Observations of the internal tide and associated mixing across the Malin Shelf

Abstract

An energetic internal tide was observed at two contrasting shelf continental sites to the west of Scotland. The first site was 5 km shoreward of the continental shelf break, and the second was 45 km directly to the east of the first site. Each site was instrumented with a seabed mounted acoustic Doppler current profiler and thermistor chain for a 2 week period in July 1996, when thermal stratification was well developed. In addition to the measurements of the evolution of the water column structure and flow field, two series of measurements of the rate of dissipation of turbulent kinetic energy were made at each of the sites. The barotropic tide was found to contain an unusually strong diurnal component, thought to be due to a coastally trapped wave; as a result, there is a significant diurnal component to the baroclinic energy spectra. Significant peaks are also present in the baroclinic energy spectra at interaction frequencies between the main semidiurnal (M 2 ) and diurnal (K 1 ) tidal constituents at both sites. Observations of profiles of the rate of dissipation of turbulent kinetic energy show a significant proportion of the measured rate of dissipation took place within the thermocline. Close to the shelf edge, detailed observations of a hydraulic jump and nonlinear internal waves are analyzed to show significantly stronger mixing at this site (characterized by diffusion coefficients of 1.6 and 6.1 × 10 −4 m 2 s −1 ) than at the site farther onshore (where the diffusion coefficient is estimated to be 0.7 × 10 −4 m 2 s −1 ).