Antarctic sea ice extent has been persistently low since late 2016,
possibly owing to changes in atmospheric and oceanic conditions.
However, the relative contributions of the ocean, the atmosphere and the
underlying mechanisms by which they have affected sea ice remain
uncertain. To investigate possible causes for this sea-ice decrease, we
establish a seasonal timeline of sea ice changes following 2016, using
remote sensing observations. Anomalies in the timing of sea ice retreat
and advance are examined along with their spatial and interannual
relations with various indicators of seasonal sea ice and oceanic
changes. They include anomalies in winter ice thickness, spring ice
removal rate due to ice melt and transport, and summer sea surface
temperature. We find that the ice season has shortened at unprecedented
rate and magnitude, due to earlier retreat and later advance. We
attribute this shortening to a winter ice thinning, in line with the
ice-albedo feedback, with ice transport playing a more minor role.
Reduced ice thickness has accelerated spring ice area removal as thinner
sea ice requires less time to melt. The consequent earlier sea ice
retreat has in turn increased ocean solar heat uptake in summer,
ultimately delaying sea ice advance. We speculate that the observed
winter sea ice thinning is consistent with previous evidence of
subsurface warming of the Southern Ocean.