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JOURNAL OF CLIMATE 
Volume 27 
Fig. 5. (a) Pointwise correlations between daily surges in Cuxhaven and gridded daily SLP 
fields from the 20CRv2 (1950-2010). (b) 95th percentile time series of daily surges in Cuxhaven 
from 1843 to 2012 in comparison to the seasonal NAO index. A 10-yr moving average is also 
shown by the thick line, (c) As in (b), but in comparison to the seasonal NSCI index [SLP 
anomalies Madrid minus SLP anomalies Stockholm, shown as gray squares in (a) together with 
the stations used for the NAO: i.e„ Reykjavik and Gibraltar], (d) 30-yr moving correlations 
between 95th percentile time series of daily surges in Cuxhaven and the seasonal NAO/NSCI 
index. The SEs of the correlation coefficients (blue and red shaded areas) have been computed 
using a bootstrap method with 500 simulations (Efron and Tibshirani 1993). All time series 
from (b) to (d) are computed for the storm surge season from October to March. 
representative for North Sea storminess, we now turn 
our attention to the long-term comparison with re 
analysis wind fields. For that purpose, we use the sta 
tistical relationship described in section 2 to test whether 
the significant trends in reanalysis wind fields, which were 
recently detected by Donat et al. (2011b), are consistent 
with the storm surge record. The evaluation of the re 
constructed surge record (i.e., based on 20CRv2 wind and 
pressure time series from the ensemble mean) confirms a 
stationary and high predictive skill back until approximately 
1910, which is indicated by the efficiency criteria shown 
in Fig. 6. Before that time, the efficiency criteria point to 
a decreasing predictive skill: that is, the R 2 decreases and 
the RMSE increases significantly. While during the cali 
bration period from 1951 to 2010 over 83% of the ob 
served variability can be explained by wind and pressure 
effects, before 1910 the performance gradually decreases 
to values between 70% and 80% with an absolute mini 
mum in 1871 of 60%. Also for the RMSE the absolute 
minimum occurs in the early decades (RMSE = 20.64 cm