Residual Circulations Due to Bottom Roughness Variability under Tidal Flows

Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight. The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies is that resolution of roughness variability is as important as resolution of topography at length scales comparable to the tidal excursion. Therefore higher-resolution models that seek to resolve flow patterns due to tidal scale topographic variability will also require similarly resolved bottom roughness variability.