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

Ruan, Xiaozhou and Thompson, Andrew F. and Taylor, John R. (2019) The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime. Journal of Physical Oceanography, 49 (2). pp. 469-487. ISSN 0022-3670. doi:10.1175/jpo-d-18-0079.1.

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The 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: H_a and H_L. The former characterizes the steady Ekman arrested state, and the latter characterizes a relaminarized state. The derivation of H_L makes use of a newly defined slope Obukhov length L_s that characterizes the relative importance of shear production and cross-slope buoyancy advection. The value of H_a can be combined with the temporally evolving depth of the mixed layer H to form a nondimensional variable H/H_a that provides a similarity prediction of the BBL evolution across different turbulent regimes. The length scale L_s 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 H/H_a and H/H_L 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.

Item Type:Article
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URLURL TypeDescription
Ruan, Xiaozhou0000-0003-1240-1584
Thompson, Andrew F.0000-0003-0322-4811
Additional Information:© 2019 American Meteorological Society. Manuscript received 19 April 2018, in final form 10 December 2018. We thank two anonymous reviewers as well as Georgy Manucharyan for helpful comments that improved this manuscript. We gratefully acknowledge support from NSF Awards OPP-1246460 and OPP-1644172.
Funding AgencyGrant Number
Subject Keywords:Bottom currents; Nonlinear dynamics; Topographic effects; Turbulence; Boundary layer; Large eddy simulations
Issue or Number:2
Record Number:CaltechAUTHORS:20190221-075418137
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Official Citation:Ruan, X., A.F. Thompson, and J.R. Taylor, 2019: The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime. J. Phys. Oceanogr., 49, 469–487,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:93014
Deposited By: Tony Diaz
Deposited On:21 Feb 2019 18:33
Last Modified:16 Nov 2021 16:55

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