Meteorological factors contributing to negative surges 
25 
4. Meteorological factors 
contributing to negative 
surges 
The dominant factor forcing water surface oscil 
lations in the Baltic Sea is strong wind. As a rule, 
offshore wind leading to falling coastal water lev 
els is less severe over land than over the sea and 
may be more or less deflected directionally, 
depending on the shape of the coastline. However, 
by choosing a suitable coastal location for the 
wind measuring station, deflections of recorded 
wind directions can be kept to a minimum. Strong 
offshore winds recorded by the coastal stations 
are normally associated with offshore storms 
accompanying fast-moving low pressure systems 
tracking across the Baltic Sea, which affect large 
sea areas. A relatively rare cause of low water 
levels, nevertheless worth mentioning, is long- 
lasting gale-force winds connected with an anti 
cyclone over Scandinavia and Eastern and Cen 
tral Europe. In a high-pressure system of this 
type, the prevailing E-SE and S winds influence 
large parts of the Baltic basin. 
All of these different wind systems, though trans 
formed and very much influenced on their track, 
develop in accordance with the prevailing pres 
sure pattern over Europe and the adjacent Atlan 
tic Ocean. 
4.1. Strong offshore wind on the 
coast 
Strong offshore wind is far less severe on the 
coast than over the sea. Coastal wind measure 
ments made during negative surges have con 
firmed that, in the large majority of cases (about 
90 %), the accompanying wind on the southern 
Baltic Sea coast came from offshore directions, 
i.e. ESE-WSW. Alongshore and random direc 
tions during periods of low water usually occur 
red immediately after the wind had veered, when 
water levels, though rising, had not yet reached 
the threshold value. 
The hydrological situation was studied at selec 
ted gauge stations in order to determine the 
distribution of wind directions. Wind directions 
were used whenever sea levels at the gauge sta 
tions had dropped to <440 cm, and one wind 
measurement made just before the water level 
dropped to the threshold value was used as well. 
Therefore, the number of wind records differs at 
the individual stations. 
The predominant wind sectors during negative 
surge events were SW-S. The frequency of these 
directions was 75-77 % in the western and cen 
tral parts of the Baltic coast (e. g. Warnemunde 
and Swinoujscie), and 62 % in the eastern part 
(Kotobrzeg). The frequency of westerly wind direc 
tions was 12-13 % in the western and central 
parts, and just below 6 % in the eastern part of 
the Baltic Sea. The percentage of southeasterly 
wind directions at the western coast scarcely 
reached 10 % but was slightly above 17 % in 
Kotobrzeg. In addition the share of east-south- 
easterly winds at Kotobrzeg was about 8%. In 
the other parts of the coast, this direction 
occurred only sporadically during approaching 
storms. Winds from the large W-NW (300°) and 
eastern (90°) sectors were extremely rare. As has 
been pointed out above, these wind directions 
were only observed occasionally while water lev 
els still were below the defined threshold value 
but, after having dropped to the minimum value, 
had already begun to rise. (Fig. 4.1 .a. to 4.1 .c). 
Fig. 4.1. Frequency distribution of wind directions at sea levels s440 cm, 1955-2005