Ice conditions in the Baltic Sea

The large north-south extent of Swedish waters leads to large temperature differences, and therefore also large differences in ice conditions.
Ice cover is to be expected during a year with normal temperature conditions, if nothing exceptional happens.

However it is not only the average temperature that affects the ice: changes in wind and temperature can make ice more or less severe even if temperatures are normal.

There can also be variations within and between different sea areas, so that ice cover could be considered mild in the Bay of Bothnia but harsh in the Skagerrak. Ice cover for each winter is classified based on the normal conditions for that particular stretch of water.

Effect of the wind

As well as air temperature, wind also affects the sea ice extent. The wind may disperse or pack the ice at sea and creates special conditions that are very significant to marine traffic, such as ice ridges and packed ice along the coast and harbour entrances.

Constant north-easterly or easterly winds, which generally bring low temperatures in winter time, drive existing (and developing) sea ice in towards the Swedish east coast where ridges form that can be hard to break through.

During these conditions the ice situation in the Bothnian Sea is more severe along the Swedish coast than the Finnish coast. Long, wide leads can appear on the Finnish side. However if the wind is westerly or south westerly the ice situation eases along the Swedish coast but becomes more compact further east.

South-westerly winds cause any ice on the west coast to form packed slush in the harbour entrances. This is especially common during the ice breakup period. In the Skagerrak the ice from the Kattegat is also driven north with the currents leading to more difficult ice conditions there.

Normal and extreme conditions

A normal ice winter needs continuous periods of high pressure weather during November and December in the Gulf of Bothnia – giving gentle winds and temperatures under –10 °C.

The ice edge advances southwards during the new year, and the maximum ice cover is often reached some time between the beginning and middle of March. By this stage, the ice covers the whole of the Bothnian Bay, most of the Bothnian Sea, the Bay of Finland and the Åland Sea, as well as the northern Baltic. Thin ice can occur along the coast as far south as the Kalmar Sound.

Over the last few years there have been a couple of very mild winters with a late start to the ice cover. However the winter of 1986/87 was a particularly harsh and icy winter.

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