Sea ice led to poleward-shifted winds at the Last Glacial Maximum: the influence of state dependency on CMIP5 and PMIP3 models

Abstract

Abstract. Latitudinal shifts in the Southern Ocean westerly wind jet could drive changes in the glacial to interglacial ocean CO 2 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.

The influence of the ocean circulation state on ocean carbon storage and CO<sub>2</sub> drawdown potential in an Earth system model

“…Löfverström et al (2014). Sime et al (2013) and Sime et al (2016) suggest that Southern Hemisphere winds will also be stronger when applying LGM boundary conditions, though they emphasise that results from different palaeo-proxies and models disagree on this. In our simulation, we intensify the wind stress in both hemispheres and this leads to decreased capacity of both the biological and the solubility pump and effectively an increase in pCO atm 2 .…”

Back to the Future: Using Long-Term Observational and Paleo-Proxy Reconstructions to Improve Model Projections of Antarctic Climate

“…Conversely, models are found to underestimate the Antarctic cooling in LGM simulations [72]. This has implications for the reliability of simulated westerlies, which are however difficult to assess in models due to uncertainties in available proxies for westerlies in the LGM [5,73,74]. Similar issues have been noted in PlioMIP experiments of the mPWP for which model disagreements in Southern Ocean SST, sea ice extent, westerly winds and the broader thermohaline circulation are again difficult to assess due to uncertainty in proxy reconstructions across these variables [75].…”

“…Paleoclimate model intercomparison projects (PMIPs) have increasingly provided data and stimulated the development of approaches for using paleoclimate simulations to improve future projections [20,80]. Examples of promising approaches that combine output from PMIP simulations and long-term climate reconstructions to constrain Antarctic climate projections include: (i) exploiting Antarctic/global warming relationship [81], (ii) utilizing cross-model correlations in simulated future and past sea ice extent (SIE) changes [81], and (iii) by assessing links between SIE and the circumpolar surface westerlies as simulated by models in a range of different past climates [73]. A key next step that would benefit the above interpretations by, for example, increasing the statistical significance of relationships across the multi-model ensemble, would be to increase the number of modelling groups participating in future PMIPs (only nine models were available for use in recent PMIP3 studies [73,81]).…”

Linking Glacial‐Interglacial States to Multiple Equilibria of Climate

“…Although the strength and position of the SH westerly winds in the glacial periods has received much attention as a possible driver of the atmospheric CO 2 change (Toggweiler, ), paleoproxy data are very uncertain (Kohfeld et al, ; Shulmeister et al, ) while simulations of the LGM show very little agreement among models (Sime et al, ). In our simulations, changes in the SH jet stream (Figure ) have little impact on CO 2 , which is mainly driven by changes in sea ice cover.…”