THE LIFE STORY OF A 1ST-YEAR SEA-ICE RIDGE

Field data are described and analyzed from all-winter monitoring of the structure and temperature of one sea ice ridge in the northern Baltic Sea in the winter of 1991. The ridge aged to 3.5 months and experienced substantial structural evolution: the consolidated layer grew to 1 m, average porosity decreased from 0.28 to 0.18, keel thickness decreased by 1 m, and the ridge geometry became smoother. The porosity decreased due to freezing and showed a persistent minimum of 0.20-0.23 in the midkeel region; the void distribution changed due to packing rearrangements of ice blocks. Ice volume changed due to thermodynamic growth and decay. Within the sail and consolidated layer the heat flow was mainly vertical varying with time according to the surface forcing; the corresponding total ice production estimated from the temperature data would be 0.14 m, a bit more than the measured ice production 0.10 m. Predictions of the consolidated layer growth, based on (a) ice surface temperature or (b) air temperature or (c) local undeformed ice growth, gave good results. In spring the ice blocks throughout the keel beneath the consolidated layer melted uniformly.