Evolving Southern Ocean overturning in warming climates

TL;DR

The Southern Ocean circulation intensifies and shifts poleward in warming climates, with eddies compensating differently in the past and future.

Contribution

The study reveals distinct eddy compensation mechanisms in past and future warming scenarios, linked to Antarctic bottom water formation.

Findings

Eulerian-mean MOC shifts poleward under high-emission scenarios, with eddy compensation maintaining intensified residual overturning.

Eddy-induced MOC is modulated by surface heat and freshwater flux changes in warming climates.

Mid-Pliocene and future warming show distinct eddy compensation patterns due to differences in buoyancy forcing and ventilation.

Abstract

The Southern Ocean Meridional Overturning Circulation (MOC) has intensified in recent decades, yet the interplay between its Eulerian and eddy components under future warming remains uncertain. Using ensemble climate simulations, here we show that the Eulerian-mean MOC shifts poleward under high-emission scenarios during the twenty-first century, with compensating eddy-induced MOC sustaining a uniformly intensified residual overturning. This response is less pronounced under low-emission scenarios with climate mitigation. Likewise, a poleward-shifted Eulerian-mean MOC occurred during the Mid-Pliocene Warm Period, but with weaker, broader eddy compensation, leading to non-uniform intensified residual overturning. Across past and future warming climates, the eddy-induced MOC is primarily modulated by surface heat flux changes at lower latitudes and by freshwater flux changes at higher latitudes over the Southern Ocean. The buoyancy forcing changes drive northward Antarctic upwelling, promoting Antarctic bottom water formation. Along with MOC changes, ventilation intensifies in the lower latitudes of the Southern Ocean as related to the subduction branch, especially during the past warm period.

Although the Southern Ocean Meridional Circulation intensifies and shifts poleward under warming periods, the eddy compensation in the past and future is distinct due to enhanced Antarctic bottom water formation and subduction branch ventilation.

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