Sea ice conditions in the Baltic Sea have been systematically monitored for more than 100 years. All sea ice-related parameters display large interannual variability, but a change towards milder ice winters has been observed over the past 100 years: in particular, the annual maximum ice extent has decreased and the length of the ice season has become shorter. There is no correlation between consecutive ice seasons because the thermal memory of the Baltic Sea is only 2-3 months. Interannual variability in sea ice conditions is principally driven by the large-scale atmospheric circulation, described by the North Atlantic Oscillation. In addition to a tendency towards milder winters, the occurrence of severe ice winters has also decreased considerably over the past 25 years.

Abstract. While variations of Baltic Sea ice extent and thickness have been extensively studied, there is little information about drift ice thickness, distribution, and its variability. In our study, we quantify the interannual variability of sea ice thickness in the Bay of Bothnia during the years 2003–2016. We use various different data sets: official ice charts, drilling data from the regular monitoring stations in the coastal fast ice zone, and helicopter and shipborne electromagnetic soundings. We analyze the different data sets and compare them to each other to characterize the interannual variability, to discuss the ratio of level and deformed ice, and to derive ice thickness distributions in the drift ice zone. In the fast ice zone the average ice thickness is 0.58±0.13 m. Deformed ice increases the variability of ice conditions in the drift ice zone, where the average ice thickness is 0.92±0.33 m. On average, the fraction of deformed ice is 50 % to 70 % of the total volume. In heavily ridged ice regions near the coast, mean ice thickness is approximately half a meter thicker than that of pure thermodynamically grown fast ice. Drift ice exhibits larger interannual variability than fast ice.

While variations of Baltic Sea ice extent and thickness have been extensively studied, there is little information about drift ice thickness, distribution, and its variability. In our study, we quantify the interannual variability of sea ice thickness in the Bay of Bothnia during the years 2003-2016. We use various different data sets: official ice charts, drilling data from the regular monitoring stations in the coastal fast ice zone, and helicopter and shipborne electromagnetic soundings. We analyze the different data sets and compare them to each other to characterize the interannual variability, to discuss the ratio of level and deformed ice, and to derive ice thickness distributions in the drift ice zone. In the fast ice zone the average ice thickness is 0.58 ± 0.13 m. Deformed ice increases the variability of ice conditions in the drift ice zone, where the average ice thickness is 0.92 ± 0.33 m. On average, the fraction of deformed ice is 50 % to 70 % of the total volume. In heavily ridged ice regions near the coast, mean ice thickness is approximately half a meter thicker than that of pure thermodynamically grown fast ice. Drift ice exhibits larger interannual variability than fast ice.Published by Copernicus Publications on behalf of the European Geosciences Union.

Abstract. While variations of Baltic Sea ice extent and fast ice thickness have been extensively studied, there is little information about the sea ice thickness distribution and its variability. In our study, we quantify the interannual variability of sea ice thickness in the Bay of Bothnia during the years 2003-2016. We use various different data sets: official ice charts, drilling data from the regular monitoring stations in the coastal fast ice zone and from helicopter-and ship-borne electromagnetic soundings. We analyze the different data sets and compare them to each other to characterize the interannual 10 variability, to discuss the ratio of level and deformed ice, and to derive ice thickness distributions in the drift ice zone. In the fast ice zone the average ice thickness is 0.58 +/-0.13 m. Deformed ice increases the variability of ice conditions in the drift ice zone where the average ice thickness is 0.92 +/-0.33 m. In heavily ridged ice regions near the coast, mean ice thickness can be even manyfold thicker than in a pure thermodynamically grown fast ice. Drift ice exhibits larger inter-annual variability than fast ice. 15