Pancake ice formation in the Weddell Sea

The ice formation resulting from two low temperature events at the Weddell Sea ice edge during April 2000 is presented. Pancake and frazil ice were sampled at seven stations at varying distances from the ice edge. The ice cover was further characterized from above, using helicopter aerial photography, and from below, using a remotely operated vehicle. Previously undescribed two-layer pancake types were observed and classified. A novel pancake growth mechanism is introduced to account for these, involving the washing of frazil ice over the pancake top surface and its subsequent freezing. The process was directly observed in ice tank experiments. Layer thicknesses seen in the field were compared to the ice growth that would occur both under calm conditions and from free-surface frazil ice growth. Classical, bottom accretion, pancake growth was found to proceed at a rate similar to that of thin congelation ice. Top-layer growth was more rapid, at approximately double the congelation rate. Overall ice volume production was similar to congelation ice for the thin pancakes considered (similar to20 cm), though subsequent thickening was expected to be faster as the rapid top-layer process continued and the equivalent congelation growth slowed. It is suggested that parameterization of this new process is important for models that aim to simulate the rapid advance and thickening of wave-influenced ice covers.