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JOURNAL OF CLIMATE 
Volume 27 
Woodworth et al. 2011). Here, we focus on a long sea 
level record observed at the tide gauge of Cuxhaven, 
which is located in the southeastern North Sea. Sea 
levels have been observed in Cuxhaven since 1843. Until 
1899, the measurements were taken as readings of high 
and low water levels 4 times per day. Since then con 
tinuous curves were registered on tidal charts and only 
the peaks were handwritten in logbooks. Digital re 
cordings are available since the mid-1990s. To get the 
full information about the tidal curves before that time, 
extensive digitization works at the German Maritime 
and Hydrographic Agency made hourly data available 
back to 1918. In the present study we combine both data 
types (i.e., tidal peaks and hourly observations) to get 
insights into the history of all available measurements. 
The datasets were carefully checked for outliers, datum 
shifts, missing values, and time drifts in earlier studies 
(Wahl et al. 2010, 2011). The overall quality was found 
to be good. For example, Dangendorf et al. (2013a) 
demonstrated a high coherence of sea levels between the 
Cuxhaven record and 12 additional records all located in 
the German Bight. 
Storm surges measured in the region can be decom 
posed into an external and a local/regional component; 
both caused by meteorological disturbances over the 
northeast Atlantic and the entire North Sea basin, re 
spectively (Miiller-Navarra and Giese 1999). Hence, it is 
reasonable to suggest that strong storms occurring over 
the northeast Atlantic or North Sea region will leave 
a fingerprint in the surge record of Cuxhaven. To illus 
trate the genesis of such a storm event. Fig. 1 shows the 
meteorological and oceanographic situation in the re 
gion during January 2007. In this month. Northern Eu 
rope was affected by a series of strong storms (Fink et al. 
2009), leading also to a series of strong storm surge 
events (Fig. 1). In total the long-term 95th percentile of 
daily surges was exceeded eight times in only 22 days. 
The genesis of storm surge events often starts with a 
low pressure system over the North Atlantic traveling 
westward to the larger Baltic and Scandinavian area. On 
their way such pressure systems may trigger waves in the 
deep ocean northeast of Scotland, which then propagate 
into the North Sea elevating the water levels in the 
German Bight approximately 15 h later (Rossiter 1958). 
Such external surges may increase a single high water 
event by up to 1 m (Bruss et al. 2010). However, the most 
important factor for the surge generation is related to 
strong local winds from northwesterly directions blow 
ing over the shallow shelf areas in the German Bight 
(depths <40 m; along the coastlines <10 m) occurring 
when the low pressure systems travel farther eastward 
into the Scandinavian/Baltic area. These winds cause 
an effect of water pile up with surges of up to more than 
four meters. In January 2007, the meteorological situa 
tion was characterized by a strong pressure gradient with 
pressure anomalies 16hPa below the long-term mean 
over Scandinavia and exceeding it by 9hPa west of the 
Iberian Peninsula (Fink et al. 2009). These conditions 
lead to the generation of a series of particular large surge 
events in the German Bight (Fig. 1). 
To study the long-term behavior of such storm 
events—our first aim in the present study—short-term 
periodic and long-term MSL changes have to be elimi 
nated from the observational data first (Pugh 2004). A 
common way of separating tides from surges is to ap 
ply a harmonic analysis to the raw data. However, this 
method has two general restrictions: 
1) First, for a harmonic analysis at least hourly obser 
vations are needed. This restriction often hampers 
the evaluation of long tide gauge measurements back 
into the eighteenth and nineteenth centuries, since 
before 1900 most observations are limited to read 
ings of tidal high and low water levels (and times). 
2) Second, for tide gauges located in shallow continen 
tal shelf seas (e.g., Cuxhaven), nonlinear shallow 
water effects often bias the harmonic representation 
of tides, leaving unwanted periodic constituents in 
values of nontidal residuals (surges) (Pawlowicz et al. 
2002 ). 
To overcome these restrictions, Horn (1948, 1960) 
developed a more sophisticated technique that allows 
accurate predictions of the astronomical tides just on 
the basis of tidal peaks. The approach is based on 
a harmonic representation of inequalities (for detailed 
information of different computation steps, see Miiller- 
Navarra 2013), usually resolving the decomposition of 
tides and surges more accurately in very shallow seas 
than the “traditional" harmonic analysis (Pansch 1989). 
The German Bight has distinct semidiurnal tides and 
under this assumption it is favorable to represent tidal 
high and low water heights and times as harmonic de 
viations of mean heights and intervals. This has also 
been best practice in the operational service of the 
German Maritime and Hydrographic Agency for de 
cades (Miiller-Navarra 2013). Additionally, as also men 
tioned above for the Cuxhaven record, digitized hourly 
observations are limited to the period from 1918 onward. 
Horn's method, however, allows us to extend the tidal 
analysis back to 1843 without being dependent on hourly 
data only available after 1918. 
After applying the tidal analysis to the raw data of 
observational peaks, we generate a surge record by sub 
tracting the astronomical tides from the original signal. 
One should note that there is a difference between surges 
generated with hourly observations and tidal peaks.