In our Letter, a conceptual error in the cross-validation approach led to an overestimation of the predictive skill of the winter (December–February) North Atlantic Oscillation (NAO) and Euro-Atlantic surface climate from Arctic sea-ice variability. The error does not affect the hindcasts based on the snow advance index.
Specifically, to produce the one-year-out cross-validated hindcasts based on Arctic sea-ice variability, we performed a maximum covariance analysis (MCA) between Arctic sea-ice concentration (SIC) anomalies and winter Euro-Atlantic sea-level pressure anomalies in the period 1979/80 to 2012/13. We then applied one-year-out cross-validation using subsets of years from the SIC time series derived from the whole period. Thereby, the regression coefficients (that is, slope and intercept) and predictor value of the statistical model were estimated assuming the knowledge of the MCA fields in the year out. This procedure overestimates the cross-validation skill.
One-year-out cross-validated hindcasts instead require cross-validation of the MCA pattern-generation in the year out, thus performing an MCA on the remaining years. Following this approach, the cross-validated skill in hindcasting the winter NAO index using September SIC over the whole Arctic is 0.08, indicating that there is no predictive skill from Arctic sea-ice variability. The cross-validated NAO skill using October or November SIC over the whole Arctic is 0.22 and 0.18, respectively, suggesting some skill.
Although our analysis reveals no skill in sea-ice-based NAO predictions with three months lead time, the limited skill from October–November sea-ice concentration supports the notion that sea-ice information should be incorporated in dynamical prediction systems to improve their skill at forecasting the surface winter climate in Europe.
Following the identification of the error in our cross-validation approach, this Letter has been retracted. We are grateful to Geert Jan van Oldenborgh (KNMI, De Bilt, The Netherlands) for identifying this error. We also thank Francisco J. Doblas-Reyes (IC3, Barcelona, Spain) for discussions.
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The online version of the original article can be found at 10.1038/ngeo2118
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García-Serrano, J., Frankignoul, C. Retraction Note: High predictability of the winter Euro–Atlantic climate from cryospheric variability. Nature Geosci 7, E2 (2014). https://doi.org/10.1038/ngeo2164
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DOI: https://doi.org/10.1038/ngeo2164
