8 
Negative Surges in the Southern Baltic Sea 
the above period, with a lower frequency in the 
last two decades than in the first decades. Under 
extreme conditions, water levels may drop more 
than 1.5 m in 12 hours. Water levels are corre 
lated with local wind patterns, and the best wind 
correlation for the GDR coast was found at 
Arkona. Based on such correlations, it has been 
possible to develop different statistical prediction 
models. A very simple model is wind surge 
curves, with water levels shown as a function of 
wind speeds and directions. For example, with 
southwesterly winds of 20 m/s at Arkona, a water 
level 1 m below normal has to be expected at 
Warnemünde. However, such simple point corre 
lation inadequately reflects the complicated nat 
ural processes taking place. Mewes (1987) 
distinguished among three different cylone paths 
causing negative surges on the coast of the 
GDR: firstly, cyclones which form in the western 
North Atlantic Ocean and track across the North 
Sea, secondly, cyclones forming roughly south of 
Iceland and tracking northeast and, thirdly, 
cyclones forming off Iceland which track along 
Scandinavia toward the Nordic Seas but whose 
centre does not cross the Baltic Sea. After the 
reunification of Germany, Baerens et al. (1995) 
studied the frequency of negative surges on the 
German Baltic Sea coast between Flensburg and 
Warnemünde without including the eastern part 
of the Baltic between Warnemünde and the 
national border of Poland. In contrast to the de 
clining number of negative surges on the coast of 
Mecklenburg-Vorpommern, their frequency in the 
period from 1900-1990 has actually increased on 
the coast of Schleswig-Holstein. This contrasting 
development appears to be linked to the direc 
tion of the coast, but its exact cause is still 
unknown. In both areas, negative storm surges 
are most frequent in the time from November 
through January. 
Water level fluctuations on the coast of Poland 
and in the eastern part of the German coast are 
thought to be closely linked, but a cross-border 
analysis of negative surges has not been carried 
out so far. Therefore, the development of nega 
tive surges in the different coastal sections and 
the exact causes of differences in the occurrence 
of negative surges in different parts of the coast 
still are not fully understood. 
1.3. Geographic and hydrodynamic 
background 
The region dealt with in this monograph is the 
south coast of the western and central parts of 
the southern Baltic Sea. The western part of the 
Baltic is shaped like a bay opening to the east, 
whose width increases from 25 nm at Wismar to 
120 nm at Kotobrzeg (Fig. 1.3. a). The western 
most part of the southern Baltic coast between 
Wismar on the Mecklenburg Bay and Cape 
Arkona on the island of Rügen extends roughly in 
a southwest to northeastward direction. This part 
of the coast, covered by the gauges at Wismar 
and Warnemünde, has a highly variable topogra 
phy with shallow water and a multitude of creeks, 
shoals, and sandbanks. 
The central section of the coast extending from 
the high chalk cliffs of Cape Arkona to 
Swinoujscie and the Odra estuary (gauges at 
Sassnitz and Swinoujscie) is oriented from north 
west to southeast and also has intricate topo 
graphic features comprising small sandy coastal 
islets, narrows, and sandbanks. In the adjacent 
Pomeranian Bay, also the seabed is highly varia 
ble, with shallow depths below 10 m prevailing. 
A particularly wide belt of shoals exists off the 
island of Uznam (Usedom), in the waters close to 
the Swinoujscie gauge, and around the island of 
Wolin. 
The rather straight eastern part of the coast 
between Wolin and Kotobrzeg runs in a west- 
southwest to east-northeast direction. Also the 
10-m-isobath, which is only one nautical mile off 
the coast, runs in a rather straight line. 
Considering the subdivision of the coast into 
three parts differing in their topography, the prob 
ability of extreme water levels occurring at Sass 
nitz would be expected to be on the same order 
as at Swinoujscie. The same holds for the 
gauges at Wismar and Warnemünde. However, 
because of the bay effect, the probability of 
occurrence of extreme sea level events decre 
ases from west to east, as is clearly shown in 
chapter 3.3. One of the main causes of this phe 
nomenon is the size of the area of open water 
relative to the coast length and the widening of 
the bay. 
The main factor influencing coastal sea levels in 
the Baltic Sea region is wind, which either 
pushes the water away from the coast or towards 
it, whereas tides are of minor importance. 
Another factor of lesser importance is the chang-