Growth and motion at the Weddell Sea ice edge

Abstract

The formation of sea ice in the presence of turbulence was studied using data from drifting buoy
deployments and ice sampling in the Weddell Sea during April 2000. The study sought to
improve understanding of pancake ice in terms of dynamics, heat fluxes, ice growth rates and
mechanisms.
Ice motion at high frequencies was examined using GPS buoy positions at a 20-minute
sampling interval. Relative motions of the buoy array were characterised by a marked
oscillation at the highest frequencies, with an RMS value two orders of magnitude higher than
previously seen in the Weddell Sea. This motion ceased overnight as the pancakes consolidated.
Wave forcing, either surface gravity or internal, was postulated as the cause. The oscillation was
found to significantly influence the proportions of pancake and frazil ice, though the nature of
the ice cover meant that ice production rates were unaffected, in contrast to the enhanced growth
this would imply for congelation ice. Momentum transfer parameters were found to be similar
to those found for the Greenland Sea Odden ice tongue.
Pancakes were found to be dominantly thickened by over-topping of the surrounding
frazil ice crystals, termed ‘scavenging’, and gave rise to distinct morphologies, which were
classified. A physical model was developed to describe the evolution of the pancake ice cover to
consolidation. Ice production in the pancake/frazil process was found to proceed at
approximately double the rate of the equivalent congelation ice cover, or 0.58 times the limiting
free-surface frazil production. It was suggested that the discrepancy will seriously impact largescale
modelling attempts to simulate heat and momentum fluxes between the ocean and
atmosphere, as well as salt rejection and subsequent water mass modification, though it is
acknowledged that further field measurements are required to place some currently empirical
parameters into a physical context.

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