Poleward shift in the Southern Hemisphere westerly winds synchronous with the deglacial rise in CO2

• Published in: Paleoceanography and Paleoclimatology
• Main Authors: Gray, William R., De Lavergne, Casimir, Jnglin Wills, Robert C., Menviel, Laurie, Spence, Paul, Holzer, Mark, Kageyama, Masa, Michel, Elisabeth
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU) 2023
• Subjects: Southern Ocean; Sea ice
• Online Access: https://archimer.ifremer.fr/doc/00842/95397/103174.pdf; https://archimer.ifremer.fr/doc/00842/95397/103175.csv; https://archimer.ifremer.fr/doc/00842/95397/103176.csv; https://archimer.ifremer.fr/doc/00842/95397/103177.csv; https://archimer.ifremer.fr/doc/00842/95397/103178.pdf; https://doi.org/10.1029/2023PA004666; https://archimer.ifremer.fr/doc/00842/95397/

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 ...