26 
Negative Surges in the Southern Baltic Sea 
4.2. Offshore storm surges in 
the wind field of a moving 
depression 
The Norwegian and North Seas, Scandinavia, and 
the Baltic Sea lie in a region of predominantly 
westerly winds that is crossed regularly by east 
ward tracking disturbances, mostly active 
depressions with frontal systems, whose origin is 
the Atlantic Ocean. Before a low-pressure system 
reaches the Baltic Sea area, winds mostly have a 
strong southerly component, but they normally 
veer after the front has passed. On the southern 
North and Baltic Sea coasts, approaching depres 
sions are preceded by offshore winds. Low-pres 
sure systems are a common occurrence in this 
area. 
The smaller depressions, often low-pressure 
troughs with atmospheric fronts which are not 
too deep and move relatively fast, are accompa 
nied by winds which, though reaching gale force, 
do not affect the sea surface long enough to 
have a major impact on coastal water levels. 
By contrast, well developed low-pressure troughs 
and their frontal systems moving across the coast 
are accompanied by gale-force backing winds as 
the fronts approach, and by veering winds after 
they have passed. This situation normally causes 
sea level oscillations. Water levels first fall mark 
edly until a sharp minimum is reached, and rise 
again as the wind veers. Stormy wind in the rear 
of such fronts causes water levels to rise, some 
times far above the levels recorded at the begin 
ning of the surge. This situation is represented by 
the mareographic curves in Figs. 5.1. b, 5.6. b 
and 5.18. b. 
Some depressions tracking east begin to slow 
down as they enter Scandinavia, still deepening. 
The pressure gradient becomes very steep and 
the wind, initially gale-force, increases in severity 
and finally reaches hurricane force. Offshore wind 
at the southern Baltic coasts causes water levels to 
fall until the wind either calms or veers. A typical 
reaction of Baltic Sea levels to such wind forcing 
is gradually falling water levels in large parts of the 
coast, often followed by sharply dropping levels as 
the storm grows to maximum force and, finally, a 
long-lasting minimum (flat section in the mareo- 
graph curve) which lasts as long as the hurricane 
continues without changing direction. When the 
wind finally veers, water levels begin to rise more 
or less rapidly (often supported by alongshore or 
onshore winds). The mareographic curves in 
Figs. 5.5. b, 5.9. b, 5.12. b, 5.19. b show events 
associated with this atmospheric situation. 
4.3. High pressure system as low 
sea level generator 
Another type of low sea level variation can be 
observed when a strong, stationary anticyclone 
covers, or oscillates over, Fennoscandia and the 
northwestern parts of Russia. In such atmospheric 
conditions, two main factors contribute to falling 
water levels: one factor is the very high hydro 
static pressure in the powerful high-pressure sys 
tem, and the other one is the wind system devel 
oping at its southwestern edge. While light to 
moderate winds with a strong northerly compo 
nent prevail over the northernmost part of the Bal 
tic Sea, winds farther south veer E-SE and in 
crease in severity, reaching gale force in places. 
This is usually due to a steepening pressure gradi 
ent caused by a series of depressions tracking 
across the area from western Europe. In the west 
ernmost part of the Baltic Sea, the Sounds, and in 
the southeastern part of the North Sea, SE-S 
winds prevail because atmospheric pressure in 
this area is lower than over Scandinavia. Similar 
configurations of pressure systems persisting long 
enough in the area-a week, two weeks, or even 
more-force surface water not only away from the 
shores but out of the sea basin through the 
Sounds. In such meteorological situations, all 
coastal water gauges in the Baltic Sea record low 
water levels. Extreme negative surge events under 
the influence of an anticyclone are very rare, how 
ever. One extreme event was recorded prior to the 
period studied in this monograph, in February 
1954. Initially, a southeasterly, moderate to strong 
air flow persisted over central Europe and the Bal 
tic Sea for eight days, from 7 to 14 February. 
Then, for another eight days, from 15 to 22 Febru 
ary, the wind system of an anticyclone over Fen 
noscandia forced surface water away from the 
eastern and southern Baltic coasts. Water levels 
dropped to about 440 cm, or even lower, in the 
central and western parts of the southern Baltic 
coast before 23 February. On 23 February, a mini 
mum of 427 cm was recorded in Warnemunde 
and, one day later, 418 cm in Kotobrzeg. This is 
one of the rare cases of storm-induced negative 
surges where water levels in the eastern part of 
the coast dropped below those in the western 
part. Besides, there was a striking resemblance of 
the mareographic curves, which ran nearly parallel 
for a very long time under the impact of the long 
period of stormy weather (by contrast, during 
quickly passing storms, water levels in different 
parts of the same coast often move in opposite di 
rections, compare Fig. 1.3. b). As the strong off 
shore wind on the southern coast continued, wa 
ter levels continued to oscillate between 420 cm 
and 450-460 cm for several days (Fig. 4.3.).