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Titel |
Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum: report of the MARGO Project |
VerfasserIn |
C. Waelbroeck, A. Paul, M. Kucera, A. Rosell-Melé, M. Weinelt, R. Schneider, A. Mix, MARGO Project Members |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250026274
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Zusammenfassung |
Reconstructions of sea surface temperature (SST) from relatively stable periods in the past,
such as the Last Glacial Maximum (LGM), represent one of the best means to constrain
climate sensitivity and provide targets to evaluate coupled atmosphere-ocean general
circulation models. The first quantitative global reconstruction of SST at the LGM was
developed by the CLIMAP (Climate: Long-Range Investigation, Mapping and Prediction)
project. Since then, there has not been any concerted effort to synthesize existing paleodata at
the global scale, although several shortcomings of CLIMAP pioneering work have become
apparent.We present a LGM global synthesis of SST reconstructions undertaken by
the MARGO (Multiproxy Approach for the Reconstruction of the Glacial Ocean
Surface) project. The objective has been to compile and analyse available estimates
of LGM SSTs based on all prevalent microfossil-based (i.e., transfer functions
based on planktonic foraminifera, diatoms, dinoflagellate cysts and radiolarians
abundances) and geochemical (i.e., alkenones and planktonic foraminifera Mg/Ca)
paleothermometers. The MARGO project approach is to argue that no current proxy method
is objectively better than another to provide an accurate picture of past SST, and that
consequently the multiproxy approach yields the least biased representation of past
reality.
As originally suggested by CLIMAP, the strongest annual mean cooling (up to -10Ë C)
occurred in the mid-latitude North Atlantic and extended into the western Mediterranean
(-6Ë C). However, in contrast to CLIMAP, MARGO data indicate that the cooling
was more pronounced in the eastern than in the western basin. The magnitude and
position of a steep temperature gradient between 60 and 45Ë N are supported by four
different proxies. In contrast with the CLIMAP reconstruction, all proxies also agree
on ice-free conditions in the Nordic Seas during glacial summer. However, large
discrepancies with respect to glacial temperatures recorded by different microfossil proxies
remain. The best convergence between the various proxy estimates occurs within the
30Ë N to 30Ë S band. Strong inter-basins differences as well as clear west-east
gradients within each basin mark the equatorial oceans, although anomalies are
smaller in the Pacific and Indian Ocean than in the Atlantic. Tropical cooling is more
extensive than that proposed by CLIMAP. Large cooling of the Eastern Boundary
Current (EBC) systems in the Southern hemisphere is reconstructed by all proxies,
making this a very robust feature of the climate and ocean circulation during the
LGM.
Existing coupled atmosphere-ocean general circulation models (AO-GCMs) simulations
for the LGM show significant disagreement with respect to the location and magnitude of the
North Atlantic cold anomaly while exhibiting stronger glacial cooling in the western than in
the eastern Atlantic (http://pmip2.lsce.ipsl.fr/). This demonstrates that the robust MARGO
North Atlantic East-West SST anomaly gradient is a good target with which the skill of
models can be evaluated. With the advent of the multi-proxy method, we have not only been
able to produce a new reconstruction of the glacial ocean surface, but also to deliver
uncertainty estimates. Taken together, this yields new observational bounds on the
sensitivity of Earth’s climate system, with the perspective of improving existing climate
models that are being used in the assessment of ongoing and future climate change. |
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