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.