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

Abstract

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 c* 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 c* 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 c* is proportional to an effective isopycnal slope s*, carrying a finite memory g of past slopes. Introducing eddy memory explains the model results and brings to light an oscillation with a period 2p ffiffiffiffiffiffiffiffiffi T E g p ' 50 yr, where the eddy diffusion time scale T E ; 10 yr and g ' 6 yr are diagnosed from the eddy-resolving model. The EM mode increases the Ekman-driven gyre variance by g/T E ' 50% 6 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.

Arctic Ocean surface geostrophic circulation 2003–2014

“…3) we obtain good agreement with the idealised experiments (Fig. 2b of Manucharyan et al, 2017) and taking L mix = 10-15 km (Nurser and Bacon, 2014), we find K = 200-500 m 2 s −1 , consistent with values predicted by Manucharyan and Spall (2016).…”

Arctic Ocean Freshwater Content and Its Decadal Memory of Sea‐Level Pressure

“…Although this oscillation is within the uncertainty of our estimate, if physical it could be related to the advection of freshwater recently exported from the Arctic back into the basin after a circuit of the subpolar gyre (Proshutinsky et al, 2015;Stewart & Haine, 2013). In higher resolution models and the real Arctic we might expect a similar oscillation due to eddy memory (Manucharyan et al, 2017), although HiGEM parameterizes rather than resolves eddies at high latitudes so this is unlikely to be important here. A third possibility is exchange between the liquid and solid freshwater reservoirs, resulting from feedbacks on ice formation and melt.…”

Partitioning of Kinetic Energy in the Arctic Ocean's Beaufort Gyre

“…Idealized numerical experiments and theory have been put forward to describe a mechanism for wind‐driven gyre equilibration by eddies that cumulatively act to flatten isopycnals and inhibit freshwater accumulation (Manucharyan et al, , ). However, likely due to the simplicity of the vertical stratification used in these idealized experiments, the generated eddies were not coherent and predominantly of the first baroclinic mode with substantially larger sizes O(100 km) compared to the observed isolated halocline eddies which have significant second baroclinic mode energy and are O(10 km) in size.…”