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JOURNAL OF CLIMATE 
Volume 27 
1880 1900 1920 1940 1960 1980 2000 
Time [yr] 
Fig. 7. (a) Box plot of residual percentile trends (20CRv2 gen 
erated surges minus observation) from all ensemble members over 
the period from 1871 to 2011. The gray shaded areas represent the 
maximal range of SEs (95 % confidence level): that is, residual trends 
are significant for all ensemble members and percentiles, (b) 30-yr 
moving trends for the 99th percentile residuals (20CRv2 generated 
surges minus obseivation). All 56 ensemble members are shown in 
orange. The ensemble mean together with the related SE's is given 
by the red line and the red shaded area, respectively, (c) As in (b), 
but for the 99.9th percentile residuals. 
observations and 20CRv2 found by Krueger et al. 
(2013a,b) over the North Atlantic region prior the 1940s. 
In contrast to this earlier study, the inconsistencies with 
surge levels at Cuxhaven are less pronounced and be 
come clearly visible only for the period before the 1910s. 
It should be noted that the surges itself differ to the 
conventional storminess proxies insofar that they mea 
sure both changes in wind speed and direction. How 
ever, there are three reasons why this does not affect our 
main conclusion that the surge record is representative 
for storminess in the region: 
1) The rank correlation (Spearman) between wind 
speeds and absolute surges has a value of 0.53, which 
is significantly different statistically from zero (see 
Fig. S2 of the supplementary material). Since during 
periods of low wind speeds the influence of bathy 
metric effects on the surge generation increases 
relative to the winds, the correlation between the 
extreme events, which are analyzed here, increases 
noticeably. For example, the correlation between the 
annual 95th percentile time series of both factors 
increases to a value of 0.64 (not shown). 
2) Dangendorf et al. (2013c) investigated changes in 
the frequency of different wind directions between 
January and March from 1871 to 2008 and could not 
find any evidence for significant changes during the 
period of interest. 
3) The fact that Krueger et al. (2013b) previously 
detected similar differences in conventional proxies 
(although for a slightly different region and with 
larger differences than detected here) and Donat 
et al. (2011b) pointed to significant trends in the wind 
speed further supports the reliability of the surge 
record as a measure of storminess. 
Additionally, one could argue that the use of a single 
grid point time series rather than the entire wind field 
over the North Sea as input for the empirical wind surge 
model could bias the results. When looking at Fig. 3 in 
Donat et al. (2011b), it can clearly be seen that the linear 
trends are evident over the entire North Sea area. In 
fact, strongest trends in wind speeds have been found for 
the grid points not used as input data in the present 
study. Therefore, using other additional grid points as 
input parameters for the wind surge formulas would lead 
to even larger inconsistencies between 20CRv2 and 
surge observations. 
On this basis, we conclude that the significant trend 
detected by Donat et al. (2011b) was less a result of the 
large decadal trends in storminess in the last decades but 
rather reflects lower occurrence of extreme values in the 
early decades of the reanalysis. The latter are supported 
by neither pressure-based storm indices (Krueger et al. 
2013a,b) nor the surge observations at Cuxhaven since 
1843. 
4. Discussion and conclusions 
We have established a new storm surge record for the 
tide gauge of Cuxhaven, extending conventional records 
back to 1843. This could be achieved by using a generally 
known but in the last decades less considered method 
(Horn 1948, 1960) that enables us to decompose mea 
surements of tidal high and low water levels into tides, 
MSL, and (skew) surges. The method can open a world 
wide available but in terms of surges unappreciated data