Processes influencing the internal tide, its higher harmonics, and tidally induced mixing on the Malin-Hebrides Shelf

Abstract

A three dimensional prognostic hydrodynamic model with a turbulence energy submodel is used to examine the spatial distribution of the internal tide in the shelf edge region off the west coast of Scotland. In the initial series of calculations presented here the model is used in a cross-shelf form. The model contains all the non-linear terms necessary to examine the generation of the higher harmonics of the tide, and the generation of very short waves on the density interface near the shelf-break. Initial calculations assuming a homogeneous sea region show a thick turbulent bottom boundary layer generated by bottom friction, which reduces the tidal current in the near-bed region, although above this the flow is essential inviscid with no vertical variation. Calculations with a specified vertical stratification reduce the thickness of the bottom turbulent layer with an associated change in current profile. In the case of specified stratification no internal tides are generated, however when the density field is allowed to evolve a significant internal tide is generated, with an associated internal shear giving rise to enhanced mixing in regions of largest shear. Results from the model show that higher harmonics of the internal tide, and short internal waves are generated above the shelf-break, where horizontal mixing processes are important. The magnitude of these short internal waves increases with increasing tidal forcing, and also depends upon the size of the horizontal diffusion term. Regions of intense surface and bed turbulence associated with the internal tide are found in the shelf-break area, suggesting that turbulence energy measurements in this location may be a good test of turbulence closure models. However the significant spatial variability associated with these regions shows that an intense measurement campaign is required to avoid significant undersampling.