Flow and particle motion induced above a tall seamount by steady and tidal background currents

The response of a stratified ocean over a tall, isolated seamount induced by the combination of slow, steady, spatially uniform inflow and weak diurnal tides is examined using a primitive-equation numerical model. Steady inflow alone forms a Taylor cap, which retains Lagrangian fluid particles over the seamount for a duration that scales inversely with the mean inflow speed U over the range U/fL=0.004−0.036 , where f is the Coriolis parameter and L is the horizontal length scale of the seamount. Fluid particles located at mid-depth and shallower over the top of the seamount are retained for up to about five advective time scales ( L/ U ). Near-bottom particles may be retained much longer. Tidal forcing alone drives a seamount-trapped wave that gives rise, through non-linear rectification, to a bottom-intensified time-mean circulation around the rim of the seamount. The magnitude of the rectified mean current varies approximately linearly with the tidal amplitude U 0 over the range U 0/ fL=(1.08−3.60) × 10 −3. The seamount-trapped wave scatters particles vertically over the seamount summit, producing net displacements over many tidal cycles of 100–300 m. The combination of both forcings produces a near superposition of the two separate responses. The tide hardly affects particle retention by the steady inflow, except near the bottom, where retention is enhanced by the bottom-intensified rectified mean flow. The steady inflow hardly affects the vertical scattering of particles by the tide, except by altering the duration for which particles remain over the seamount and are thereby subject to vertical scattering.