Observational Evidence for a Regime Shift in Summer Antarctic Sea Ice

Abstract In recent years, the Southern Ocean has experienced extremely low sea ice cover in multiple summers. These low events were preceded by a multidecadal positive trend that culminated in record high ice coverage in 2014. This abrupt transition has led some authors to suggest that Antarctic sea ice has undergone a regime shift. In this study we analyze the satellite sea ice record and atmospheric reanalyses to assess the evidence for such a shift. We find that the standard deviation of the summer sea ice record has doubled from 0.31 million km 2 in 1979–2006 to 0.76 million km 2 for 2007–22. This increased variance is accompanied by a longer season-to-season sea ice memory. The atmosphere is the primary driver of Antarctic sea ice variability, but using a linear predictive model we show that sea ice changes cannot be explained by the atmosphere alone. Identifying whether a regime shift has occurred is difficult without a complete understanding of the physical mechanism of change. However, the statistical changes that we demonstrate (i.e., increased variance and autocorrelation, and a changed response to atmospheric forcing), as well as the increased spatial coherence noted by previous research, are indicators based on dynamical systems theory of an abrupt critical transition. Thus, our analysis is further evidence in support of a changed Antarctic sea ice system. Significance Statement In recent years, there have been several summers with extremely low Antarctic sea ice cover, including consecutive record lows in February 2022 and February 2023. Since then, the 2023 winter has seen a remarkably low sea ice growth with an anomaly far below expected climatology. This has led researchers to question whether there has been a regime shift, and we assess the observational evidence for such a shift. In the last decade or so, the variability of summer sea ice has almost doubled, accompanied by a much longer sea ice memory from season to season. These statistical changes, as well an increased spatial coherence noted by other researchers, are consistent with theoretical indicators of a critical transition, or regime shift.