Variability of tidal currents in a wide strait: A comparison between drifter observations and numerical simulations

The spatial variability of semidiurnal tidal currents throughout the waters of the northwest coast of Canada is investigated by examination of velocities of surface drifters recording half-hourly Loran-C positions. Time series are examined to remove any records with significant inertial currents. The remaining records are segmented into sequential 25-hour blocks, detrended, and analyzed for semidiurnal energy. Tidal current ellipses are computed and compared with two different numerical models: a finite element barotropic simulation, and a finite difference baroclinic simulation that was initialized with horizontally uniform density surfaces. Models and observations show the dominance of negative rotation (clockwise rotary) with the amplification of currents shoreward of the continental slope. In shallow waters both models agree well with the observations. Along the edge of the continental shelf in waters deeper than 200 m the baroclinic model provides better agreement with observations because of its ability to simulate internal tides. However, observations also reveal that internal tidal currents at and seaward of the shelf break may be irregular because of the presence of plumes and jets, and a baroclinic model with horizontally uniform stratification cannot reproduce local features in these jets and plumes. Internal tidal currents in Dixon Entrance exceed those simulated by the baroclinic model by a significant degree. This is likely due to the constraints of the density field used to initialize the model and to terrain smoothing in its 5-km grid.