Observations of short‐period ice floe accelerations during leg II of the Polarbjørn drift

During leg II of the Polarbjørn drift, we measured the orthogonal components of acceleration in the vertical and horizontal directions for the frequency interval 0.04–5 Hz, and the compass heading, at three sites adjacent to the ship. During days 310–329, the ice surrounding the ship underwent a transition from large floes with scales of 1–10 km, to small, broken floes with scales of 10–100 m. The deformation and fracture associated with this transition generated significant accelerations which were associated with ice shear, ridging, and ice floe collisions. For the shearing between two large floes, we observed stick‐slip accelerations with 0.4‐Hz frequencies and 3 mm s−2 amplitudes. For ridging, we observed two kinds of behavior, depending on floe size. For ridging of large kilometer‐scale floes, we observed 1‐Hz sinusoidal oscillations with 10 mm s−2 amplitudes, while for 100‐m‐scale floes, the spectra of the accelerations were white. When the field of 50‐ to 100‐m‐scale floes was divergent, ice collisions with amplitudes as large as 150 mm s−2 dominated; for two of the three sites these collisions excited the resonant bobbing and rocking frequency of the floe. Our observations ended on day 329, when the ice converged, the floes were apparently crushed, and the accelerometers ceased operation.