Eddy saturation and frictional control of the Antarctic Circumpolar Current

Abstract

The Antarctic Circumpolar Current is the strongest current in the ocean and has a pivotal impact on ocean stratification, heat content, and carbon content. The circumpolar volume transport is relatively insensitive to surface wind forcing in models that resolve turbulent ocean eddies, a process termed “eddy saturation.” Here a simple model is presented that explains the physics of eddy saturation with three ingredients: a momentum budget, a relation between the eddy form stress and eddy energy, and an eddy energy budget. The model explains both the insensitivity of circumpolar volume transport to surface wind stress and the increase of eddy energy with wind stress. The model further predicts that circumpolar transport increases with increased bottom friction, a counterintuitive result that is confirmed in eddy‐permitting calculations. These results suggest an unexpected and important impact of eddy energy dissipation, through bottom drag or lee wave generation, on ocean stratification, ocean heat content, and potentially atmospheric CO2.

Baroclinic Adjustment and Dissipative Control of Storm Tracks

“…(1) and (2) is analogous to an ecological predator-prey relationship, whereby baroclinicity (prey) is periodically eroded by bursts of eddy heat flux (predator) that mixes temperature horizontally downgradient. This relationship maintains the system in a state that oscillates between being marginally stable and marginally unstable with respect to the intense bursts in storm-track activity (Novak et al 2017). As Ambaum and Novak (2014) noted, the value of baroclinicity around which the system oscillates between marginal stability and instability is equal to the eddy dissipation constant D in Eq.…”

“…The setting of 20 equally spaced sigma levels and T42 horizontal resolution (corresponding to 2.8158) was used, since this resolution was found to be sufficient for the study of similar midlatitude dynamics in a similar GCM by Chen et al (2007). Additionally, PUMA with this resolution was found to produce realistic storm tracks (e.g., Fraedrich et al 2005), which exhibit the predator-prey-like oscillations in baroclinicity and heat flux that were observed in the North Atlantic and North Pacific (Novak et al 2017). All experiments were run for 21 years of perpetual equinox.…”

“…This process is called ''eddy saturation.'' Recent work of Marshall et al (2017) has also shown that changes in the bottom drag (via which eddy energy dissipates in the time-varying picture) only affect the large-scale baroclinicity in steady state, while eddy energy remains largely unaffected. Thus, Marshall et al (2017) conclude that their results are consistent with the Ambaum-Novak model predictions except for the limiting cases of vanishing friction and vanishing wind stress.…”

“…Focusing therefore on this baroclinic eddy-mean flow interaction, Ambaum and Novak (2014) proposed a heuristic model that was later found to reproduce some detailed properties of the temporally oscillating behavior of the North Atlantic and North Pacific storm tracks (Novak et al 2017). The model is a two-dimensional dynamical system:…”

“…The baroclinic characteristics relate to the thermal properties of the mean flow, such as the mean meridional temperature gradient (which, by thermal wind balance, is proportional to the vertical shear of the mean flow). It is the interaction between the eddies and the baroclinic characteristics of the mean flow that is often seen as the primary control of midlatitude storm tracks (e.g., Pedlosky 1992;James 1994;Novak et al 2017).…”