface wind stress or derived using a two-equation turbulence model. Resulting M4tidal distributions computed with the various viscosity formulations are compared with each other. The spatial distribution the M4tidal current in the case of the flow dependent viscosity, shows a significant enhancement of the surface M4tide due to non-linear coupling between the surface shear and the tidally dependent viscosity. In deep water this is the dominant mechanism giving rise to an enhancement in near surface M4current. In shallow water, however, where other non-linear processes are important in generating the M4tide, namely changes in water depth and friction, these are significant as well as the non-linear surface shear effect, in changing the M4tidal distribution. However, when an eddy viscosity formulation depending upon flow and surface wind stress is used, the non-linear coupling mechanism producing an enhanced M4tidal surface current is reduced, and no increase in M4surface current occurs.Calculations with the two equation turbulence model, do not produce an increase in the near surface M4tide, as the near surface viscosity in this model is determined principally by the wind stress rather than the tidal current. Some conclusions concerned with developing a simple eddy viscosity model having similar characteristics to the turbulence energy model, and experimental data sets for validation are considered at the end of the paper.
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