Poleward Shift in the Southern Hemisphere Westerly Winds Synchronous With the Deglacial Rise in CO2

Abstract The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO 2 over glacial‐interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ 18 O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid‐Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO 2 ( R 2 = 0.98). Experiments with a 0.25° resolution ocean‐sea‐ice‐carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO 2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO 2 rise, and suggest outgassing of natural CO 2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.