Meteorological  factors  contributing  to  negative  surges

25

4.  Meteorological  factors
contributing  to  negative
surges
The  dominant  factor  forcing  water  surface  oscillations ­
  in  the  Baltic  Sea  is  strong  wind.  As  a  rule,
offshore  wind  leading  to  falling  coastal  water  levels ­
  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  longlasting
  gale-force  winds  connected  with  an  anticyclone ­
  over  Scandinavia  and  Eastern  and  Central ­
  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  transformed ­
  and  very  much  influenced  on  their  track,
develop  in  accordance  with  the  prevailing  pressure ­
  pattern  over  Europe  and  the  adjacent  Atlantic ­
  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  measurements ­
  made  during  negative  surges  have  confirmed ­
  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  directions ­
  during  periods  of  low  water  usually  occurred ­
  immediately  after  the  wind  had  veered,  when
water  levels,  though  rising,  had  not  yet  reached
the  threshold  value.
The  hydrological  situation  was  studied  at  selected ­
  gauge  stations  in  order  to  determine  the
distribution  of  wind  directions.  Wind  directions
were  used  whenever  sea  levels  at  the  gauge  stations ­
  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  central ­
  parts  of  the  Baltic  coast  (e.  g.  Warnemunde
and  Swinoujscie),  and  62  %  in  the  eastern  part
(Kotobrzeg).  The  frequency  of  westerly  wind  directions ­
  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-southeasterly
  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  levels ­
  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