Stratification and Mean Flow Effects on Diurnal Tidal Currents off Vancouver Island

Abstract

A series of numerical experiments with a three-dimensional, baroclinic model were conducted to study the influences of density stratification and wind-driven currents on the K1 tide over the continental shelf off Vancouver Island. The region is one of anomalously large diurnal tidal currents due to the generation of coastally trapped waves at the entrance to Juan de Fuca Strait. Model results are compared with data obtained from a number of moorings deployed over the shelf, including a line extending for over 300 km in the alongshore direction. The results show that inclusion of stratification significantly improves the representation of K1 currents in the model, particularly with respect to the alongshore phase propagation of the clockwise and counterclockwise rotary components. With homogeneous water, the coastally trapped waves propagate relatively slowly and are dissipated before reaching Brooks Peninsula. In contrast, the ambient stratification permits coastally trapped motions to propagate beyond Brooks Peninsula, in agreement with observations. The seasonal variability of the K1 currents is also examined from long records obtained at a number of sites over the shelf. The observations indicate a seasonal modulation in the phase of the rotary components that increases in amplitude with distance from the generation region of the shelf waves. Attempts to model the observed summer/winter phase difference by including a seasonal-mean wind stress are discussed.