The mid-latitudes of the Southern Hemisphere are characterized by strong westerly winds circulating in a relatively narrow band of latitudes. These zonal winds play an important role in ocean circulation and influence the oceans' capacity to store carbon. These winds are therefore also thought to play a key role in regulating atmospheric CO2 during past glacial-interglacial cycles. However, past variations in their position and strength remain poorly understood.
A compilation of d18O measurements of planktonic foraminiferal shells from across the Southern Ocean has enabled us to trace past variations in surface temperatures (SST) in this ocean during the last deglaciation. These measurements were complemented by "emergent" relationships derived from a set of climate models, which enabled us to reconstruct changes in westerly winds in the Southern Hemisphere. During the Last Glacial Maximum (LGM), around 20,000 years ago, these winds were positioned around 4.8° further north than during the Middle Holocene, dated at around 6,500 years before the present (remember that the Holocene is the interglacial period we've been in for around 10,000 years). Their strength was then reduced by around 25%.
Simulations calculated with an ocean-ice-carbon model with a resolution of 0.25° suggest that the more northerly positioning of these winds during the LGM had reduced the vertical overturning rate of the Southern Ocean, resulting in a suppression of the degassing of CO2 stored deep in this ocean.
CO2 outgassing from the Southern Ocean is likely to increase as westerly winds shift towards the South Pole due to anthropogenic warming.
Contact : William Gray, LSCE
Hemisphere westerly winds synchronous with the deglacial rise in CO2. Paleoceanography and Paleoclimatology, 38, e2023PA004666. https://doi.org/10.1029/2023PA004666
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