Abstract
AbstractThe 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.
Highlights
The Antarctic Circumpolar Current (ACC) is driven by wind and buoyancy forcing [e.g., Rintoul and Naveira Garabato, 2013] with a modest contribution from remote diapycnal mixing [Munday et al, 2011]
Understanding the processes that control eddy saturation is important because the ACC volume transport is closely tied to global ocean stratification [Gnanadesikan and Hallberg, 2000; Karsten et al, 2003; Munday et al, 2011] and thereby to ocean heat and carbon storage [Ferrari et al, 2014; Munday et al, 2014; Watson et al, 2015; Lauderdale et al, 2016]
The majority of ocean circulation models used for climate projections do not resolve eddies and show much greater sensitivity of the ACC volume transport and overturning to the surface wind stress [Farneti and Delworth, 2010; Farneti et al, 2015; Bishop et al, 2016; Gent, 2016], calling into question the ability of current coupled climate models to reliably predict future ocean heat and carbon uptake [e.g., Le Quéré et al, 2007; Farneti et al, 2010]
Summary
The Antarctic Circumpolar Current (ACC) is driven by wind and buoyancy forcing [e.g., Rintoul and Naveira Garabato, 2013] with a modest contribution from remote diapycnal mixing [Munday et al, 2011]. Numerous studies have shown that its equilibrium volume transport is much less sensitive to the surface wind stress in eddy-saturated models with resolved, rather than parameterized, turbulent ocean eddies [Straub, 1993; Hallberg and Gnanadesikan, 2001; Tansley and Marshall, 2001; Hallberg and Gnanadesikan, 2006; Munday et al, 2013]. The majority of ocean circulation models used for climate projections do not resolve eddies and show much greater sensitivity of the ACC volume transport and overturning to the surface wind stress [Farneti and Delworth, 2010; Farneti et al, 2015; Bishop et al, 2016; Gent, 2016], calling into question the ability of current coupled climate models to reliably predict future ocean heat and carbon uptake [e.g., Le Quéré et al, 2007; Farneti et al, 2010]. The aims of this study are to explain the physics of eddy saturation and to demonstrate that this leads to antifrictional control—stronger dissipation results in a stronger ACC—with important implications for ocean heat and carbon content
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