The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesised to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ18O 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 (LGM; 20 ka) relative to the mid-Holocene (6.5 ka). Climate models from the Palaeoclimate Modelling 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 CO2 (R2=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 CO2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO2 rise, and suggest outgassing of natural CO2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.

Key Points

we use planktic foraminiferal δ18O and climate models to infer deglacial changes in the Southern Hemisphere surface westerlies

we estimate the westerlies were ∼5° equatorward and ∼25% weaker at the LGM; their poleward shift over deglaciation mirrors the rise in CO2

experiments with a 1/4° ocean-sea-ice-carbon model indicate increased oceanic carbon storage with equatorward shifted westerlies

Plain Language Summary

The mid-latitudes of the Southern Hemisphere are characterised by a band of strong westerly winds. These winds play an important role in driving the circulation of the deep ocean and are thought to influence the oceans’ ability to store carbon. Understanding how the westerlies have varied in the past is challenging as we have few methods to track the winds directly. Here we use oxygen isotopes in foraminiferal shells to track changes in the broad-scale pattern of sea surface temperature across the Southern Ocean, which we link to changes in the winds using climate models. We find the westerly winds were displaced around 5° equatorward during the cold climate of the last ice age, and that the poleward shift in the winds we observe as the earth warmed out of the ice age bears an uncanny resemblance to the increase in atmospheric CO2. We then force the winds in a climate model towards the equator in a similar manner to the shift we observe in the ice age, and find the model stores more carbon in the ocean. Our results support a link between shifts in the Southern Hemisphere westerly winds and atmospheric CO2.