A numerical study of the tidal circulation and buoyancy effects in a Scottish fjord: Loch Torridon.

[1] The tidal and buoyancy-driven circulation in Loch Torridon, a Scottish fjord, is investigated using a three-dimensional, hydrostatic, primitive equation model. The predictions from the model have been quantitatively compared to available data gathered between 1998 and 2001. The model performed well in reproducing the tidal characteristics and density evolution of the fjord over the 3-month simulation period. Results show that tidal currents are dominated by the constraint imposed on the flow by the shallow sill separating the upper and middle basins: the flow is strongly accelerated during flood and ebb tides, and significant residual currents are generated that may influence exchange and residence times of the middle basin. A strong baroclinic response to the flow over the sill is predicted throughout the fjord, with a propagating linear M-2 tide displacing isopycnals vertically by up to 32 m. At the sill the formation of stationary lee waves on both flood and ebb tides is predicted, with the wave generated during flood tide being notably larger as the growth of the ebb wave is inhibited by deep dense water. This allows more energy to transfer into the seaward propagating internal tide. As the tidal flow weakens, the lee waves propagate over the sill and upstream. Estimates of the total vertical turbulent buoyancy flux in the fjord are made, and the distribution of mixing over a tidal cycle is shown; vertical mixing is seen to be highly spatially and temporally variable and strongly linked to the presence and dissipation of lee waves near the sill.