In 2012, 2013 and 2014, the overall Antarctic sea ice extent reached well above the long-term average (since 1979 when satellite observations began) of 18.5 × 106 square kilometres (km2). In fact, each of these years set new successive daily records for the greatest ice extent since 1979 (Figure ANT10) (Massom et al. 2013a, b, 2014, 2015). The mechanisms responsible for each of these anomalous extents were, however, quite different, and their examination underlines the complexity of sea ice formation and decay.
In 2012, the net Antarctic sea ice extent remained close to, or slightly above, average for much of the year until late summer. During August and September, strong winds caused a rapid expansion of the ice edge and pushed the ice extent to a new record high. In 2013, different mechanisms appear to have been involved in achieving the subsequent record extent; the early advance of sea ice in the western Ross Sea sector was associated with colder than average sea surface temperatures (SSTs) originating in that region. When it reached lower latitudes, this cold SST anomaly tongue turned eastwards in the Antarctic Circumpolar Current, and wrapped around the northern edge of the sea ice to aid its further expansion as the year progressed. Colder than average SSTs off the ice edge in the Weddell Sea delayed sea ice retreat in late 2013 and drove early advance during 2014. Indeed, greater than average sea ice extent in the Weddell Sea was the main contributor to the well above average net Antarctic sea ice extent in early 2014. As the 2014 season progressed, sea ice expansion pushed further to the east, so that much of the Indian Ocean sector experienced well above average ice coverage for the rest of 2014. During mid-2014, wind-driven ice advance in the western Pacific Ocean and Ross Sea also contributed to the record sea ice extent in 2014. It is currently unknown whether the patterns of sea ice expansion and decay in 1 year influenced the pattern of extent in the following year.
These 3 years of successive record-breaking sea ice extents were followed by an equally interesting year in 2015. For much of early 2015, daily sea ice extents were well above average (Figure ANT10), but there was a dramatic slowing of ice expansion in June. This coincided with the development of a strong El Niño event in the tropical Pacific Ocean, which transported warmer than average waters westwards. El Niño–Southern Oscillation (ENSO) events influence large-scale variability in atmospheric patterns at polar southern latitudes, which in turn determine where sea ice expands more or less rapidly (Yuan 2004). Because 2012–14 was not significantly influenced by the ENSO, more localised (high-latitude) processes tended to dominate the growth and retreat of sea ice at that time. However, with the development of an El Niño in 2015, there was a rapid transition to a different atmospheric synoptic pattern and associated ice growth conditions. In the subsequent months of 2015, as the sea ice distribution responded to these new growth conditions, net sea ice retreated to close to average extents.