The ERA-40 and ERA-Interim data used in this study have been obtained from the ECMWF data server. This research was supported by funding provided by the Climate and Large-Scale Dynamics Program of the U.S. National Science Foundation. We greatly appreciate the constructive criticism, as well as the many helpful comments and suggestions provided by the reviewers.
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Note: one of these winters is the period January–February 1979, which is included because a major warming occurred in late winter.
The average duration for nonevent winters is only slightly reduced compared with all winters, by less than 1 day over the range of heat flux thresholds considered.
Because of the relatively small number of degrees of freedom for both the minor and the major warming ensembles, the difference between the minor and major warming curves is only statistically significant at the 90% confidence level for a range of heat flux thresholds around the climatological mean value.
The ramping is a sinusoidal increase (decrease) from 0 m (the final amplitude) to the starting amplitude (0 m) over the 50 days of ramping.