Variability of sea-ice in the northern Weddell Sea during the 20th century

The record of winter fast-ice in the South Orkney Islands, northern Weddell Sea, Antarctica, is over a century long and provides the longest observational record of sea-ice variability in the Southern Hemisphere. Here we present analyses of the series of fast-ice formation and breakout dates from 1903 to 2008. We show that over the satellite era (post-1979), the timing of both final autumn formation and complete spring breakout of fast-ice is representative of the regional sea-ice concentrations (SIC) in the northern Weddell Sea, and associated with atmospheric conditions in the Amundsen Sea region to the west of the Antarctic Peninsula. Variation in the fast-ice breakout date is influenced by the intensity of the westerly/north-westerly winds associated with the Southern Annular Mode (SAM). In contrast, the date of ice formation displays correlations with regional oceanic and sea-ice conditions over the previous 18 months, which indicate a preconditioning during the previous summer and winter, and exhibits variability associated with variation in tropical Pacific sea-surface temperature (i.e., the El Niño-Southern Oscillation, ENSO). A reduction in fast-ice duration at the South Orkney Islands around the 1950s was associated with both later formation and earlier breakout. However, there were marked changes in variability (with periodicities of 3-5, 7-9, and 20 years) in each of the series and in their relationships with ENSO and SAM, indicating the need for caution in interpreting changes in ice conditions based on shorter-term satellite series. Key Points Analyses of a 106 year record of fast-ice from the Southern Ocean Timings of formation and breakout are affected by different processes Frequencies of variability change with no trend over last 30 years