Extended Data Fig. 9: Meridional overturning circulation in the Southern Ocean.

Shown are the Eulerian-mean meridional overturning stream function (Sv, 10⁶ m³ s⁻¹) in depth coordinates in the perturbation experiments representing the average between model years 100-200 for a) interglacial (PID_PERT), b) intermediate (40ka_PERT) and c) glacial maximum (LGM_PERT) conditions (cf. Extended Data Fig. 6). Positive (negative) values indicate clockwise (anticlockwise) circulation. The latitudinal variation in zonal wind-stress induces upwelling between the easterly and westerly wind maxima, and downwelling outside this corridor. Upwelled water can be returned as part of the wind-driven cells south or north of the Antarctic divergence (Ant. Div.) that is linked to a southbound Ekman transport under the influence of Polar easterlies and a northbound Ekman transport under the influence of the westerlies. Along these transport pathways heat and freshwater fluxes at the surface are part of the generalized circulation pattern that feed into the lower or upper limb of the recirculation, respectively. However, the actual upwelling transport largely occurs along surfaces of equal density, with wind and eddy processes playing a central role¹⁶. To illustrate such an isopycnal upwelling pathway, we additionally show the σ₃ density surface (black line) that characterizes conditions at 3000 m depth and 30°S and upwells towards the latitude belt of the Antarctic Divergence that is subject to strong mixing during austral fall and winter seasons⁷⁸.