Abstract
Abstract. Latitudinal shifts in the Southern Ocean westerly wind jet could drive changes in the glacial to interglacial ocean CO2 inventory. However, whilst CMIP5 model results feature consistent future-warming jet shifts, there is considerable disagreement in deglacial-warming jet shifts. We find here that the dependence of pre-industrial (PI) to Last Glacial Maximum (LGM) jet shifts on PI jet position, or state dependency, explains less of the shifts in jet simulated by the models for the LGM compared with future-warming scenarios. State dependence is also weaker for intensity changes, compared to latitudinal shifts in the jet. Winter sea ice was considerably more extensive during the LGM. Changes in surface heat fluxes, due to this sea ice change, probably had a large impact on the jet. Models that both simulate realistically large expansions in sea ice and feature PI jets which are south of 50° S show an increase in wind speed around 55° S and can show a poleward shift in the jet between the PI and the LGM. However, models with the PI jet positioned equatorwards of around 47° S do not show this response: the sea ice edge is too far from the jet for it to respond. In models with accurately positioned PI jets, a +1° difference in the latitude of the sea ice edge tends to be associated with a −0.85° shift in the 850 hPa jet. However, it seems that around 5° of expansion of LGM sea ice is necessary to hold the jet in its PI position. Since the Gersonde et al. (2005) data support an expansion of more than 5°, this result suggests that a slight poleward shift and intensification was the most likely jet change between the PI and the LGM. Without the effect of sea ice, models simulate poleward-shifted westerlies 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 the LGM winds were more likely to have also been shifted slightly poleward.
Highlights
The concentration of CO2 in the atmosphere decreases by ∼ 90 parts per million between warm interglacial and cold glacial climate states due to oceanic storage of the excess carbon (Sigman et al, 2010)
We focus in this study on the PI and Last Glacial Maximum (LGM) CMIP5-PMIP3 simulations
In terms of PI to LGM jet shifts, if we apply a linear least-squares fit, we find that a 1◦ difference in the sea ice edge suggests a −0.85◦ shift in the 850 hPa jet (r = −0.80; N = 5)
Summary
The concentration of CO2 in the atmosphere decreases by ∼ 90 parts per million between warm interglacial and cold glacial climate states due to oceanic storage of the excess carbon (Sigman et al, 2010). Völker and Köhler (2013) show that when the atmospheric jet shifts poleward, summer sea ice extends, likely due to enhanced heat loss to the atmosphere Both dust and buoyancy forcing may provide an additional means for jet changes to influence glacial to interglacial climate shifts. Existing analyses of PMIP2 and PMIP3 LGM simulation ensembles show considerable inter-model disagreement in PI to LGM southern hemispheric jet changes (Rojas, 2013; Chavaillaz et al, 2013) This is despite the fact that most CMIP5 models exhibit a poleward shift, and all models a strengthening, of the surface jet from 1900 to 2100 (Bracegirdle et al, 2013). We investigate past-cooling LGM state dependency, sea ice, and changes in the Southern Ocean westerly wind jet using CMIP5-PMIP3 output
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