Repository logo
 

The evolution and arrest of a turbulent stratified oceanic bottom boundary layer over a slope: Downslope regime

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Ruan, X 
Thompson, AF 
Taylor, JR 

Abstract

jats:titleAbstract</jats:title>jats:pThe dynamics of a stratified oceanic bottom boundary layer (BBL) over an insulating, sloping surface depend critically on the intersection of density surfaces with the bottom. For an imposed along-slope flow, the cross-slope Ekman transport advects density surfaces and generates a near-bottom geostrophic thermal wind shear that opposes the background flow. A limiting case occurs when a momentum balance is achieved between the Coriolis force and a restoring buoyancy force in response to the displacement of stratified fluid over the slope: this is known as Ekman arrest. However, the turbulent characteristics that accompany this adjustment have received less attention. We present two estimates to characterize the state of the BBL based on the mixed layer thickness: jats:italicH</jats:italic>jats:subjats:italica</jats:italic></jats:sub> and jats:italicH</jats:italic>jats:subjats:italicL</jats:italic></jats:sub>. The former characterizes the steady Ekman arrested state, and the latter characterizes a relaminarized state. The derivation of jats:italicH</jats:italic>jats:subjats:italicL</jats:italic></jats:sub> makes use of a newly defined slope Obukhov length jats:italicL</jats:italic>jats:subjats:italics</jats:italic></jats:sub> that characterizes the relative importance of shear production and cross-slope buoyancy advection. The value of jats:italicH</jats:italic>jats:subjats:italica</jats:italic></jats:sub> can be combined with the temporally evolving depth of the mixed layer jats:italicH</jats:italic> to form a nondimensional variable jats:italicH</jats:italic>/jats:italicH</jats:italic>jats:subjats:italica</jats:italic></jats:sub> that provides a similarity prediction of the BBL evolution across different turbulent regimes. The length scale jats:italicL</jats:italic>jats:subjats:italics</jats:italic></jats:sub> can also be used to obtain an expression for the wall stress when the BBL relaminarizes. We validate these relationships using output from a suite of three-dimensional large-eddy simulations. We conclude that the BBL reaches the relaminarized state before the steady Ekman arrested state. Calculating jats:italicH</jats:italic>/jats:italicH</jats:italic>jats:subjats:italica</jats:italic></jats:sub> and jats:italicH</jats:italic>/jats:italicH</jats:italic>jats:subjats:italicL</jats:italic></jats:sub> from measurements will provide information on the stage of oceanic BBL development being observed. These diagnostics may also help to improve numerical parameterizations of stratified BBL dynamics over sloping topography.</jats:p>

Description

Keywords

Bottom currents, Nonlinear dynamics, Topographic effects, Turbulence, Boundary layer, Large eddy simulations

Journal Title

Journal of Physical Oceanography

Conference Name

Journal ISSN

0022-3670
1520-0485

Volume Title

49

Publisher

American Meteorological Society