Ice dynamics in the Bothnian Bay inferred from ADCP measurements

A bottom mounted ADCP has monitored the ice motion and thickness in Bothnian Bay, Baltic Sea during the entire winter season 2004. The ADCP was deployed at 20 m depth at Falkensgrund well outside the land fast ice zone. The data shows that the ice motion is primarily driven by the wind but with a clear influence of internal ice stresses. The ice stresses become more dominant as the ice grow thicker with increasing number of observations with nearly stationary ice for relatively high wind speeds. A clear dependence of the ice/wind speed ratio to wind shifts is detected with higher ratio in the new wind direction. The effect of strain hardening is also seen in several events as decreasing ice speed, sometimes to zero, in spite of constant wind speed and wind direction. A rough force balance computation gives a compressive ice strength of about 9 × 10⁴ Nm⁻², which is much larger than normally used in numerical ice models. The ice thickness data show numerous ice ridges with ice draft well above 1 m passing the instrument. The ridges make up a large portion, 30–50%, of the total ice volume showing that dynamical processes are important for the total ice production in the Bothnian Bay.