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Science 3 April 2009:
Vol. 324. no. 5923, pp. 78 - 80
DOI: 10.1126/science.1166349
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Reports

Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly

Valérie Trouet,1* Jan Esper,1,2 Nicholas E. Graham,3,4 Andy Baker,5 James D. Scourse,6 David C. Frank1

The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña–like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA.

1 Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Zürcherstrasse 111, 8903 Birmensdorf, Switzerland.
2 Oeschger Centre for Climate Change Research, Erlachstrasse 9a, 3012 Bern, Switzerland.
3 Hydrologic Research Center, 12780 High Bluff Drive, Suite 250, San Diego, CA 92130–2069, USA.
4 Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093–0225, USA.
5 School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, UK.
6 School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5AB, UK.

* To whom correspondence should be addressed. E-mail: trouet{at}wsl.ch

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly.
M. E. Mann, Z. Zhang, S. Rutherford, R. S. Bradley, M. K. Hughes, D. Shindell, C. Ammann, G. Faluvegi, and F. Ni (2009)
Science 326, 1256-1260
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Science. ISSN 0036-8075 (print), 1095-9203 (online)