Seasonal and long term variation 
19 
In conclusion, there is no clear temporal trend in 
the recorded low sea level values, and low sea 
level values are independent of annual mean sea 
levels. There are years in which very low sea lev 
els were recorded which lasted only for a short 
time, and other years in which the annual mini 
mum was not particularly low but remained at 
that level much longer. Clear differences have 
been found in the distribution of low water levels 
along the coast: there is not only a linear trend 
with lower minimum values in the west than in 
the east, but there are also differences in the time 
series. 
Fig. 3.1. c shows the annual number of hours 
during which water levels were below 440 cm. As 
this is only a different representation of the data 
shown in Fig. 3.1. a, the above results apply. 
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 
year 
Fig. 3.1. c Long term variation of low sea level events s440 cm at the Wismar, 
Warnemünde, Sassnitz, Swinoujscie and Kotobrzeg gauge stations 
3.2. Seasonal distribution 
In the mean annual cycle, there is a pronounced 
difference among monthly distributions, as can be 
seen in the mean monthly sea level distribution at 
Warnemünde in Fig. 3.2. a or in the January and 
June distributions for all five stations in Fig. 3.2. b 
and c. The monthly mean and median values vary 
by 12-16 cm, the maximum is reached in the sum 
mer months, and the minimum is reached in win 
ter or spring. An even more important difference 
regarding extreme sea levels is found in the width 
of the monthly distributions. In summer, e.g. June, 
the distribution is quite narrow, with a standard 
deviation around 12 cm, and shows a pronounced 
maximum, with about 34 % of all values spread 
±5 cm about the mean. The lowest June value on 
record is 436 cm at Wismar. The winter distribu 
tion, e.g. January, is much wider, with a standard 
deviation around 28 cm and a less pronounced 
maximum, with only 16 % of all values spread 
±5 cm about the mean. The lowest value ever re 
corded, 335 cm in Wismar, is much lower than the 
June value, and this holds for all five stations. 
A clearer picture of the annual cycle of low sea 
levels is given in Fig. 3.2. d, which shows the 
seasonal distribution of sea levels below 440 cm 
for all five stations. The majority of negative surge 
events occurred in December and January. This 
is the stormiest season, with frequent strong 
southwesterly winds (180-270°) in the southern 
Baltic Sea pushing water away from the shore. 
This explains the high occurrence of water levels 
below 440 cm during the months from October 
to February. The largest total number of negative 
surge events was recorded at the Wismar water 
gauge, the only station at which such events 
have been recorded in all the months of the year. 
In Warnemünde, June was the only month in 
which water levels below 440 cm were never 
recorded; Sassnitz, Swinoujscie and Kotobrzeg