Across-sill circulation near a tidal mixing front in a broad fjord

Abstract

The Clyde Sea is a broad fjord located on the southwest coast of Scotland. Cross-boundary circulation processes are investigated using observations over the shallow sill that separates the stratified Clyde Sea from the well-mixed North Channel of the Irish Sea. A simple method for determining the amplitude and phase of the M 2 tidal constituent and the spatial residual current field from shipboard acoustic Doppler profiler (ADCP) data is presented. Tidal results using this method compare well with established model results, and also agree sufficiently well with nearby moored tidal observations. Using new, high-resolution cross-sill sections observed with an undulating conductivity, temperature, depth profiler, the presence of a high-gradient tidal mixing front is confirmed, indicating steeply sloping (1:250) isopycnals near the surface expression of the front. Repeated alongsill sections throughout a tidal period show a consistent temperature and salinity structure, with a sloping interface (1:700) separating heavy, high-salinity water in the lower water column toward the southeastern sill from less-dense water over the northwestern sill. A geostrophic interpretation of the observed density field supports the existence of a northwestward directed, alongsill jet near the frontal outcrop, and a residual across-sill exchange modified by the earth's rotation. However, residual across-sill currents, measured both by the shipboard ADCP and moored instruments, indicate flows additional to the across-sill circulation predicted by the density structure. Subtidal currents from three sill moorings reveal barotropic motions, which are likely responsible for frontal displacements, and depth-dependent exchanges that are partially wind-driven. The barotropic flows are of unknown origin and are not spatially correlated between mooring locations, but appear periodic in nature and are an order of magnitude larger than flows required to account for the observed variations in subtidal sea level at the sill. The depth-dependent across-sill exchange resembles a classical density-driven circulation, which is enhanced when the wind is directed parallel to the sill toward the southeast, and reduced when the wind is directed into the bay. The substantial sampling effort provides clear evidence for the spatial complexity of the circulation over the sill; however, temporal undersampling prevented conclusive quantification of forcing mechanisms responsible for the observed across-sill circulation.