Sea ice thickness distribution and ice keel characteristics in the Bothnian Bay based on high resolution ADCP measurements

Abstract. The northern part of the Baltic Sea, specifically the Bothnian Bay, is covered by drifting pack ice each winter. This requires icebreaker assistance for commercial shipping and presents challenges for planned offshore structures, such as wind farms. During the 2023/2024 ice season, a high-resolution dataset of ice draft was collected in the Bothnian Bay using an Acoustic Doppler Current Profiler (ADCP) located at 20 m depth in the central region of the bay. The integrated ice motion resulted in that draft data was gathered along a 737 km track. The ice draft analysis revealed a significant spatial frequency of deformed ice in March and April, with 60% of the ice having a draft greater than 1 m, 4 % exceeding 4 m, and 0.1 % surpassing 11 m. Additionally, the volume of deformed ice with a draft greater than 1 m accounted for over 90 % during March and April, indicating that dynamic ice production largely outweighed thermodynamic growth. Among the 5500 individually identified ice keels, 20 % had a keel depth larger than 5 m and 2 % a depth larger than 10 m. There was also an indication that one keel reached all the way down to the instrument at 20 m depth. Comparison with satellite data showed a qualitative agreement between the spatial variability in reflectance, represented by the image gradient, and the measured ice draft gradient. It is difficult to verify if the 2023/2024 ice season was extreme or normal with respect to the frequency and thickness of ice keels but here are indications that the relatively early freezup in late December provided preconditions for above average deformation and ridge formation.