Distribution, morphology and structure of sea ice pressure ridges in the Baltic Sea

Abstract

The first ridges in winter were observed to build in the northeastern part of the Bothnian Bay in November. The probability of occurrence of ridges was the highest in March which varied yearly up to 90-100% in the Bothnian Bay and around 10-20% in the northern parts of the Baltic Proper. The last ridges were found in the central part of the Bothnian Bay in May. Ridges increased level ice thickness on average by 6-14 cm equivalent ice thickness in the Bothnian Bay, and 2-4 cm in the Bothnian Sea and in the Gulf of Finland. The maximum average equivalent thickness was found in northeastern Bothnian Sea, which was 80 cm in March. During severe winters the amount of ridges was from 5 to 10 times bigger than in an average winter. Ridges were commonly found in as south as the latitude of Gotland and Gulf of Riga during severe winters.

An ideal conclusive model of pressure ridges was made according to morphological field measurements carried out in 73 ridge sails (mean height 0.8 m) and 14 entire ridges (mean height 1.3 m and depth 8.4 m). The ice properties, salinity, density and brine content in ridges were measured and microscale structure of the samples were studied.

The sail height and keel depth correlated. The mean ratio of keel depth to sail height was 6.3 varying between 2.6–10. The keel width could not be determined according to the sail dimensions. Mean ratio of keel to sail width was 12.5.

The mean slope angle of the sails higher than 0.5 m was 21°. The mean keel slope angle was 29° and the mean slope angle of their sails was 26°. Accumulated snow next to ridge sails smoothened the surface profile 4°. The sail height did not correlate with the steepness of its slopes. The average total porosity of the ridges could be kept as a constant value of 30%. The sail porosity was smaller (20%) than the keel porosity (30%). The frozen nucleus inside the ridge was averagely 0.3 times thicker than level ice around the ridge, although the deviation was large. It was mainly formed from rafted ice blocks which had frozen together.

The ice ridges were composed of ice blocks which were 20 cm thick on average. The mean ratio of length to longer the longest axis was. The ice block width and length correlated and their ratio was 1.6. The length followed lognormal distribution, while the thickness fitted best to Erlang distribution. The thicker the deformed ice sheet became, the higher the ridge sails were. There was no correlation between the keel size and the ice block thickness, which was measured from the ridge sails. The ice block in the ridge sails were orientated so that their mean dip was 39°. There was no correlation between the ice block sizes and their dips.

The driving force was estimated to be in the order of the magnitude of the forces to create a potential energy and overcome fiction in the ridges, what seems to be a limiting factor for the ridge size.

The shape of the pressure ridges in the Baltic Sea did not differ from the ridges in the Arctic Seas, except that they were generally smaller and had formed of thinner ice. However, the ratio of the keel depth to the sail height was bigger in the Baltic Sea. The correlation between the ice block dimensions and sail size were very similar in the Baltic and Arctic.