The Southern Ocean has shown little warming south of the Antarctic Circumpolar Current (ACC) over recent decades, and Antarctic sea-ice cover has been modestly expanding 1 . Along the northern flank of the ACC, however, the upper ocean has been warming rapidly 2, 3 . Using observations and general circulation model simulations, we show that these patterns -of delayed warming south of the ACC and enhanced warming to the north -are fundamentally shaped by the Southern Ocean's meridional overturning circulation: wind-driven upwelling of unmodified water from depth damps warming around Antarctica; greenhouse gas induced heat uptake is largely balanced by anomalous northward heat transport; and heat is preferentially stored along the northern flank of the ACC, where surface waters are subducted.Further, we find that these processes are primarily due to passive advection of the anomalous warming signal by climatological ocean currents; changes in atmospheric and oceanic circulations play a secondary role. These findings suggest that the Southern Ocean responds to greenhouse gas forcing on the timescale over which the deep ocean waters that are upwelled 1

Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing 1-3 , where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch 4-6 . While brine rejection from sea ice is thought to contribute to the lower branch 7 , the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport 8,9 . Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework 10 to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with di erential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ∼22 × 10 6 m 3 s −1 . These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.The Southern Ocean State Estimate (SOSE) is an ice/ocean data assimilation produced for the time period January 2005 through December 2010. (See Methods and Supplementary Information for SOSE details and validation.) The bulk freshwater fluxes at the ocean surface south of 50 • S, as estimated by SOSE, are summarized in Fig. 1a. When sea ice forms, nearly all of the salt remains behind in the underlying seawater (a process called brine rejection); when the ice melts, liquid freshwater is returned to the ocean. Direct open-ocean precipitation minus evaporation (0.28 fwSv) and glacial ice melt (0.05 fwSv) are both smaller than net sea-ice melt (0.50 fwSv) and brine rejection (0.36 fwSv). (Bulk freshwater volume fluxes are given in units of freshwater sverdrups 11 , 1 fwSv = 10 6 m 3 freshwater s −1 3.15 × 10 4 Gt freshwater per year.) Melt exceeds brine rejection because sea ice incorporates snowfall at a rate of 0.14 fwSv. Moreover, wind-driven sea-ice transport creates a freshwater conveyor belt from the Antarctic coast to the open ocean 12 , leading to sharp gradients in freshwater flux. The spatial structure of the sea-ice redistribution is assessed in Fig. 1b by comparing the annual mean freshwater flux leaving the atmosphere, land and glaciers (left panel) with that entering the ocean (right panel); the difference is due to sea-ice freshwater redistribution (middle panel, vectors show the ice thickness transport). From the atmosphere, widespread precipitation over the Southern Ocean leads to a broadly distributed downward freshwater flux with a characteristic magnitude of 0.5 m yr −1 , and glacial ice melt provides a stronger freshwater source near the Antarctic coas...

The dynamics of the lower cell of the meridional overturning circulation (MOC) in the Southern Ocean are compared in two versions of a global climate model: one with high-resolution (0.1°) ocean and sea ice and the other a lower-resolution (1.0°) counterpart. In the high-resolution version, the lower cell circulation is stronger and extends farther northward into the abyssal ocean. Using the water-mass-transformation framework, it is shown that the differences in the lower cell circulation between resolutions are explained by greater rates of surface water-mass transformation within the higher-resolution Antarctic sea ice pack and by differences in diapycnal-mixing-induced transformation in the abyssal ocean. While both surface and interior transformation processes work in tandem to sustain the lower cell in the control climate, the circulation is far more sensitive to changes in surface transformation in response to atmospheric warming from raising carbon dioxide levels. The substantial reduction in overturning is primarily attributed to reduced surface heat loss. At high resolution, the circulation slows more dramatically, with an anomaly that reaches deeper into the abyssal ocean and alters the distribution of Southern Ocean warming. The resolution dependence of associated heat uptake is particularly pronounced in the abyssal ocean (below 4000 m), where the higher-resolution version of the model warms 4.5 times more than its lower-resolution counterpart.