Three dimensional effects of Ekman adjustment on instabilities within seamount bottom boundary layers.

Boundary currents flowing past seamounts precondition the downwelling bottom boundary layer (BBL) along the seamount slopes towards developing one or more type of instability associated with the potential vorticity (PV) reversing sign. We show using idealized submesoscale and BBL-resolving ROMS simulations that a useful characterization of the instabilities can be obtained by analyzing the sources of their turbulence kinetic energy (TKE) and the structure of PV within the BBL. These are in turn found to depend both on the non-dimensional seamount height Nh_s/af as well as its lateral aspect ratio β=b/a which measures the along-flow seamount length b to its cross-slope width a. N, h_s and f are respectively the constant background stratification, seamount height and Coriolis frequency. Increasing Nh_s/af has the effect of reducing the bottom stress through the thermal wind shear induced by sloping isopycnals within the BBL, which opposes the ageostrophic BBL shear. This dynamical mechanism, referred to as Ekman adjustment, suppresses BBL turbulence and dissipation. However the preconditioning due to Ekman adjustment simultaneously renders the BBL susceptible to centrifugal, symmetric and mixed centrifugal-symmetric-gravitational instability modes which equilibrate and dissipate over long downstream distances. The most intense turbulent dissipation in our simulations is seen in the wake of circular seamounts (β=1) , and is identified as arising primarily from a centrifugal mode of instability. It produces dissipation rates over 3.5 times higher than would be expected over a flat bottom, and persists over a downstream distance that is an order of magnitude larger than the seamount length itself. For elongated seamounts, i.e. β>>1, along-slope Ekman adjustment gives rise to a BBL configuration that develops a hybrid centrifugal-symmetric-gravitational mode which is only modestly dissipative relative to a pure centrifugal mode, with normalized, area-averaged dissipation rates around 0.5 for β=8 and β=16, compared to 3.5 for β=1.