Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly

摘要

The North Atlantic Oscillation (NAO)-the main synoptic mode of atmospheric circulation and climate variability in the North Atlantic/European sector-has a substantial influence on marine and terrestrial ecosystems and regional socio-economic activity (1). Because the instrumental record of the NAO is relatively short (2), there is a need for information on its long-term variability to better understand climate variability in the more distant past and to assess NAO predictability. An array of multiproxy approaches has been applied to extend the NAO record back in time (3); however, these only extend to 1400 A.D. and do not span the MCA (4), a period (～800-1300) marked by a wide range of changes in climate globally, including apparent relative warmth over the North Atlantic/European sector and much of the extra-tropical Northern Hemisphere (5). The MCA is the most recent natural counterpart to modern warmth and can therefore be used to test characteristic patterns of natural versus anthropogenic forcing.