This research was supported by the Office of Science (BER) of the U.S. Department of Energy. The authors thank three anonymous reviewers for their useful comments that led to improvement of the manuscript. The Reanalysis-I and Reanalysis-II and the 20CR data were obtained from the NOAA ESRL/PSD Internet archive (www.esrl.noaa.gov/psd), and the two ECMWF reanalysis datasets were obtained from the ECMWF data portal (http://data.ecmwf.int/data).
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