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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 continuous
curves were registered on tidal charts and only
the peaks were handwritten in logbooks. Digital recordings
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 decomposed
into an external and a local/regional component;
both caused by meteorological disturbances over the
northeast Atlantic and the entire North Sea basin, respectively
(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 illustrate
the genesis of such a storm event. Fig. 1 shows the
meteorological and oceanographic situation in the region
during January 2007. In this month. Northern Europe
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 blowing
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 situation
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 eliminated
from the observational data first (Pugh 2004). A
common way of separating tides from surges is to apply
a harmonic analysis to the raw data. However, this
method has two general restrictions:
1) First, for a harmonic analysis at least hourly observations
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 readings
of tidal high and low water levels (and times).
2) Second, for tide gauges located in shallow continental
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 deviations
of mean heights and intervals. This has also
been best practice in the operational service of the
German Maritime and Hydrographic Agency for decades
(Miiller-Navarra 2013). Additionally, as also mentioned
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 subtracting
the astronomical tides from the original signal.
One should note that there is a difference between surges
generated with hourly observations and tidal peaks.
