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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 2 | Copyright
Clim. Past, 14, 157-174, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Feb 2018

Research article | 09 Feb 2018

Insights into Atlantic multidecadal variability using the Last Millennium Reanalysis framework

Hansi K. A. Singh1, Gregory J. Hakim2, Robert Tardif2, Julien Emile-Geay3, and David C. Noone4 Hansi K. A. Singh et al.
  • 1Pacific Northwest National Laboratory, US Department of Energy, Richland, WA, USA
  • 2Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
  • 3Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA
  • 4College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, USA

Abstract. The Last Millennium Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually resolved proxy data over years 0–2000CE. We use the LMR to examine Atlantic multidecadal variability (AMV) over the last 2 millennia and find several robust thermodynamic features associated with a positive Atlantic Multidecadal Oscillation (AMO) index that reveal a dynamically consistent pattern of variability: the Atlantic and most continents warm; sea ice thins over the Arctic and retreats over the Greenland, Iceland, and Norwegian seas; and equatorial precipitation shifts northward. The latter is consistent with anomalous southward energy transport mediated by the atmosphere. Net downward shortwave radiation increases at both the top of the atmosphere and the surface, indicating a decrease in planetary albedo, likely due to a decrease in low clouds. Heat is absorbed by the climate system and the oceans warm. Wavelet analysis of the AMO time series shows a reddening of the frequency spectrum on the 50- to 100-year timescale, but no evidence of a distinct multidecadal or centennial spectral peak. This latter result is insensitive to both the choice of prior model and the calibration dataset used in the data assimilation algorithm, suggesting that the lack of a distinct multidecadal spectral peak is a robust result.

Publications Copernicus
Short summary
The Atlantic Multidecadal Oscillation (AMO) is prominent in the climate system. We study the AMO over the last 2000 years using a novel proxy framework, the Last Millennium Reanalysis. We find that the AMO is linked to continental warming, Arctic sea ice retreat, and an Atlantic precipitation shift. Low clouds decrease globally. We find no distinct multidecadal spectral peak in the AMO over the last 2 millennia, suggesting that human activities may have enhanced the AMO in the modern era.
The Atlantic Multidecadal Oscillation (AMO) is prominent in the climate system. We study the...