Baltic Sea - Sea Ice

Sea Ice

On the long-term average, the Baltic Sea is ice-covered for about 45% of its surface area at the maximum annually. The ice-covered area during such a typical winter includes the Gulf of Bothnia, the Gulf of Finland, Gulf of Riga, Väinameri in the Estonian archipelago, the Stockholm archipelago and the Archipelago Sea. The remainder of the Baltic itself does not freeze during a normal winter, with the exception of sheltered bays and shallow lagoons such as the Curonian Lagoon. The ice reaches its maximum extent in February or March; typical ice thickness in the northernmost areas in the Bothnian Bay, the northern basin of the Gulf of Bothnia, is about 70 cm (28 in) for landfast sea ice. The thickness decreases farther south.

Freezing begins in the northern extremities of Gulf of Bothnia typically in middle of November, reaching the open waters of Bothnian Bay in early January. The Bothnian Sea, the basin south Kvarken, freezes on average in late February. The Gulf of Finland and the Gulf of Riga freeze typically in late January. In 2011, the Gulf of Finland was completely frozen on 15 February.

The ice extent depends on whether the winter is mild, moderate or severe. Severe winters can lead to ice formation around southern Sweden and even in the Danish straits. According to William Derham, during the severe winters of 1703 and 1708 the ice cover permeated as far as the Danish straits, parts of the Gulf of Bothnia and Gulf of Finland, in addition to coastal fringes in more southerly locations such as the Gulf of Riga. This description would actually have meant that the whole of the Baltic Sea had been covered with ice.

It is known that since 1720, the Baltic Sea has frozen over entirely only 20 times. The most recent case was in early 1987, which was the most severe winter in Scandinavia since that date. The ice then covered 400 000 km2. During the winter of 2010/11, which was quite severe compared to those of the last decades, the maximum ice cover was 315 000 km2, which was reached on 25 February 2011. The ice then extended from the north down to the northern tip of Gotland, with small ice free areas on its either side, and the east coast of the Baltic Sea was covered by an ice sheet ca. 25–100 km (16–62 mi) wide all the way down to Gdańsk. This was brought about by a stagnant high-pressure area that lingered over central and northern Scandinavia from ca. 10 to 24 February. After this, strong southern winds pushed the ice further into the north, and much of the waters north of Gotland were again free of ice, which had then packed against the shores of southern Finland. The effects of the afore-mentioned high-pressure area did not reach the southern parts of the Baltic Sea, and thus the entire sea did not freeze over. However, floating ice was additionally observed near Świnoujście harbour in January 2010.

In the recent years prior to 2011, Bothnian Bay and the Bothnian Sea were frozen with solid ice near the Baltic coast and dense floating ice far from it. In 2007 there was almost no ice formation except for a short period in March.

During winter, fast ice, which is attached to the shoreline, develops first, rendering the ports unusable without the services of icebreakers. Level ice, ice sludge, pancake ice or rafter ice form in the more open regions. The gleaming expanse of ice is similar to the Arctic, with wind-driven pack ice and ridges up to 15 m. Offshore of the landfast ice, the ice remains very dynamic all year, and it is relatively easily moved around by winds and therefore forms pack ice, made up of large piles and ridges pushed against the landfast ice and shores.

In spring, the Gulf of Finland and the Gulf of Bothnia normally thaw in late April, with some ice ridges persisting until May in the eastern extremities of the Gulf of Finland. In the northernmost reaches of the Bothnian Bay, ice usually stays until late May; by early June it is practically always gone.

The ice cover is the main habitat for two larger animal species. They are the grey seal (Halichoerus grypus) and the Baltic ringed seal (Pusa hispida botnica) that both feed underneath and breed on the ice. Of these two seals, only the Baltic ringed seal suffers when there is not adequate ice in the Baltic Sea, as it feeds its young only on ice. The grey seal is adapted to reproducing also with no ice in the sea. The sea ice also harbours several species of algae that live in the bottom and inside brine pockets in the ice.