Extending Residual-Mean Overturning Theory to the Topographically Localized Transport in the Southern Ocean

Abstract

Abstract The Southern Ocean plays a major role in global air–sea carbon fluxes, with some estimates suggesting it contributes to up to 40% of the oceanic anthropogenic carbon dioxide uptake, despite only comprising about 20% of oceanic surface area. Thus, the Southern Ocean overturning, the circulation that transports tracers between the surface and deep ocean interior, is particularly important for climate. Recent studies show that vertical velocities and tracer transport are largest just downstream of bottom topography; these quantities are related to the overturning, but provide incomplete information about the net Lagrangian transport, usually described with the residual-mean theory in a zonally integrated sense. This study uses an idealized Southern Ocean–like channel model with particle tracking to visualize the thickness-weighted velocities that capture the net overturning transport of a parcel, connecting residual-mean overturning theory to the three-dimensional, localized nature of the overturning. From this, we split the flow into three main drivers of transport: a wind-driven Ekman pumping into or out of a density layer, and standing eddies and transient eddies, both of which are localized near the topography. In this framework, the three-dimensional overturning circulation is not a small residual between the eddy and Eulerian-mean transport. The existence of a ridge weakens the response of the overturning to changes in wind, especially in the lower cell. This local understanding of the overturning framework suggests that careful modeling and sampling of specific regions near topography in the Southern Ocean are vital to understand climate sensitivity, transport, carbon export, and connections with the oceans to the north.