A model-tested North Atlantic Oscillation reconstruction for the past millennium

Abstract

The North Atlantic Oscillation (NAO) is the major source of variability in winter atmospheric circulation in the Northern Hemisphere, with large impacts on temperature, precipitation and storm tracks1, and therefore also on strategic sectors such as insurance2, renewable energy production3, crop yields4 and water management5. Recent developments in dynamical methods offer promise to improve seasonal NAO predictions6, but assessing potential predictability on multi-annual timescales requires documentation of past low-frequency variability in the NAO. A recent bi-proxy NAO reconstruction7 spanning the past millennium suggested that long-lasting positive NAO conditions were established during medieval times, explaining the particularly warm conditions in Europe during this period; however, these conclusions are debated. Here, we present a yearly NAO reconstruction for the past millennium, based on an initial selection of 48 annually resolved proxy records distributed around the Atlantic Ocean and built through an ensemble of multivariate regressions. We validate the approach in six past-millennium climate simulations, and show that our reconstruction outperforms the bi‐proxy index. The final reconstruction shows no persistent positive NAO during the medieval period, but suggests that positive phases were dominant during the thirteenth and fourteenth centuries. The reconstruction also reveals that a positive NAO emerges two years after strong volcanic eruptions, consistent with results obtained from models and satellite observations for the Mt Pinatubo eruption in the Philippines8,9.

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Figure 1: Proxy selection.
Figure 2: Ensemble NAO reconstructions.
Figure 3: Validation of the ensemble NAO reconstructions.
Figure 4: NAO response to large volcanic eruptions.

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Acknowledgements

The work of P.O., D.S., V.M.-D. and M.C. has been financially supported by the French ANR CEPS project Green Greenland (ANR-10-CEPL-0008). F.L. and C.R. acknowledge the support of the NCCR-Climate, funded by the Swiss National Science Foundation (grant number 200020 147174). P.Y. acknowledges funding from the Swedish Research Council (grant number C0629701). We thank the paleoclimate community, in particular the PAGES2k Network, and all the institutions involved in the CMIP5/PMIP3 initiative for making available proxy data and model outputs. We also thank F. González-Rouco and M. Montoya for discussions at an earlier stage of the analysis; M. Khodri for feedback on the volcanic reconstructions; and E. Hawkins for help and advice on the statistical analysis.

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Authors

Contributions

P.O. gathered and prepared the data with the help of M.C., and developed the ensemble reconstruction and part of the statistical analyses. F.L. prepared the pseudo-proxies from reanalyses and models and carried out the perfect-model study. D.S. analysed the impact of external forcings on the NAO. All authors helped in conceiving the study, discussing ideas and writing the paper.

Corresponding author

Correspondence to Pablo Ortega.

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The authors declare no competing financial interests.

Additional information

Data will be made available at http://www.ncdc.noaa.gov/data-access/paleoclimatology-data.

Extended data figures and tables

Extended Data Figure 1 Verification of the pseudo-reconstructions.

Box-and-whisker plots showing the correlations between the six pseudo-reconstructed NAOmc ensembles and the corresponding true simulated NAOs, in three independent subsets of years (from left to right for each ensemble): the 117 years selected for calibration (specific to each ensemble realization), the remaining 30 years that need to be validated (following the original strategy), and a long validation period from ad 1000–1822 (possible thanks to the perfect-model approach). In this latter case, since the period is the same for all the ensemble realizations, the correlation is also calculated for the ensemble mean (filled coloured dots). Red horizontal lines indicate the first significant correlation coefficient (P < 0.05) of the ensemble.

Extended Data Figure 2 Comparison of three alternative reconstructions.

The figure shows probability density functions (PDFs) for the 50-year moving-window correlations between the simulated NAO and three alternative pseudo-reconstructions in the ensemble of PMIP3 simulations (Extended Data Table 3): a, NAOTrouet; b, NAOmc; c, NAOLehner. Vertical dotted lines represent the median correlation for the ensemble. Each coloured line represents a different pseudo-reconstructed NAO ensemble.

Extended Data Figure 3 NAO–pseudo-proxy relationships.

af, Correlations between the simulated NAO and the associated pseudo-proxies in the PMIP3 runs. Pseudo-proxies are defined from a pure climatic signal (coloured asterisks), and subsequently perturbed with AR(1) noises (thin green lines). For the perturbed pseudo-proxy definitions, ten different realizations of the noise are considered. The correlations between the actual proxies and NAOVinther in the common period ad 1823–1969 are also shown for comparison (thick black crosses).

Extended Data Table 1 Proxy description
Extended Data Table 2 Description of the reanalyses.
Extended Data Table 3 Description of the simulations.
Extended Data Table 4 Principal-component (PC) contributions to the ensemble reconstructions.
Extended Data Table 5 Description of the NAO/sea-level pressure (SLP) records used for validation.
Extended Data Table 6 The 11 largest volcanic eruptions between ad 1049 and ad 1969 in three alternative reconstructions.
Extended Data Table 7 Eleven large volcanic eruptions common to the three alternative reconstructions.

Supplementary information

Supplementary Data

This file contains the main code used to produce the model-constrained NAO ensemble reconstruction. (PDF 88 kb)

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Ortega, P., Lehner, F., Swingedouw, D. et al. A model-tested North Atlantic Oscillation reconstruction for the past millennium. Nature 523, 71–74 (2015). https://doi.org/10.1038/nature14518

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