Wind‐driven freshwater buildup and release in the Beaufort Gyre constrained by mesoscale eddies

Abstract

Recently, the Beaufort Gyre has accumulated over 20,000 km3 of freshwater in response to strong anticyclonic atmospheric winds that have prevailed over the gyre for almost two decades. Here we explore key physical processes affecting the accumulation and release of freshwater within an idealized eddy‐resolving model of the Beaufort Gyre. We demonstrate that a realistic halocline can be achieved when its deepening tendency due to Ekman pumping is counteracted by the cumulative action of mesoscale eddies. Based on this balance, we derive analytical scalings for the depth of the halocline and its spin‐up time scale and emphasize their explicit dependence on eddy dynamics. Our study further suggests that the Beaufort Gyre is currently in a state of high sensitivity to atmospheric winds. However, an intensification of surface stress would inevitably lead to a saturation of the freshwater content—a constraint inherently set by the intricacies of the mesoscale eddy dynamics.

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

“…Unlike the ACC, which might be in a so-called eddy saturation regime with weak equilibrium sensitivity of its transport and density structure (Böning et al 2008;Tansley and Marshall 2001;Hallberg and Gnanadesikan 2001;Munday et al 2013;Abernathey and Cessi 2014), the Beaufort Gyre appears to be highly sensitive to variations in the Ekman pumping (Proshutinsky et al 2002;Manucharyan and Spall 2016). This implies that a continuing melting of the sea ice and/or changes in the atmospheric winds that alter the Ekman pumping would lead to substantial changes in the halocline depth and hence in the freshwater budget of the entire Arctic Ocean.…”

“…Marshall and Radko (2003) demonstrate that the equilibrium isopycnal slope s of an eddying baroclinic current is directly related to K. In addition, Manucharyan et al (2016) show that the temporal variability of s also depends on the sensitivity of eddy diffusivity to halocline slope dK/ds (see section 4). Thus, climate models that use too small (large) values of eddy diffusivity would result in too deep (shallow) halocline.…”

“…Since the Arctic Ocean is rapidly changing, it is important to understand mechanisms behind the transient gyre adjustment. Manucharyan et al (2016) point out that the eddy field also plays a key role in Ekman-driven gyre variability. Because of the tendency of the eddy streamfunction to oppose the Ekman transport, a large-scale circulation (i.e., averaged over small-scale eddies) has to be a stable dynamical system that equilibrates on a time scale inversely proportional to the eddy diffusivity K (Manucharyan et al 2016).…”

“…Manucharyan et al (2016) point out that the eddy field also plays a key role in Ekman-driven gyre variability. Because of the tendency of the eddy streamfunction to oppose the Ekman transport, a large-scale circulation (i.e., averaged over small-scale eddies) has to be a stable dynamical system that equilibrates on a time scale inversely proportional to the eddy diffusivity K (Manucharyan et al 2016). These theoretical conclusions rely upon the use of a conventional Gent-McWilliams (GM) eddy parameterization (Gent and McWilliams 1990;Gent et al 1995) that takes the isopycnal eddy thickness fluxes to be proportional to large-scale halocline thickness gradients:…”

“…A recent study, however, emphasized internal dynamics of the gyre and demonstrated that the large-scale halocline deepening due to Ekman pumping is counteracted by the cumulative action of mesoscale eddies (Manucharyan and Spall 2016), a dynamical balance similar to that of the Antarctic Circumpolar Current (ACC; Marshall and Radko 2003) or the Weddell Gyre (Su et al 2014).…”