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Bottom Boundary Potential Vorticity Injection from an Oscillating Flow: A PV Pump

Ruan, Xiaozhou and Thompson, Andrew F. (2016) Bottom Boundary Potential Vorticity Injection from an Oscillating Flow: A PV Pump. Journal of Physical Oceanography, 46 (11). pp. 3509-3526. ISSN 0022-3670. doi:10.1175/JPO-D-15-0222.1.

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Oceanic boundary currents over the continental slope exhibit variability with a range of time scales. Numerical studies of steady, along-slope currents over a sloping bathymetry have shown that cross-slope Ekman transport can advect buoyancy surfaces in a bottom boundary layer (BBL) so as to produce vertically sheared geostrophic flows that bring the total flow to rest: a process known as buoyancy shutdown of Ekman transport or Ekman arrest. This study considers the generation and evolution of near-bottom flows due to a barotropic, oscillating, and laterally sheared flow over a slope. The sensitivity of the boundary circulation to changes in oscillation frequency ω, background flow amplitude, bottom slope, and background stratification is explored. When ω/f ≪ 1, where f is the Coriolis frequency, oscillations allow the system to escape from the steady buoyancy shutdown scenario. The BBL is responsible for generating a secondary overturning circulation that produces vertical velocities that, combined with the potential vorticity (PV) anomalies of the imposed barotropic flow, give rise to a time-mean, rectified, vertical eddy PV flux into the ocean interior: a “PV pump.” In these idealized simulations, the PV anomalies in the BBL make a secondary contribution to the time-averaged PV flux. Numerical results show the domain-averaged eddy PV flux increases nonlinearly with ω with a peak near the inertial frequency, followed by a sharp decay for ω/f > 1. Different physical mechanisms are discussed that could give rise to the temporal variability of boundary currents.

Item Type:Article
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URLURL TypeDescription
Ruan, Xiaozhou0000-0003-1240-1584
Thompson, Andrew F.0000-0003-0322-4811
Additional Information:© 2016 American Meteorological Society. (Manuscript received 18 November 2015, in final form 1 September 2016) We thank two anonymous reviewers as well as Jessica Benthuysen, Leif Thomas, Andrew Stewart, Georgy Manucharyan, and Kaushik Srinivasan for helpful comments and conversations that improved this manuscript. We gratefully acknowledge support from NSF Award OPP-1246460.
Funding AgencyGrant Number
Subject Keywords:Boundary currents; Potential vorticity; Boundary layer; Differential equations; Numerical analysis/modeling
Issue or Number:11
Record Number:CaltechAUTHORS:20161222-102809867
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Official Citation:Ruan, X. and A. Thompson, 2016: Bottom Boundary Potential Vorticity Injection from an Oscillating Flow: A PV Pump. J. Phys. Oceanogr., 46, 3509–3526, doi: 10.1175/JPO-D-15-0222.1.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73142
Deposited By: George Porter
Deposited On:22 Dec 2016 19:13
Last Modified:11 Nov 2021 05:12

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