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Is the sea ice coming to the Baltic Sea
early this winter ? 

Posted 10 October 2010


The period and extent of sea icing in Northern Europe may give interesting climatic change indications, and more important which influence may human activities have had. The last winter 2009/2010 was the coldest since 1987, put the sea ice cover remained far behind (see: Fig.). Did the high number of 2000 sizable merchant ships navigating the Baltic Sea prevented to forming of sea ice? The winters succeeding the last winter severely had been four very cold winters in the 1940s. While the ice cover was about 50%, in 1987 it reached almost 90% and in the first three World War II years and 1946/47 the Baltic Sea was covered completely.

A fully ice cover happened in the winter 1939/40, the first time since 1883. To the people it came out of the blue. No one had predicted a unusual winter, no one had imaged an extreme winter. Such assertion for the winter to come has now been made by Polish scientists: They that the Atlantic Gulf Current lost strength, and reduce the usual heat supply for a warmer Europe considerably. While little can be done about that, recalling a bit how the ice developed in the extraordinary winter 1939/40 is also some sort of preparation. As source we use the monthly weather reports by the Swedish Meteorological Service[1]


Throughout the autumn 1939 high pressure prevailed all over Scandinavia, and a widely persistent wind flow from the north. The icing of seas and rivers started in the northern und middle part of Norrland, in the north of the NW of Svealand in the mid of October,  about 2 weeks earlier then usually. The first ice reports from the ports Toe and Lulea came in on the 2nd of November. It remained modest throughout the month, and only since the 27th of November sea ice appeared in other ports, including Hasparand and Karlsborg. On the 30th November the Soviet Union ambushed Finland, by land, sea and air, which increased the activities at sea in the Gulf of Finland tremendously, while any naval activities in the Gulf of Bothnia remained negligible. 

During a few cold periods until 21st December the ice extended south and reached Kalmarsund by the end of the month. Further details by a brief extraction from a book[2]:

___”In Hanko/Finland (at the west entrance to the Gulf of Finland), icing started on December 27, 1939; solid ice formed on January 4, 1940; end of ice came on May 7, 1940.; at almost the same time as Helsinki. However, on January 15, 1940 the Gulf of Finland was still open as far East as the median of Pellinki. Ice then formed rapidly[3]. Just to remind. Although the Gulf of Bothnia is far in the North it is with over 200 metres – in the Baltic Sea area -the deepest water, holding considerable heat for considerable time even in cold winters. An ‘ice-bridge’ between Turku and the island of Åland formed on January 6/7, 1940, which is about 2 ½ weeks earlier than usual.”

Attention should be drawn to tow interesting distinct deviation from the average. 

  1. The early ice forming inland , coastal areas, and Åland islands, which was presumably due to the generally low sea surface temperatures (and inflow of cold air from the Arctic) due to naval activities throughout Northern Europe, and
  2. The long lasting open sea in the Gulf of Finland, which indicates that due to military activities a high mixing of water took place preventing the formation of ice. 

The sea ice closed around Gotland in early February, and it become the first winter since 1883 that covered the whole Baltic Sea with ice about two weeks later. If the coming winter will be again severe, it will be interesting to compare the developments with those which happened 71 years ago.  

Further reading:
Book section 2_17: Baltic Sea paved way for extreme winter

Short texts: 


Figure 1 & 4: The BALTIC SEA PORTAL is maintained by the Finnish Environment Institute, the Finnish Meteorological Institute and the Ministry of Environment in Finland

[1] Statens Meteorologisk Hydrografiska Anstalt, Argång 21 & 23.  

[2] At page 92 of “Climate Change & Naval War” 2006: