Dynamical contrasts between pancake and pack ice, investigated with a drifting buoy array

Abstract

The motion of pancake ice was investigated using an array of specialised drifting buoys, deployed into the advancing ice edge of the Weddell Sea in April 2000. The buoys remained in the ice as the pancakes consolidated into a coherent ice sheet, and the study examined the contrasts in dynamics for equivalent periods before and after consolidation. Drift velocities were largely determined by the meridional component, perpendicular to the ice edge. Prior to consolidation, these showed significantly elevated magnitudes at high frequencies (periods shorter than six hours). Scalar velocities were higher than previously reported values, reducing with time and distance from the ice edge. The same trends were not evident from in situ wind data. Derivation of momentum transfer parameters (wind factor, turning angle) was hampered by a lack of reliable wind directions from the outermost buoys, however. Relative motions between buoys were investigated using differential kinematic parameters. These displayed high amplitude, high frequency oscillations in unconsolidated ice, with RMS invariant values up to two orders of magnitude higher than normally reported for Weddell Sea pack ice. The values were found to be strongly dependent on sampling interval, increasing further at intervals less than one hour. In situ winds did not display an equivalent variation, suggesting that wind-forcing was not responsible, and translation under wave action, either internal or surface gravity, was postulated as the forcing. Copyright 2006 by the American Geophysical Union.