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

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 CO2 over glacial-interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal delta O-18 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 degrees (2.9-7.1 degrees, 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 CO2 (R-2 = 0.98). Experiments with a 0.25 degrees 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.

Automated summary

The Southern Hemisphere westerly winds have a significant impact on the deep ocean circulation and carbon storage. This study reconstructs the changes in the Southern Hemisphere surface westerlies during the last deglaciation using planktic foraminiferal delta O-18 data and climate models. The results show a 4.8-degree equatorward shift and a 25% weakening of the westerlies during the Last Glacial Maximum compared to the mid-Holocene. Climate models underestimate this shift. According to the reconstruction, the poleward shift in the westerlies closely correlates with the rise in atmospheric CO2.