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- Sime, Louise C., et al.
(författare)
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Southern Hemisphere westerly wind changes during the Last Glacial Maximum : model-data comparison
- 2013
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 64, s. 104-120
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Hemisphere (SH) westerly winds are thought to be critical to global ocean circulation, productivity, and carbon storage. For example, an equatorward shift in the winds, though its affect on the Southern Ocean circulation, has been suggested as the leading cause for the reduction in atmospheric CO2 during the Last Glacial period. Despite the importance of the winds, it is currently not clear, from observations or model results, how they behave during the Last Glacial. Here, an atmospheric modelling study is performed to help determine likely changes in the SH westerly winds during the Last Glacial Maximum (LGM). Using LGM boundary conditions, the maximum in SH westerlies is strengthened by similar to+1 m s(-1) and moved southward by similar to 2 degrees at the 850 hPa pressure level. Boundary layer stabilisation effects over equatorward extended LGM sea-ice can lead to a small apparent equatorward shift in the wind band at the surface. Further sensitivity analysis with individual boundary condition changes indicate that changes in sea surface temperatures are the strongest factor behind the wind change. The HadAM3 atmospheric simulations, along with published PMIP2 coupled climate model simulations, are then assessed against the newly synthesised database of moisture observations for the LGM. Although the moisture data is the most commonly cited evidence in support of a large equatorward shift in the SH winds during the LGM, none of the models that produce realistic LGM precipitation changes show such a large equatorward shift. In fact, the model which best simulates the moisture proxy data is the HadAM3 LGM simulation which shows a small poleward wind shift. While we cannot prove here that a large equatorward shift would not be able to reproduce the moisture data as well, we show that the moisture proxies do not provide an observational evidence base for it.
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| 2. |
- Sime, Louise C., et al.
(författare)
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Warm climate isotopic simulations: what do we learn about interglacial signals in Greenland ice cores?
- 2013
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 67, s. 59-80
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of Last Interglacial stable water isotopes in ice cores show that central Greenland delta O-18 increased by at least 3 parts per thousand compared to present day. Attempting to quantify the Greenland interglacial temperature change from these ice core measurements rests on our ability to interpret the stable water isotope content of Greenland snow. Current orbitally driven interglacial simulations do not show delta O-18 or temperature rises of the correct magnitude, leading to difficulty in using only these experiments to inform our understanding of higher interglacial delta O-18. Here, analysis of greenhouse gas warmed simulations from two isotope-enabled general circulation models, in conjunction with a set of last Interglacial sea surface observations, indicates a possible explanation for the interglacial delta O-18 rise. A reduction in the winter time sea ice concentration around the northern half of Greenland, together with an increase in sea surface temperatures over the same region, is found to be sufficient to drive a >3 parts per thousand interglacial enrichment in central Greenland snow. Warm climate delta O-18 and delta D in precipitation falling on Greenland are shown to be strongly influenced by local sea surface condition changes: local sea surface warming and a shrunken sea ice extent increase the proportion of water vapour from local (isotopically enriched) sources, compared to that from distal (isotopically depleted) sources. Precipitation intermittency changes, under warmer conditions, leads to geographical variability in the delta O-18 against temperature gradients across Greenland. Little sea surface warming around the northern areas of Greenland leads to low delta O-18 against temperature gradients (0.1-0.3 parts per thousand. per degrees C), whilst large sea surface warmings in these regions leads to higher gradients (03-0.7 parts per thousand per degrees C). These gradients imply a wide possible range of present day to interglacial temperature increases (4 to >10 degrees C). Thus, we find that uncertainty about local interglacial sea surface conditions, rather than precipitation intermittency changes, may lead to the largest uncertainties in interpreting temperature from Greenland ice cores. We find that interglacial sea surface change observational records are currently insufficient to enable discrimination between these different delta O-18 against temperature gradients. In conclusion, further information on interglacial sea surface temperatures and sea ice changes around northern Greenland should indicate whether +5 degrees C during the Last Interglacial is sufficient to drive the observed ice core delta O-18 increase, or whether a larger temperature increases or ice sheet changes are also required to explain the ice core observations. (c) 2013 Elsevier Ltd. All rights reserved.
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