Journal
JOURNAL OF PHYSICAL OCEANOGRAPHY
Volume 47, Issue 4, Pages 855-866Publisher
AMER METEOROLOGICAL SOC
DOI: 10.1175/JPO-D-16-0194.1
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Funding
- Stanback Postdoctoral Fellowship Fund at Caltech
- Howland Postdoctoral Program Fund at WHOI
- NSF [PLR-1415489, OCE-1232389, OCE-1235488, ACI-1053575]
- NSF OPP Award [PLR-1313614, PLR-1203720]
- Directorate For Geosciences
- Office of Polar Programs (OPP) [1415489] Funding Source: National Science Foundation
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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 psi* 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 psi* 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 km(3) freshwater content overshoot) and cannot be explained by the GM parameterization. An improved parameterization is developed, such that psi* is proportional to an effective isopycnal slope s*, carrying a finite memory gamma of past slopes. Introducing eddy memory explains the model results and brings to light an oscillation with a period 2 pi root T-E gamma approximate to 50 yr, where the eddy diffusion time scale T-E similar to 10 yr and gamma approximate to 6 yr are diagnosed from the eddy-resolving model. The EM mode increases the Ekman-driven gyre variance by gamma/T-E approximate to 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.
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