Nonlinear stratified spindown over a slope

Abstract

Nonlinear stratified spindown of an along-isobath current over an insulated slope is shown to develop asymmetries in the vertical circulation and vertical relative vorticity field. During spindown, cyclonic vorticity is weakened to a greater extent than anticyclonic vorticity near the boundary because of buoyancy advection. As a consequence, Ekman pumping is weakened over Ekman suction. Momentum advection can weaken Ekman pumping and strengthen Ekman suction. Time-dependent feedback between the geostrophic flow and the frictional secondary circulation induces asymmetry in cyclonic and anticyclonic vorticity away from the boundary. Buoyancy advection over a slope can modify the secondary circulation such that anticyclonic vorticity decays faster than cyclonic vorticity outside the boundary layer. In contrast, momentum advection can cause cyclonic vorticity to spin down faster than anticyclonic vorticity. A scaling and analytical solutions are derived for when buoyancy advection over a slope can have a more significant impact than momentum advection on these asymmetries. In order to test this scaling and analytical solutions, numerical experiments are run in which both buoyancy and momentum advection are active. These solutions are contrasted with homogeneous or stratified spindown over a flat bottom, in which momentum advection controls the asymmetries. These results are applied to ocean currents over continental shelves and slopes.

Role of tides on the formation of the Antarctic Slope Front at the Weddell‐Scotia Confluence

“…Ekman layer dynamics in the benthic layer also contribute to tilting of isopycnals over a slope [e.g., Wåhlin et al ., ; Benthuysen et al ., ]. Given an initial along‐slope mean flow, regions of convergence and divergence arise from variations in the strength of the bottom Ekman layer, caused by variations in the geostrophic flow above the bottom boundary layer [ Benthuysen and Thomas , ]. The convergence/divergence regions then act to tilt the isopycnals through the generation of vertical velocities sustaining, in turn, the geostrophic transport.…”

The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime

“…In other words, the interior velocity field is a part of the solution of the BBL system and cannot be viewed as a background forcing independent of BBL processes. By shaping the background mean flow, at least close to the ocean bottom, BBL dynamics may influence the interior circulation beyond classic Ekman spinup and spindown processes (Thomas and Rhines 2002;Benthuysen and Thomas 2013;Ruan and Thompson 2016).…”

Potential vorticity diagnostics based on balances between volume integral and boundary conditions

“…Concerning modification of PV in the bottom boundary layer, the net modification of PV is also a function of time (Benthuysen and Thomas, 2013): the velocity and stratification in the bottom boundary layer do not reach instantaneously their equilibrium value (Benthuysen and Thomas, 2012). Thus, the final modification of PV along a boundary depends on the time a fluid parcel will remain in contact with the boundary layer.…”