A study of sea ice dynamic events in a small bay

In February 2002, a series of interesting sea ice dynamic events took place in Pärnu Bay, a very small basin of 15 km across in the Gulf of Riga, Baltic Sea. The opening of the bay faces SSW. The fast ice sheet was compressed, broken into small floes of typical size of no more than 20 m and moved together with the drift ice on 5 February driven by a strong SSW wind. The ice was then immobile for a week in a SW wind. After that, the wind turned northwestly with mild velocities and part of the ice cover drifted out of Pärnu Bay, so that about half of the basin opened while on the other side heavy rubble formation occurred. The thickness of the ice was from 20 to 40 cm on the coast to 5–15 cm towards the ice edge and the highest wind speed was about 20 ms- 1. For this paper, we performed a scale analysis to estimate the strength of the thin fast ice and applied a fine resolution ice model to investigate the sea ice dynamics. The compressive strength of the fast ice is found to be within 30 and 60 kPa, and the breakage of the thin fast ice fits into the drift ice theory. The standard strength parameter of the dynamic ice model, P⁎ = 30 kPa, is found to be a reasonable estimate of the compressive strength for the drift ice in these events. The severe fast ice breaking process produced drift ice composed of small blocks, mostly of no more than 20 m. Such drift ice keeps the standard compressive strength but seems to have noticeably less shear strength.