Atmospheric drivers of Antarctic sea ice extent summer minima

Abstract. Understanding the variability of Antarctic sea ice is still a challenge. After decades of modest growth, an unprecedented minimum in the sea ice extent (SIE) was registered in summer 2017, and, following years of anomalously low SIE, a new record was established in early 2022. These two memorable minima have received great attention as single cases, but a comprehensive analysis of summer SIE minima is currently lacking. Indeed, other similar events are present in the observational record, although minor compared to the most recent ones, and a full analysis of all summer SIE minima is essential to separate potential common drivers from event-specific dynamics, in order to ultimately improve our understanding of the Antarctic sea ice and climate variability.

In this work, we examine sea ice and atmospheric conditions during and before all summer SIE minima over the satellite period up to 2022. We use observations and reanalysis data and compare our main findings with results from an ocean-sea ice model (NEMO-LIM) driven by prescribed atmospheric fields from ERA5. Examining SIE and sea ice concentration (SIC) anomalies, we find that the main contributors to the summer minima are the Ross and Weddell sectors. However, the two regions play different roles and the variability of the Ross Sea seems to explain most of the minima, with typical negative SIE anomalies about twice the ones in the Weddell Sea. Furthermore, the distribution of SIC anomalies is also different: in the Weddell Sea, they exhibit a dipolar structure, with increased SIC next to the continent and decreased SIC at the sea ice margin, while the Ross Sea displays a more homogenous decrease. We also examine the role of wintertime sea ice conditions before the summer SIE minima and find mixed results depending on the period: the winter conditions seem relevant in the most recent events, after 2017, but marginal for previous years. Next, we consider the influence of the atmosphere on the SIE minima, which appears to play a major role: after analyzing the anomalous atmospheric circulation during the preceding spring, we find that different large-scale anomalies can lead to similar regional prevailing winds that drive the summer minima. Specifically, the SIE minima seem to be associated with dominant north-westerly anomalous winds in the Weddell Sea, while a south-westerly anomalous flow prevails in the Ross Sea. Finally, we investigate the relative contribution of dynamic (e.g. ice transport) and thermodynamic (e.g. local melting) processes to the summer minima. Our results suggest that the exceptional sea ice loss in both the Ross and Weddell sectors is dominated by thermodynamic processes, while dynamics are also present but with a minor role.