The ice season of 2011/2012 was average but very short
In statistics based on the largest ice extent, the ice season of 2011/2012 was average, but in other terms it was rather undemanding. The winter was shorter than average, as it started exceptionally late, and the last pieces of ice disappeared earlier than average. Because of heavy snow falls on the ice, the ice thicknesses did not reach average figures.
In early November, the surface temperatures of sea water surrounding Finland were between six and ten degrees. The water temperatures were over a degree warmer than the long-term average. In November, the average air temperature in the sea areas was around four degrees higher than average, and in the Bay of Bothnia, up to five degrees higher. As the average air temperatures mainly ranged around seven degrees, this did nothing to cool the sea water.
As November turned into December, there was no ice yet in the Finnish sea areas, and the sea surface temperatures in all sea areas were from one to three degrees higher than the long-term averages.
December was exceptionally mild in the entire country. In the Finnish sea areas, the average air temperature was from over three to nearly seven degrees higher than the reference period temperatures. At the turn of the year, sea surface temperatures in Finnish sea areas remained from one to two degrees higher than average, and ice was only found in the far reaches of the Bay of Bothnia and sheltered bays in the Quark archipelago.
As regards temperature conditions, January was clearly divided into two different periods: the early part was milder than usual, while the latter half of the month was distinctly colder than normal. In mid-January, a cold period of a few days occurred, but the weather really took a turn for the colder in the beginning of the last week of the month. The publication of daily ice reports began on 9 January, or nearly six weeks later than average and later than ever before.
In early January, ice was only found in the northernmost sheltered bays of the Bay of Bothnia, and the extent of the ice cover was less than 4 000 km². Ice formation in the Gulf of Bothnia started from three to six weeks later than average. The first freezing in the Gulf of Finland occurred from two to four weeks later than average.
As the weather continued mild, the extent of the ice cover only grew slowly. In mid-January, the ice-covered area was only 22 000 km². As the weather took a turn for the colder, the area covered by ice rapidly started expanding. As February came in, the extent of the ice cover was 80 000 km², and the cold weather continued.
Wintery weather had set in in late January, and the cold conditions continued in February. In the Finnish sea areas, February was from less than a degree to two degrees colder than average. This cold spell also caused the sea ice cover to expand, and the ice winter reached its peak on 11 February with the ice extending over an area of 179 000 km². This took place two weeks earlier than average1. At that time, there was ice in the Bay of Bothnia and the Quark. In the south-western part of the Bay of Bothnia there was a large area of open water, while ice was found in the coastal areas of the Sea of Bothnia. The Archipelago Sea was completely ice-covered, and in the Gulf of Finland, the western margin of the ice field could be traced from Bengtskär to Osmussaar. The Gulf of Riga was throughout covered by thin ice, and the coastal areas of the Baltic Sea Proper basin were ice-covered. Thin new ice was also found in Kattegat and the Danish straits.
After this, the winds veered south-westerly, and the ice-covered area started declining as ice fields were compressed by the wind. As a result, vessel traffic had to be stopped at times, as the ice was being strongly compressed by the wind. In early March, ice only occurred in an area of approximately 106 000 km². In March, the ice was further compressed, and the area covered by ice was reduced. More severe sub-zero temperatures occurred from time to time, causing new ice to form in open sea areas. As winds continued to blow, this thin ice was compacted at the edge of the close ice field as brash ice barriers that slowed down shipping.
Towards the end of March, ice on the southern and western coasts had started to rot. The area covered by ice had declined to 50 000 km², and spring was on its way. Regardless of this, the ice winter continued harsh both in the eastern end of the Gulf of Finland, east of Gogland, and in the north-eastern reaches of the Bay of Bothnia – the ice fields were hard and heavily ridged.
In April, the weather on land was colder than usual. In sea areas, April was slightly warmer than average, excluding the Bay of Bothnia, where the weather was somewhat colder than average. During April the sea ice was drifting with the winds, and the spring caused the ice to rot and thaw out. By mid-April, the ice cover had reduced to an area of 30 000 km². In the Gulf of Finland, ice was only found east of Hamina, and in the Bay of Bothnia, mainly north of Pietarsaari; there was also ice in the Quark archipelago.
The spring continued cool, slowing down the melting of the ice. In early May, ice only remained in the area of the Bay of Vyborg and, in the Bay of Bothnia, north of Raahe. The Gulf of Finland became free of ice during the first weekend in May, as the last of the ice melted around the mouth of the Bay of Vyborg.The coastal ice in the Bay of Bothnia was shattered, and the ice that drifted to the open sea melted quickly. The ice in the Bay of Bothnia melted and the final ice report and chart of the winter were published on 15 May. Some fragments of ice remained for a few days in the most sheltered areas between islands, but the winter finally ended and summer began when the Bay of Bothnia was ice free on 19 May.
The publishing of ice reports ended some week and a half earlier than average. Due to its late start, the bygone winter was the shortest ever registered – only 127 ice reports were issued, while the average number is 179.
The time of the final melting of the ice varied in comparison to the statistical average. In the coastal areas of the Bay of Bothnia, the final disappearance of ice occurred a few days earlier than average. In the Sea of Bothnia, it melted slightly later than average. In the Gulf of Finland, the time of its disappearance varied from two weeks earlier than normal to almost the normal time.
The duration of the ice winter in the Bay of Bothnia was from over four weeks to nearly six weeks shorter than average. In the Sea of Bothnia and the Gulf of Finland, the ice winter was from nearly two weeks to three weeks shorter than average. In the northern Baltic, at Utö, there was no ice last winter. The maximum thickness of the fast ice in Finnish territorial waters was 50–60 cm in the Bay of Bothnia, 40–50 cm in the Sea of Bothnia, 35–35 cm in the Archipelago Sea and 25–60 cm in the Gulf of Finland. The pelagic ice thickness varied between 20–70 cm in the Bay of Bothnia, 5–30 in the Sea of Bothnia and 20–45 cm in the Gulf of Finland.
Jouni Vainio and Ilkka Matero