It is thought that wind changes over the Southern Ocean may have been critical in driving changes in CO2 between cold ice-world and warm-world climates.
Because of inconsistencies between the simulated wind changes in different models, changes in these winds have been difficult to interpret. Here we investigate this problem of inconsistency in model results.
We find that what we call 'state-dependence', or the simulation of the Southern Ocean winds in the present warm climate, is important in determining wind changes in the models. Alongside this state dependence, the simulation of Southern Ocean sea-ice changes is key.
Observations of cold-world sea-ice change suggest that sea ice occurred more than 5° equatorward of the present day wintertime sea-ice edge at 21,000 years ago. Without this more extensive sea ice, models simulate poleward-shifted westerly winds in warming climates and equatorward-shifted westerlies in colder climates. However, the feedback of sea ice counters and reverses the equatorward trend in cooler climates so that 21,000 years ago, winds were more likely to have also been shifted slightly poleward.
This finding will help in understanding the large increase in atmospheric CO2 from 21,000 years ago until the pre-industrial period.
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Louise C. Sime et al. Sea ice led to poleward-shifted winds at the Last Glacial Maximum: the influence of state dependency on CMIP5 and PMIP3 models, Climate of the Past (2016). DOI: 10.5194/cp-12-2241-2016