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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  pressure ­
  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  slowmoving
  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  Baltic ­
  Sea,  westerly  and  northwesterly  wind  directions ­
  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  accumulated ­
  at  the  coast  began  flowing  back,  causing ­
  water  levels  to  fall  slowly.  As  the  wind  continued ­
  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