What was the contribution of the ocean to the 2016 Antarctic sea ice extent drop and was this event unprecedented?

The 2016 Antarctic sea ice extent drop was a rapid decline that led to persistent low sea ice conditions. The drop followed a period of stability and even slight sea ice extent increase: this sudden change from extreme high values to an absolute minimum, which occurred in less than two years, is distinctive of this event, together with the lack of recovery that ensued.

Even though new record lows have been recently established, there is still a lot to uncover on the dynamics of the 2016 drop. While the event was triggered by atmospheric forcing, the potential preconditioning role of the ocean is unclear. To shed light on it, we perform sensitivity experiments with a fully-coupled regional climate model, which is re-initialized in January 2016 using different ocean and sea ice conditions but keeping boundary forcings in the atmosphere and ocean unchanged. We show that the state of the Southern Ocean in early 2016 does not determine whether the drop occurs or not, but indeed has an impact on its amplitude, regional characteristics and subsequent sea ice recovery.

This dramatic drop is unprecedented in our current observational record, but it is possible that other similar events have occurred before the beginning of the satellite era. We explore this possibility in a new spatial reconstruction of atmospheric and sea ice variables covering 1958-2023, built using a data assimilation method that combines station-based observations and results from large ensembles of simulations. Additionally, we force a sea ice-ocean model with the atmospheric reconstruction to estimate the oceanic variations. Our results indicate that a drop similar to the 2016 one occurred at the end of the 1970s. We then compare the spatial patterns and changes during the two events and investigate the links between the sea ice, ocean and atmosphere.