Depth-Dependent Studies of Tidally Induced Residual Currents on the Sides of Georges Bank

Abstract

Using a depth-dependent tidal model, the tidally induced residual currents on the northern and southern sections of Georges Bank are computed and the effects of various physical parameters on the current are examined. Because of significant on-bank Stokes' drift, the rigid-lid approximation is not appropriate for the shallow region of the southern section. Also, in the same shallow region, the curvilinear structure of isobaths significantly increases the one-/off-bank residual flow in the lower/upper portion of the water column, and thus enhances the upwelling near the shallow end of the section. Although the amplitudes of the residual current and the center of gyres in the cross-bank circulation vary significantly with the values and forms of the vertical eddy viscosity and the stratification in the water column, the overall residual circulation is generally insensitive to these physical parameters. The only exception is that, for a sharp density front or a large horizontal variation in the vertical eddy viscosity, an additional tidally forced cross-bank Lagrangian gyre can be induced in the middle portion of the northern section. In comparison to the wintertime tidally induced residual currents, the summertime cross-isobath residual gyres are generally stronger and extend over larger areas, especially toward the shallow portion of the bank. In stratified cases, the cross-bank Eulerian residual current near the frontal region of the northern section can be very significant (up to ≈ 7 cm s?1). The computed along-bank residual currents are generally consistent with the observations. However, the strong on-bank residual current (≈0.7 cm s?1) at 75 m of Station A on the southern section, which is estimated from the observed low-frequency modulation of residual current, cannot be reproduced by the model.