Effects of Mean and Tidal Current Interaction on the Tidally Induced Residual Current

Abstract

A weakly nonlinear depth-dependent tidal model has been developed to study the effects of mean and tidal current interaction on the tidally induced residual current. The model is applied to the northern and southern sides of Georges Bank. The weakly nonlinear approximation is reasonable for the southern section, but only marginally applicable to the deep water, area of the northern section. For the depth-averaged tidal current, only the amplitudes of the along-isobath component are significantly affected by the interaction. The effects on the amplitudes decrease from the surface. For the cross-isobath tidal current, its changes in the upper portion of the water column generally correspond positively with the change in the along-isobath tidal current, and those in the lower portion correspond negatively. The mean and tidal current interaction increases the along-isobath residual current near the deep ends of both sections and in most of the shallow portion of the southern section. Although the effects of the interaction on the along-isobath residual current are small, a condition required for the validity of the weakly nonlinear approximation, the effects on the cross-isobath residual current are found to be very significant in many areas. This is because the cross-isobath residual component is generally an order of magnitude smaller than the along-isobath residual component; thus a small change in the latter component can generate a significant change in the former component. The increase/decrease in the along-isobath residual current due to the interaction is found to increase the onbank/offbank flow in the upper portion of the water column, and the offbank/onbank flow in the lower portion. The overall response of the residual current to the interaction is qualitatively the same for different values and forms of the vertical eddy viscosity. The response of the depth-averaged residual current to the interaction is also insensitive to the depth-dependent structure of the tidal and residual current. Dynamical interpretations for various effects of the mean and tidal current interaction on the tidal and residual currents are presented.