Drag-Controlled Regime Transitions in the Eddy Saturation Mechanism of the Antarctic Circumpolar Current

TL;DR

This study investigates how drag strength influences eddy saturation mechanisms in the Antarctic Circumpolar Current using an idealized model, revealing a threshold-dependent transition between different controlling processes.

Contribution

It identifies a threshold in drag strength that determines whether eddy saturation is driven by standing meanders or a combination of meanders and eddy diffusivity adjustments.

Findings

Below threshold, eddy saturation involves both meanders and diffusivity adjustments.

Above threshold, eddy saturation is governed solely by standing meander adjustments.

Changes in drag strength can explain variations in eddy saturation mechanisms across studies.

Abstract

Eddy saturation -- the weak sensitivity of Antarctic Circumpolar Current (ACC) transport to wind stress -- is a fundamental feature of Southern Ocean dynamics, yet the processes that maintain this state remain debated. Previous studies have proposed different mechanisms, including adjustments of eddy diffusivity and standing meanders, but the conditions under which each mechanism dominates are unclear. Here we use an idealized reentrant channel model to examine how drag strength controls the eddy saturation. When the wind strength relative to friction is below a certain threshold, eddy saturation is governed by a combination of standing meander and eddy diffusivity adjustments; once the threshold is exceeded, it is governed solely by standing meander adjustment. These results suggest that changes in drag strength may account for the divergent eddy saturation mechanisms reported across studies.