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Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre

Manucharyan, Georgy E. and Thompson, Andrew F. and Spall, Michael A. (2017) Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre. Journal of Physical Oceanography, 47 (4). pp. 855-866. ISSN 0022-3670. doi:10.1175/JPO-D-16-0194.1.

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Mesoscale eddies shape the Beaufort Gyre response to Ekman pumping, but their transient dynamics are poorly understood. Climate models commonly use the Gent–McWilliams (GM) parameterization, taking the eddy streamfunction Ψ* to be proportional to an isopycnal slope s and an eddy diffusivity K. This local-in-time parameterization leads to exponential equilibration of currents. Here, an idealized, eddy-resolving Beaufort Gyre model is used to demonstrate that Ψ* carries a finite memory of past ocean states, violating a key GM assumption. As a consequence, an equilibrating gyre follows a spiral sink trajectory implying the existence of a damped mode of variability—the eddy memory (EM) mode. The EM mode manifests during the spinup as a 15% overshoot in isopycnal slope (2000 km3 freshwater content overshoot) and cannot be explained by the GM parameterization. An improved parameterization is developed, such that Ψ* is proportional to an effective isopycnal slope s*, carrying a finite memory γof past slopes. Introducing eddy memory explains the model results and brings to light an oscillation with a period 2π√T_Eγ ≈ 50 yr, where the eddy diffusion time scale T_E ~ 10 yr and γ ≈ 6 yr are diagnosed from the eddy-resolving model. The EM mode increases the Ekman-driven gyre variance by γ/T_E ≈ 50% ± 15%, a fraction that stays relatively constant despite both time scales decreasing with increased mean forcing. This study suggests that the EM mode is a general property of rotating turbulent flows and highlights the need for better observational constraints on transient eddy field characteristics.

Item Type:Article
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Manucharyan, Georgy E.0000-0001-7959-2675
Thompson, Andrew F.0000-0003-0322-4811
Additional Information:© 2017 American Meteorological Society. Manuscript received 23 August 2016, in final form 4 February 2017. Published online: 12 April 2017. GEM acknowledges the Stanback Postdoctoral Fellowship Fund at Caltech and the Howland Postdoctoral Program Fund at WHOI. MAS was supported by NSF Grants PLR-1415489 and OCE-1232389. AFT acknowledges support from NSF OCE-1235488. GEM thanks Glenn Flierl for discussions during the 2014 Geophysical Fluid Dynamics Summer School at WHOI. The authors acknowledge the high-performance computing support from Yellowstone provided by the NCAR CIS Laboratory, sponsored by the NSF. This work used the Extreme Science and Engineering Discovery Environment (XSEDE; Towns et al. 2014), which is supported by NSF Grant Number ACI-1053575.The manuscript benefited from discussions at the annual Forum for Arctic Modeling and Observing Synthesis (FAMOS) funded by the NSF OPP Awards PLR-1313614 and PLR-1203720.
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Woods Hole Oceanographic InstitutionUNSPECIFIED
Subject Keywords:Arctic; Eddies; Ekman pumping/transport; Mesoscale processes; Parameterization; Multidecadal variability
Issue or Number:4
Record Number:CaltechAUTHORS:20170926-125017588
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81837
Deposited By: Tony Diaz
Deposited On:26 Sep 2017 19:58
Last Modified:15 Nov 2021 19:46

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