Tidal mixing and cross-frontal particle exchange over a finite amplitude asymmetric bank: A model study with application to Georges Bank

Tidal mixing, internal wave bares, and cross-bank particle transport over a finite amplitude asymmetric bank are examined using a two-dimensional prim itive equation ocean model with Mellor and Yamada (level 2.5) turbulent clo sure. Driven by the surface M-2 tide, the model results show that tidal mix ing exhibits temporal and spatial asymmetries on southern and northern flan ks of the bank. It is strongest near the bottom around maximum on-bank tida l flow as a result of gravitational instability when denser water is advect ed upslope over lighter water. A sharp thermal depression occurs on the ste ep northern flank, which produces large current shear and strong tidal mixi ng throughout the upper 50 m of the water column. Dissipation also exhibits a strong tidal variation, with the largest values (of order 10(-5) W/kg) o ccurring near-bottom around maximum on- and off-bank tidal flow. Dissipatio n generally decreases upward, with a distinct phase lag in the vertical. Fl uid particles are advected upslope near the bottom in the upper slope regio n (depth <150 m) on both flanks, with some particles moving across the tida l mixing fronts near the bottom. The near-bottom residual Lagrangian curren t is opposite in direction to the residual Eulerian current on the northern flank due to strong nonlinearity over the steep bottom slope. The mean ups lope advection of fluid particles near the bottom on both flanks is consist ent with model passive tracer experiments, suggesting that strong tidal for cing of a stratified fluid over the bank can provide one physical mechanism responsible for high concentrations of nutrients and hence phytoplankton a t the fronts on Georges Bank. The model predictions of eddy viscosity and t urbulent dissipation rates an in good agreement with estimates based on rec ent current and microstructure measurements made on Georges Bank.