Severe ice season in 2010/2011

Following a normal October, sea surface temperatures in the areas surrounding Finland were in line with long-term averages. At the beginning of November, the seawater actually warmed up a bit, but then began to cool down again after the first week of the month. A thin layer of ice formed in Kemi inner harbour and the innermost bays of Replot in mid-November. At that point sea surface temperatures were equivalent to long term averages.

The end of November was unusually, even exceptionally, cold. The cold period that began in November continued in December and the amount of sea ice began to increase. As mid-December approached, there was ice in all Finnish coastal areas. The thick snow cover in southern Finland prevented the ice from thickening and the ice was dangerously thin under the snow. Immediately after mid-December, strong winds "cleaned out" the sea areas and reduced the ice-covered area. Freezing temperatures during the Christmas week caused the ice-covered area to expand rapidly again. December was exceptionally cold in the sea areas surrounding Finland.

Frosty weather continued at the beginning of the new year. The ice-covered area expanded to more than 165 000 km² on January 4. At that time, ice covered the entire Bay of Bothnia and the Quark as well as the Archipelago Sea. The thickness of the fast ice varied from about half a metre in the northern parts of the Bay of Bothnia to ten centimetres in the southern coastal areas.

After this, the frosty weather began to ease its grip and the southerly winds compressed the ice fields. On January 11, the ice-covered area had decreased to less than 100 000 km². The coldest period in January alternated with milder periods after this. The ice-covered area expanded and contracted according to the cold weather and strong winds. The ice-covered area was 150 000 km² on the last day of January. At this stage of the winter the ice situation corresponded quite closely to the average.

February was clearly divided into two parts in terms of temperature conditions. The weather at the beginning of the month was mild and windy, which caused the ice in the Gulf of Finland to drift into the sea areas east of the island of Gogland. While traffic to harbours in southern Finland continued with very little need for icebreaker assistance, Russian traffic had to cope with large problems – at one point more than 100 vessels were waiting for icebreaker assistance.

The weather got colder in the middle of February, and the latter half of the month turned out to be exceptionally cold in the majority of the country. This cold period also caused the amount of sea ice to increase rapidly, and the peak of the ice winter was reached on February 25, when ice covered an area of 309 000 km² This event occurred at nearly the average time. At that point, both the Gulf of Bothnia and the Gulf of Finland were completely covered by ice. The Gulf of Riga and the northern Baltic Sea were also completely ice-covered. The ice edge ran from Öland to Gotland and from the northern tip of Gotland south to Rozewie on the coast of Poland. In the southern Baltic Sea, there was new ice off the coasts of Poland and the former East Germany, as well as in Oresund and on the Swedish west coast.

At the end of February, the winds began to blow from the south and became stronger. Pressure in the ice fields was detected in all Finnish sea areas, and at times it reached dangerously intense levels. When the situation was at its worst, dozens of commercial vessels were waiting for icebreaker assistance on both sides of the Quark and ships were assisted one at a time.

The situation eased temporarily at the beginning of March as the winds calmed. However, this did not last long, as south-westerly winds strengthened on March 3 and the ice began to drift east. Pressure developed once again in the ice fields, causing severe compression and a rapid decrease in the size of the ice-covered area.

On March 10, the extent of the ice-covered area was 165 000 km². The winds blew from the Swedish side for another week and the ice in the Gulf of Bothnia was pushed against the Finnish coast. This caused great difficulties for winter navigation, as icebreakers had to assist the vessels one by one. Russian traffic in the eastern Gulf of Finland experienced so much difficulty that a nuclear-powered icebreaker had to be called in from the Arctic Ocean.

In mid-March, the winds calmed briefly and the frosty weather caused new ice to form in open places and cracks in the ice fields. Signs of spring appeared at the end of March and the ice-covered area decreased steadily. Even gentle winds caused pressure in the heavily ridged ice fields.

The average temperature in April was unusually high in nearly all parts of the country. Rainfall at the beginning of April darkened the ice and the coastal ice began to rot. Very warm weather at Easter (21–25 April) melted the ice on the southern coast. The coastal areas of the Gulf of Finland were almost completely free of ice, and the ice that remained was dangerously rotten. In contrast, ice in the pelagic areas of the eastern parts of the Gulf of Finland continued to disturb Russian sea traffic. The coastal ice in the Sea of Bothnia north of Uusikaupunki rotted, but farther off the shore a heavily ridged 20-30 kilometre wide ice field interfered with shipping traffic. The ice winter also continued in the Bay of Bothnia although the coastal ice slowly began to darken and subsequently rot. The ice in the pelagic areas was still strong.

The beginning of May was warm in many places, causing the ice to rot and melt. On May 14, the Gulf of Finland was ice-free and the last pieces of ice in the Sea of Bothnia also melted at the end of the following week. In mid-May, the ice in the Bay of Bothnia was mainly on the Finnish side of the fishery zone limit. Although the ice in the archipelago areas had rotted and, in many areas, also melted, ice in the pelagic area still interfered with shipping in many places. The winter finally ended and summer began when the Bay of Bothnia was ice-free on May 25.

The time of the final melting of the ice varied greatly in comparison to average values. In the coastal waters of the northern Bay of Bothnia, the date of the final disappearance of ice took place from a few days earlier to about one week later than average. Melting in the Sea of Bothnia took place from approximately one week to nearly three weeks later than on average, and in the Gulf of Finland from average times to some two weeks later than normal.

The ice disappeared more than one week later than average in the northern Baltic Sea; however, ice formation also began nearly one month earlier than usual.

The duration of the ice winter in the northern Bay of Bothnia was nearly two weeks shorter than average. The ice winter in the southern Bay of Bothnia and Vaasa Archipelago was from one and a half weeks to nearly a month longer than average. The ice winter in the Sea of Bothnia, northern Baltic Sea and Gulf of Finland was from two weeks (in the capital region) to more than six weeks (in the Rauma region) longer than average.

The maximum thickness of the fast ice was 40–80 cm in the Bay of Bothnia, 35–75 cm in the Sea of Bothnia, 25–55 cm in the Archipelago Sea and 25–65 cm in the Gulf of Finland. The thickness of the pelagic ice was 35–85 cm in the Bay of Bothnia, 20–50 cm in the Sea of Bothnia, 25–60 cm in the Gulf of Finland, 10–25 cm in the Sea of Åland and 5–30 cm in the northern Baltic Sea.

Jouni Vainio and Ilkka Matero