58 
Negative Surges in the Southern Baltic Sea 
5.18. December 2001 
Meteorological situation 
On 20 December, moderate northwesterly air 
flow prevailed over Scandinavia and the Baltic 
Sea. In the course of this day, a low pressure 
centre moved southeast from the Norwegian 
Sea, deepening and meandering as it crossed 
southern Norway and southern Sweden, where it 
arrived on 21 December at 00 UTC. It took 
another 24 hours for this slow-moving low pres 
sure centre to cover the distance from southern 
Sweden to the southern Gulf of Riga. On 
22 December, the centre backed east-northeast, 
continuing on its way across western Russia, 
where it filled. The deep low pressure trough and 
the frontal system that accompanied the slow- 
moving centre provoked a northerly storm on 
20 December which, in the afternoon and 
evening of that day, backed west to southwest 
over the western and southern parts of the Baltic 
Sea. As the southwesterly storm increased to 
7-9 Bft over the southwestern basins of the Bal 
tic Sea, westerly and northwesterly wind direc 
tions still prevailed in its northeastern part. It was 
not until the morning of 21 December that the 
wind gradually veered northwest in the whole area, 
beginning in the eastern basins of the Baltic Sea. 
Hydrological response of sea level 
In the afternoon of 20 December, the northerly 
storm backed west, now blowing alongshore and 
calming down slightly in the southwestern part of 
the Baltic Sea. The water masses that had accu 
mulated at the coast began flowing back, caus 
ing water levels to fall slowly. As the wind contin 
ued backing in offshore directions, water levels 
dropped at a faster rate, and the minima in this 
section of the coast were reached between 06 
and 12 UTC on 21 December, beginning with the 
eastern gauges. The minimum of 427 cm at 
Kotobrzeg was recorded between 05 and 
06 UTC, 393 cm at Swinoujscie around 06 UTC, 
407 cm at Sassnitz around 07 UTC, 375 cm at 
Warnemunde and 356 cm at Wismar, both 
between 08 and 09 UTC. 
The minima were rather flat because the wind did 
not increase and veer to onshore directions until 
around noon, when water levels began to rise 
again. 
Fig. 5.18. a Route of the depression centre from 06 UTC on 20 to 12 UTC on 22 December 2001, 
pressure pattern and wind field over the Baltic Sea on 21 December 2001, 00 UTC