Meteorol.  Z.  (Contrib.  Atm.  Sci.)
PrePub  Article.  2020

S.  Schwegmann  &  J.  Holfort:  Baltic  sea  ice  volume  1982-2019

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winter  ASIC  variability

winter  ASIV  variability

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(j

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40-20-




Central  8ay  of  Bothnia
Central  Gulf  of  Finland

1985  1990  1995  2000  2005  2010  2015

Figure  2:  Temporal  variability  of  total  winter  ASIC  and  ASIC,  exemplarily  for  the  central  Bay  of  Bothnia  (2:
(representing  near  coastal  ice,  21°  E/62.5°  N)  and  the  central  Gulf  of  Finland  (26°E/60°N).

be  quite  high,  depending  on  the  winter  strength,  and
differs  regionally,  as  Fig.  2  illustrates.  Shown  are  the
SIC  and  SIV  accumulated  over  the  entire  winter  for  each
winter  season,  exemplarily  for  a  grid  cell  in  the  central
Bay  of  Bothnia,  central  Gulf  of  Finland  and  at  Kaskinen,
which  represents  a  grid  cell  close  to  the  coast  in  the
Sea  of  Bothnia.  As  is  expected,  ASIC  and  ASIV  are  the
highest  in  the  central  Bay  of  Bothnia,  as  in  this  northern
most  region,  ice  growth  stalls  early  and  ice  disappears
only  in  the  end  of  the  winter.  The  other  two  positions
show  varying  patterns:  in  some  years,  Kaskinen,  which
is  further  north  than  the  Gulf  of  Finland,  has  less  ice,  in
other  years,  more  ice  can  be  found  compared  to  the  Gulf
of  Finland.  This  reflects  the  impact  of  the  underlying
atmospheric  patterns  on  ice  growth,  as  in  some  years,
cold  air  is  advected  from  the  eastern  continent  (earlier
ice  growth  in  the  Gulf  of  Finland)  and  in  other  years
from  the  Polar  Regions  (earlier  ice  growth  in  the  Gulf  of
Bothnia).
2.2  Sea  surface  and  air  temperatures
Sea  surface  temperature  (SST)  data  was  obtained  from
the  Copernicus  Marine  Environment  Monitoring  Service ­
  Baltic  Sea-  Sea  Surface  Temperature  Reprocessed
data  set  (H0yer  and  Karagali,  2016).  These  data  arc
available  for  the  period  January  1982  to  December  2011.
SST  calculations  arc  based  on  infrared  satellite  observations ­
  from  NOAA  AVHRR  and  ERS/Envisat.  Spatial
resolution  is  with  0.03°  x  0.03°  higher  than  in  other  SST
products,  temporal  resolution  is  24  hours.  In  order  to  account ­
  for  sea  ice  in  winter  months,  which  has  a  much
lower  temperature  than  the  water,  a  sea  ice  concentration
mask  based  on  the  high  resolution  ice  information  from
the  Swedish  Meteorological  and  Hydrological  Institute
(SMHI)  has  been  used  to  set  grid  cells  with  more  than
30  percent  of  sea  ice  to  a  constant  SST  of  -1  °C.  For
more  detailed  information  on  the  SST  data  set,  please
see  H0yer  and  Karagali  (2016).  As  we  have  analyzed
monthly  accumulated  SIC  and  SIV  and  their  changes  in
this  study,  we  calculated  monthly  fields  from  the  daily
SST  before  trends  were  calculated.

&

N),  Kaskinen

For  an  analysis  of  the  atmospheric  drivers  of  sea  ice
changes  we  have  examined  monthly  mean  2-m  air  temperatures ­
  from  the  National  Center  for  Environmental
Predictions/National  Center  for  Atmospheric  Research
(NCEP/NCAR,  hereinafter  referred  to  as  NCEP  data)
Reanalysis  1  Project  (Kalnay  etal.,  1996).  The  respective ­
  mean  fields  in  the  NCEP  data  arc  composites  of
observations  -  when  and  where  available  -  and  model
simulations  (where  no  observations  arc  available).These
data  have  a  coarser  resolution  (2.5°  x  2.5°)  than  the  sea
ice  and  SST  data.  However,  the  temporal  resolution  covers ­
  the  entire  study  period  in  contrast  to  other  products
with  higher  spatial  resolution,  like  ECMWF  ERA-40
(only  to  2002)  or  COSMO-REA  (only  from  1995).
3  Observed  changes  in  sea  ice  coverage
and  accumulated  sea  ice  volume
The  trends  in  winter  seasons  ASIC  and  ASIV  for  the
Baltic  Sea  over  the  whole  period  arc  shown  in  Fig.  3,  the
30-year  period  from  1982  to  2011  shows  similar  trends.
In  most  regions,  the  trend  is  negative.  Trends  in  ASIC
vary  between  -33  %  m  2  dec  1  to  +41  %  m  2  dec  1 ,
with  the  majority  of  grid  cells  showing  a  decrease  between ­
  0  %  m  2  dec -1  and  18  %m  2  dec -1 .  The  strongest
decrease  is  observed  in  the  Gulfs  of  Bothnia,  Finland
and  Riga.  In  the  central  Baltic  Sea,  trends  arc  lower,  certainly ­
  due  to  the  fact  that  the  absolute  values  arc  smaller
to  staid  with.  Along  the  coasts,  there  arc  regionally  some
grid  cells  which  show  an  increase  in  ASIC,  which  is
caused  by  a  very  low  data  coverage  in  the  beginning  of
the  observation  period.  Some  positive  trends  arc  most
probably  due  to  the  fact,  that  with  newer  satellite  data
new  ice  is  better  detected;  and  with  100  %  concentration ­
  have  a  strong  impact  on  the  trend.  Due  to  the  small
thickness  of  new  ice,  the  effect  is  not  very  pronounced
on  the  sea  ice  volume.
ASIV  shows  in  most  regions  a  decrease  and  the
highest  trends  in  the  same  regions  as  ASIC.  Mostly,
the  trends  vary  between  -22.5  m  and  11.5  m  per  decade