Ice season 2012/2013 was average
The Baltic Sea ice season 2012/2013 was average but the turning point of the winter was late. The peak of the ice winter was reached on March 15, when ice covered an area of 177 000 km².
Ice formation in the innermost bays of the northern part of the Bay of Bothnia began with the end of November. At the end of the month there was 1–5 cm thick new ice in the inner archipelago of the Bay of Bothnia. When November turned to December, the weather cooled off and ice formation increased. December was colder than usual. The extent of ice increased steadily and reached 69 000 km² at 26th December. After this, the weather became milder.
The weather began to cool off again at the end of the first week of January and the extent of ice increased. At 25th January the ice cover had extended to 152 000 km². In the last week of January the strong winds moved the ice fields and the mild weather melted ice. The extent of ice was by the end of January only 84 000 km².
In the beginning of February the weather continued similar – at night new ice was formed which the winds then broke during the day. The extent of ice did not really enlarge. For the first three weeks of February the situation remained similar. Towards the end of February the weather cooled off and new ice formed in the Gulf of Finland. The 26th February the extent of ice had expanded to 161 000 km².
In the beginning of March cold arctic air started to flow to Scandinavia and the extent of ice began to grow. Whole March was extremely cold. In the middle of March the extent of ice reached 177 000 km², which was the maximum of the ice winter. From then on the cold nights formed new ice but sunny days melted them and the extent of ice did not enlarge any more.
In April the ice started to melt fast because ice in the open sea areas had not grown very thick either in spite of the cold March. At the end of April the ice extent was only 28 000 km².
The last ice melted from the southern Bay of Bothnia on May 4th. From the Gulf of Finland the ice disappeared permanently May 6th. The Bay of Bothnia was ice-free on May 31st.
The duration of the ice winter in the Bay of Bothnia was a little shorter than average. In the Quark and the northern part of Sea of Bothnia the ice winter became 2–3 weeks shorter than average but four weeks longer than average in the southern part of Sea of Bothnia. In the Gulf of Finland the ice winter was one and a half to four weeks longer than average. At Utö in the northern Baltic Sea no permanent ice cover appeared during the last winter. Regarding its duration the ice winter was average but the turning point of the winter was experienced exceptionally late, at the end of March.
The maximum thickness of the fast ice was 60–75 cm in the Bay of Bothnia, 55 cm in the Sea of Bothnia, 45 cm in the Archipelago Sea and 40–60 cm in the Gulf of Finland. The thickness of the pelagic ice was 20–70 cm in the Bay of Bothnia, 10–40 cm in the Sea of Bothnia and 5–45 cm in the Gulf of Finland.
Jouni Vainio and Patrick Eriksson