Baroclinic tidal currents in the Gareloch, Scotland

Acoustic doppler current profiler, moored current meter and thermistor data were collected in the Gareloch, a sea loch on the west coast of Scotland, for 4 days during June 2001. The loch is approximately 8 km long, 40m deep in the interior and is separated from the Clyde estuary by a sill at a depth of 16m with respect to mean sea level. The sill is situated in the Narrows at the mouth of the loch where the tidal currents are barotropic in character and reach maximum speeds of about 0.3 m/s. In the interior of the loch, where the barotropic tidal currents should not exceed 0.02–0.03 m/s, the flow was markedly baroclinic and reached speeds of 0.15 m/s in the surface layer. The surface and bottom layer flows displayed a 180 phase difference, with the lower layer flow being directed landward during the flood tide. The tidal currents did not display the expected semi-diurnal motion that is characteristic of the Clyde Sea and estuary. Instead, the motion was dominated by short period oscillations that appeared to be seiche related rather than due to shallow water tidal effects. A strong inflow event, observed at the mouth of the loch during a flood tide, was consistent with the presence of a density-driven current superimposed on the tidal flow. The thermistor data showed that the loch was thermally stratified with a surface layer of thickness 8–10 m. Simulations with a two-layered model that allowed for variable channel width and water depth reproduced the observed phase difference between the surface and bottom layers. The model results suggested that the short period variability in the observed currents was related to interfacial oscillations generated by the strong tidal flow above the sill.