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.
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
