An exceptional winter sea-ice retreat/advance in the Bellingshausen Sea, Antarctica

The exceptional sea-ice retreat and advance that occurred in the Bellingshausen Sea, Antarctica during August 1993 was the largest such winter event in this sector of the Antarctic during the satellite era. The reasons for this fluctuation of ice are investigated using passive microwave satellite imagery, ice motion vectors derived from the satellite data, in-situ meteorological reports and near-surface winds and temperatures from the European Centre for Medium-range Weather Forecasts (ECMWF) numerical weather prediction model. The ice edge retreat of more than 400 km took place near 80degreesW from approximately 1-15 August, although the southward migration of the ice edge was not continuous and short periods of advance were also recorded. Between 16 August and 2 September there was almost continuous sea-ice recovery. The rate of change of the ice edge location during both the retreat and advance phases significantly exceeded the southward and northward velocity components of ice within the pack, pointing to the importance of ice production and melting during this event. During the month, markedly different air masses affected the area, resulting in temperature changes from +2degreesC to -21degreesC at the nearby Rothera station. 'Bulk' movement of the pack, and compaction and divergence of the sea ice, made a secondary, but still significant, contribution to the observed advance and retreat. The ice extent fluctuations were so extreme because strong meridional atmospheric flow was experienced in a sector of the Southern Ocean where relatively low ice concentrations were occurring. The very rapid ice retreat/advance was associated with pronounced low-high surface pressure anomaly couplets on either side of the Antarctic Peninsula.

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