Reconstruction  of  Annual  Maximum  Ice  Extent  in  the  Baltic  Sea  1660-2005

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Reconstruction  of  Annual  Maximum  Ice  Extent  in  the  Baltic  Sea
1660-2005
Lars  Axell  and  Katarina  Lindquist,
Swedish  Meteorological  and  Hydrological  Institute  (SMHI),  Sweden
E-mail:  Lars.Axell@smhi.se

Abstract
The  annual  maximum  ice  extent  A max  in  the  Baltic  Sea  is  an  important  indicator  of  regional
winter  climate  in  northern  Europe.  Apart  from  modern-day  operational  ice  charts,  proxy  data
such  as  coastal  observations,  mean  winter  air  temperature,  and  ice  break-up  dates  have  been
used  to  reconstruct  A max  back  to  1660.
A  consolidated  time  series  was  constructed  by  giving  the  proxy  time  series  different  weights.
These  weights  were  calculated  as  inversely  proportional  to  the  mean  square  errors  of  the  time
series  during  the  calibration  period  1961-1990.
The  consolidated  time  series  indicates  a  large  interannual  as  well  as  interdecadal  variability.  In
spite  of  the  recent  mild  winters  in  the  Baltic  Sea  region,  the  large  interannual  variabilitiy  of  A max
makes  the  calculated  trend  over  moving  30-year  windows  mostly  insignificant  in  a  statistical
sense.