The ice winter 2008/2009 was mild and shorter than average
October was warmer than normal, and the sea water temperature was at the end of October, in places, a degree above average. November also had milder than normal thermal conditions. Freezing in the northern part of the Gulf of Bothnia began shortly after mid-November, which was about three weeks later than normal.
In early December, there was ice only in the northern part of the inner archipelago in the Bay of Bothnia. Like autumn, December was clearly milder than average and there was hardly any increase in the amount of ice. At the end of the year, there was ice only in the northern part of the Gulf of Bothnia and in the inner archipelago of Kokkola and Vaasa. In the Gulf of Finland, there was only a little ice off Vyborg and St. Petersburg.
In early January, the weather got colder and ice formed in the northernmost parts of the Gulf of Bothnia and the innermost Gulf of Finland. Off Kemi and Oulu, the ice was 10-30 cm thick. At the bottom of the Gulf of Bothnia, the ice conditions were two to three weeks later than long-term averages. Equally, the surface temperatures of the water in the Finnish coastal areas were about one degree higher than normal.
There was a frost spell lasting about a week, after which the weather began to get milder. There was another cold spell in the middle of the month, followed again by milder weather. On the 30th of January, cold air from the north-east flowed over Finland. However, January was milder than normal. At the end of January, there was ice everywhere in the Bay of Bothnia, except for in the open sea areas in the central and southern parts of the bay. The Quark was covered with thin new ice, similarly to regions off the coast of the Sea of Bothnia. In the Gulf of Finland, there was ice in the inner archipelago. Off Helsinki the freezing began in the beginning of February, six week later than normal. In the eastern part of the open Gulf of Finland, new ice stretched from the east to island Mosthnyj. Seawater temperatures were still half to one degree higher than normal.
February turned out to be a regular winter month. In mid-January the weather got momentarily colder and the amount of ice increased. On February 20th, the ice-covered area expanded to 110,000 km2, which was the winter's maximum. This happened one week earlier than normal.
At the beginning of March, a high pressure zone stretched from the Baltic Sea over Finland to the Kola Peninsula. At that time, the weather was quite cold. The high pressure eventually gave way and was followed by an influx of milder air from the southwest. On March 21st, the winds weakened and turned to the northwest, whereby cold air spread over Finland and the amount of ice began to increase again. Considering the season, it was cold in Finland between the 23rd and 27th of March. At that time, the ice-covered area reached 86,000 km2. At the end of March, milder air began to spread from the south and the weather warmed up considerably.
Early April was warm for the time of year and the ice became rotten, melting quickly in the Gulf of Finland and the Sea of Bothnia. The Gulf of Finland and the Sea of Bothnia were ice-free on the 19th of April and the remaining ice in the Gulf of Finland, in the Gulf of Vyborg, melted on May 2nd.
The coastal areas of the Sea of Bothnia had open water on May 19th. At that time, there was still 20–50 cm thick drift ice in the open northern part of the Gulf of Bothnia in an area marked off by the line Ulkokalla – Kemi 1 – Malören – Farstugrunden – Falkensgrund – Ulkokalla. In the Bay of Bothnia, the rest of the ice melted away in late May and the Bay of Bothnia was ice-free on May 27th.
In the northern Bay of Bothnia the duration of the ice winter was three weeks shorter than average, in the southern Bay of Bothnia four weeks and in the Quark almost five weeks shorter than average. In the Sea of Bothnia and the Archipelago Sea the duration of the ice winter was about three weeks shorter and in the Gulf of Finland roughly six weeks shorter than average.
The maximum thickness of fast ice in the Finnish parts of the Bay of Bothnia was 50–70 cm, in the Sea of Bothnia 15–45 cm, in the Archipelago Sea 10–30 cm and in the Gulf of Finland 15–30 cm. The thickness of ice in the open sea of the Bay of Bothnia varied between 20 and 50 cm, in the Sea of Bothnia between 3 and 10 cm and in the Gulf of Finland between 10 and 25 cm.
Jouni Vainio and Patrick Eriksson