A Numerical Study of Stratified Tidal Rectification over Finite-Amplitude Banks. Part I: Symmetric Banks

Abstract

Tidal rectification over a two-dimensional finite-amplitude symmetrical bank is studied using the Blumberg and Mellor primitive equation coastal ocean circulation model (ECOM-si). In the homogeneous case, the nonlinear interaction of tidal currents with the variable bottom topography generates an along-isobath residual circulation over the bank, which tends to increase as either the slope or height of the bank is increased. In the stratified case, internal waves at tidal and higher frequencies are generated over the sloping sides of the bank. Tidal mixing occurs in the bottom boundary layer, leading to horizontal tidal mixing fronts. The resulting stratified tidal rectification associated with the tidal mixing front, the generation of internal tides, and the modification of internal friction due to stratification leads to a subsurface intensification of the along-isobath residual current at the front and at the top of the bottom mixed layer over the slope, and a cross-bank double cell circulation pattern centered at the front near the shelf break. Model results for tidal mixing are in reasonable agreement with a simple energy argument in which the thickness of the tidal mixed layer is proportional to the magnitude of the tidal current and inversely to stratification.