Abstract

Tidal mixing, internal wave bores, and cross-bank particle transport over a finite amplitude asymmetric bank are examined using a two-dimensional primitive equation ocean model with Mellor and Yamada (level 2.5) turbulent closure. Driven by the surface M2 tide, the model results show that tidal mixing exhibits temporal and spatial asymmetries on southern and northern flanks of the bank. It is strongest near the bottom around maximum on-bank tidal flow as a result of gravitational instability when denser water is advected upslope over lighter water. A sharp thermal depression occurs on the steep northern flank, which produces large current shear and strong tidal mixing 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) occurring near-bottom around maximum on- and off-bank tidal flow. Dissipation generally decreases upward, with a distinct phase lag in the vertical. Fluid particles are advected upslope near the bottom in the upper slope region (depth <150 m) on both flanks, with some particles moving across the tidal mixing fronts near the bottom. The near-bottom residual Lagrangian current is opposite in direction to the residual Eulerian current on the northern flank due to strong nonlinearity over the steep bottom slope. The mean upslope advection of fluid particles near the bottom on both flanks is consistent with model passive tracer experiments, suggesting that strong tidal forcing of a stratified fluid over the bank can provide one physical mechanism responsible for high concentrations of nutrients and hence phytoplankton at the fronts on Georges Bank. The model predictions of eddy viscosity and turbulent dissipation rates are in good agreement with estimates based on recent current and microstructure measurements made on Georges Bank.

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