Diurnal-Period Continental Shelf Waves along Vancouver Island: A Comparison of Observations with Theoretical Models

Abstract

Current and sea level data from the Coastal Oceanic Dynamics Experiment conducted of Vancouver Island from May 1979 to September 1980 reveal shelf waves of diurnal period whose motions dominate the flow over the continental shelf. In this paper, current meter data from mooring lines off Brooks Peninsula, Estevan Point and Carmanah Point are compared with theoretical solutions for a combination of free shelf waves and barotropic Kelvin waves over uniform alongshore topography. Use of Brink's scheme for baroclinic shelf waves gives close agreement between the modeled and observed K1-period horizontal currents over the continental margin off central Vancouver Island and accurately predicts vertical velocities and sea levels. The shelf waves am shown to be entirely dominated by the first baroclinic mode. Wavelength derived from the model closely approximate wavelenghts computed from the alongshore change in phase of the observed diurnal current ellipses. The baroclinic model successfully predicts the longer wavelength off southern Vancouver Island and the shorter wavelength at K1 compared to O1 periods. The model's principal failure is its inability to account for the seasonal variation in observed wavelengths. Incorporation of a mean alongshore current with cross-shelf shear into a model for barotropic shelf waves suggests that this variation arises through Doppler shifting of the wave frequency. A comparison with currents off Carmanah Point is less satisfactory presumably because of the proximity of the current meters to shallow banks and a canyon. Similarly, irregular bottom topography near Brooks Peninsula precludes meaningful comparisons.