112 Eddy Memory Mode of Low-frequency Ocean Variability

Tuesday, 27 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Georgy E. Manucharyan, California Institute of Technology, Pasadena, CA; and A. F. Thompson and M. A. Spall

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 spin-up as a 15% overshoot in isopycnal slope and cannot be explained by the GM-parameterization. We developed an improved parameterization, making psi* proportional to an effective isopycnal slope, s*, that carries a finite memory gamma of past slopes. The memory explains the isopycnal overshoot and a lagged eddy kinetic energy response, bringing to light an oscillation with a period 2pi (Te gamma)^0.5 = 50 years, where the eddy-diffusion time-scale Te = 10 years and γ = 6 years are diagnosed from the eddy-resolving model. The EM-mode increases the Ekman-driven gyre variance by gamma/Te = 50 ± 15% – a fraction that stays relatively constant despite both time-scales decreasing with increasing mean forcing. We expect the eddy memory to be a fundamental property of rotating turbulent flows and highlight the need for better observational constraints on transient eddy-field characteristics.
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