Origin of the trends in Antarctic sea ice extent over the period 1958-2023

The Antarctic sea ice extent has displayed several regime shifts over the past 65 years. Consistent lines of evidence indicate a decline in Antarctic sea ice extent from 1958 to 1978, which precedes the availability of continuous satellite observations. Subsequently, there was a significant sea ice expansion over 19792015 before the large drop observed in the past few years that led to record lows. The origin of those shifts and contrasting trends are analyzed here using a new reconstruction of atmospheric temperature, sea level pressure and sea ice extent spanning the period 1958-2023. We employ a data assimilation method that combines long simulations as well as large ensembles performed with climate models with long-term station-based records of temperature and sea level pressure at mid and high latitudes of the Southern Hemisphere. The reconstruction is thus totally independent from sea ice extent observations that are used as validation, showing the good performance of the method. In contrast to previous reconstructions and estimates, reconstructing simultaneously the atmospheric circulation, temperature, and sea ice extent ensures compatibility among the variables and thus a more straightforward dynamical interpretation. Our reconstruction indicates that no change in the reconstructed atmospheric circulation could be directly related to the regime shifts in sea ice extent trends but the covariance structure of the temperature strongly varies across periods, with more homogenous temperature anomalies for the early and recent periods and a more complex spatial pattern for the years 19792015. This might suggest a time-dependent contribution of the ocean, which will be further analyzed in a simulation performed with the NEMO model driven by the ERA5 reanalysis.