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Persistent positive North Atlantic oscillation mode dominated the Medieval Climate Anomaly
Valérie Trouet1, Jan Esper, Nicholas E Graham
1Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Zürcherstrasse 111, 8903 Birmensdorf, Switzerland. trouet@wsl.ch
Science (New York, N.Y.)
|April 4, 2009
Summary
The Medieval Climate Anomaly (MCA) featured a persistent positive North Atlantic Oscillation (NAO). This climate shift, linked to La Niña conditions, contrasts with the subsequent Little Ice Age (LIA).
Area of Science:
- Paleoclimatology
- Climate Dynamics
- Oceanography
Background:
- The Medieval Climate Anomaly (MCA) was a significant pre-industrial warm period in Europe.
- The precise drivers of the MCA and its transition into the Little Ice Age (LIA) remain incompletely understood.
Purpose of the Study:
- To reconstruct the North Atlantic Oscillation (NAO) over a 947-year period.
- To investigate the relationship between NAO dynamics and the MCA/LIA climate transition.
- To explore global climate shifts potentially linked to North Atlantic circulation.
Main Methods:
- Development of a 947-year multidecadal NAO reconstruction.
- Analysis of climate model outputs and proxy data for comparative reconstructions.
- Synthesis of globally distributed proxy data to identify large-scale climate patterns.
Main Results:
- A persistent positive phase of the NAO was identified during the MCA.
- A distinct shift towards weaker NAO conditions was observed leading into the LIA.
- Global proxy data indicate a broader climate transition from MCA to LIA, potentially influenced by La Niña-like conditions.
Conclusions:
- The MCA was characterized by a strongly positive NAO, suggesting a specific atmospheric circulation pattern.
- The transition to the LIA involved a weakening of the NAO, indicating significant climate variability.
- Intensified Atlantic meridional overturning circulation during the MCA may have amplified global climate shifts, including La Niña-like conditions.
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