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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 6 | Copyright
Clim. Past, 11, 939-958, 2015
https://doi.org/10.5194/cp-11-939-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 Jun 2015

Research article | 24 Jun 2015

Reconciling reconstructed and simulated features of the winter Pacific/North American pattern in the early 19th century

D. Zanchettin1,2, O. Bothe3, F. Lehner4,a, P. Ortega5, C. C. Raible4, and D. Swingedouw6 D. Zanchettin et al.
  • 1Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany
  • 2Department of Environmental Sciences, Informatics and Statistics, University of Venice, Calle Larga Santa Marta, Dorsoduro 2137, 30123 Venice, Italy
  • 3HZG, Helmholtz Center Geesthacht, Max Planck Strasse 1, 21502 Geesthacht, Germany
  • 4Climate and Environmental Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
  • 5LOCEAN-IPSL/UPMC, 4, place Jussieu, Paris, France
  • 6UMR CNRS 5805 EPOC – OASU – Université de Bordeaux, Allee Geoffroy St Hilaire, 33615 Pessac CEDEX, France
  • acurrently at: National Center for Atmospheric Research, Boulder, USA

Abstract. Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows for assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early 19th-century positive PNA anomaly emerges as a potentially reliable feature, although the pseudo-reconstructions are subject to a number of sources of uncertainty and deficiencies highlighted especially at multidecadal and centennial timescales. The pseudo-reconstructions demonstrate that the early 19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early 19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, which is unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early 19th-century external forcing and background climate state.

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A discrepancy exists between reconstructed and simulated Pacific North American pattern (PNA) features during the early 19th century. Pseudo-reconstructions demonstrate that the available PNA reconstruction is potentially skillful but also potentially affected by a number of sources of uncertainty and deficiencies especially at multidecadal and centennial timescales. Simulations and reconstructions can be reconciled by attributing the reconstructed PNA features to internal variability.
A discrepancy exists between reconstructed and simulated Pacific North American pattern (PNA)...
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