The Response of a Strongly Stratified Fjord to Energetic Tidal Forcing

The response of Upper Loch Linnhe, a strongly stratified Scottish fjord, to energetic tidal forcing is studied by new analysis of two observational data sets. Using a ship-mounted Acoustic Doppler Current Profiler (ADCP) and an undulating Conductivity-Temperature-Depth (CTD) we are able to repeatedly sample over a tidal cycle at sufficiently high spatial and temporal resolutions to clearly define the tidal variability in the vertical and horizontal. Away from the near-sill region, horizontal velocities are generally orientated along the longitudinal axis of the fjord. Analysis of the along-fjord velocities shows that at the M2-frequency there is a strong 1st mode internal tide with maximum horizontal velocity amplitudes of 23·4 cm s 1. The observed vertical structure of the complex amplitudes of the horizontal velocities are in close agreement with normal mode theory. Along-fjord variations of the fitted amplitude and phase lags from further analysis of the two independent data sets show that the 1st mode internal tide is behaving as a standing wave. Within the near-sill region, the circulation is complicated by the formation of a horizontal gyre and a large perturbation in the density field on the flood tide. These features are a response to the flood tide currents advecting denser water into the fjord in the form of a strong jet. Due to the narrow entrance sill being off-centre to the axis of the fjord, the strong flood tide jet gives rise to eddy formation. As the incoming flood tide water is denser than the resident surface water of the fjord the tidal jet sinks and advects the resident water landwards giving rise to the observed density perturbations.