Formation of taylor caps over a tall isolated seamount in a stratified ocean

A primitive equation numerical model is used to examine various aspects of the formation of Taylor caps over a tall isolated seamount in a steady, rotating, nearly inviscid, stratified flow. The flow is characterized by four nondimensional parameters: the Rossby number, Ro = U/fL; the Burger number, S = NH/fL; the fractional seamount height, δ = hm/H; and the aspect ratio, Δ = H/L. Here U is the uniform inflow velocity,/the Coriolis parameter, L the horizontal length scale of the seamount, N the initial buoyancy frequency, H the ocean depth away from the seamount, and hm the maximum height of the seamount above the otherwise flat bottom. For both unstratified (S = 0) and stratified (S= 1) flows over a tall seamount which ultimately form a Taylor cap, the initial response is similar to that found in previous studies which considered short seamounts ((δ1) in weakly nonlinear flows (Ro1). Two eddies form over the seamount and co-rotate clockwise around the seamount until one is swept away with the imposed inflow, leaving the other eddy trapped over the seamount to form a Taylor cap. The resulting steady flow is asymmetric with substantially enhanced velocities on the left side of the seamount (looking downstream). For steady, unstratified flows (S = 0) over tall seamounts, Taylor caps occur only for weaker inflows (smaller Rossby numbers) than would be predicted from quasigeostrophic (QG) theory. This is consistent with the generation of large vertical velocities by the compression and stretching of fluid columns passing over the tall seamounts, in clear violation of the assumptions of QG theory. Taylor caps in steady, stratified flows (S= 1) are bottom-trapped with size and shape which are sensitive to the strength of the inflow as well as the stratification. The vertical excursions of water parcels are greatly reduced compared with unstratified flows. Over short seamounts (δ<0.4), the transition from a flow with a Taylor cap to one without a Taylor cap occurs at slightly smaller Rossby numbers than would be predicted by QG theory. Over taller seamounts (δ=0.4), some inflows generate internal lee waves which appear to destroy the fluid trapping of the Taylor cap, producing flows in which water parcels are temporarily trapped over the seamount before being swept away by the imposed inflow. This temporary trapping occurs only over mid-size seamounts (0.4<δ<0.7) within a range of moderately strong inflows (0.1 <Ro<0.2) and does not occur at all over tall seamounts (δ<0.7). For seamounts which are taller than δ≍0.4, true Taylor caps (i.e. permanent trapping of water parcels) occur only with rather weak inflows (Ro<0.15).