Detection of climate-induced change in marine ecosystems requires a knowledge of the underlying variability of the environment. This paper uses a range of datasets to investigate the interannual variability in Southern Ocean sea-ice dynamics. We present the first analysis of a series of fast-ice duration data from Signy Island, which we have cross-calibrated and combined with an earlier series from the same island group. The combined series covers the period from 1903 to the present day. The analyses indicate that there has been a long term decline in the duration of sea-ice at the South Orkney Islands in the north-west Weddell Sea. This change has not been a simple linear decline but appears to have been the result of a reduction in the duration of fast-ice during the 1940s and 1950s. There was a pronounced sub-decadal year cycle in fast-ice duration at the South Orkney Islands from the mid-1960s to 1990. In recent years this cyclicity has broken down and fast-ice duration has been greater than expected. Analyses of satellite data have shown that fast-ice duration at Signy Island reflects the larger-scale ice dynamics of the Weddell Sea. Investigation of the Weddell Sea ice dynamics in relation to circumpolar ice extents indicates that the position of anomalies in the maximum sea-ice extent field precesses around the Antarctic continent with a period of approximately 7–9 years. Analysis of atmospheric and oceanic connections with the sea ice variability show that there are signals in both regimes. This environmental variability has significant implications for ecosystem function and the detection of short-term and long-term ecological change.