Antarctic sea ice plays a crucial role in the global climate system. Its annual minimum typically occurs in February/March. The record low sea ice extent observed in February 2023 (1.77 million square kilometers, 36% below the 1979-2022 average) followed exceptionally low coverage in late 2022 and January 2023. This exceptionally low extent persisted into autumn and winter, raising questions about the underlying drivers. The study investigates the hypothesis that sustained ocean warming, particularly in the Southern Ocean's subsurface, has contributed to a shift in the Antarctic sea ice state, representing a change from the previously observed gradual multi-decadal increase followed by a steep decline in 2016. This shift is further supported by a change in the seasonal persistence of sea ice, indicating a possible regime shift in the Antarctic sea ice system. Understanding the implications of this low sea ice state and the underlying mechanisms is crucial for comprehending future climate change scenarios and their impacts on the Antarctic ecosystem and global climate.

Previous research has explored the multi-decadal increase in Antarctic sea ice, attributing it to factors such as changes in high-latitude winds and Antarctic meltwater. Positive ocean-sea ice feedback mechanisms were also suggested to have played a role in sustaining the high sea ice coverage. However, the abrupt transition to a new low sea ice state in 2016 remains a significant subject of investigation. While individual extreme sea ice events have been linked to anomalous atmospheric circulation, such as the strong meridional circulation associated with zonal wave number three in spring 2016 and the strong Amundsen Sea Low circulation in spring 2021, there is a growing consensus that Southern Ocean warming must also be considered. Existing literature highlights the warming trends in the Southern Ocean's subsurface, largely attributed to increasing greenhouse gases, with a secondary influence from ozone depletion and internal climate variability. Studies have suggested a connection between the observed subsurface ocean warming and the recent decline in Antarctic sea ice, but the study adds a crucial temporal dimension to this relationship.

The study uses data from the National Snow and Ice Data Center (NSIDC) for Antarctic sea ice extent (daily, five-day, and monthly), covering November 1978 to June 2023. To analyze the sea ice extent anomalies, the authors subtract the 1979-2022 climatology for monthly and five-day data, and calculate daily minimum anomalies using the 1979-2022 annual average. They employ change point analysis using the R packages `changepoint` and `strucchange` to identify significant shifts in the mean and variability of the sea ice extent anomaly time series. The analysis detects two change points, in August 2007 and August 2016, dividing the record into three distinct periods with statistically different means. The study also incorporates data from Argo floats for Southern Ocean temperature (January 2004 to May 2023), calculating temperature anomalies relative to the 2004-2022 climatology. Hovmöller diagrams are used to show the temporal evolution of longitudinal sea ice extent and ocean temperature at different depths. A composite analysis, using Welch’s unequal variance t-test with a Bonferroni correction, assesses the significance of sea ice concentration and ocean temperature changes across different periods. Additionally, the study examines the relationships between maximum sea ice coverage in austral spring and minimum sea ice coverage in the following summer, and between minimum summer sea ice coverage and the subsequent spring maximum, to analyze sea ice persistence characteristics. Data on large-scale climate modes (Southern Annular Mode, Southern Oscillation Index, Interdecadal Pacific Oscillation, Indian Ocean Dipole, Atlantic Multidecadal Oscillation) are also used to investigate their potential influence. Finally, surface wind stress and Ekman pumping are calculated using data from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation atmospheric reanalysis to understand the role of ocean dynamics.

The study's key findings are: 1. **Record low Antarctic sea ice extent:** The Antarctic sea ice extent reached a record low in February 2023, with the lowest monthly mean anomalies in January and February. This exceptionally low sea ice persisted through the first half of 2023, resulting in the largest negative areal extent anomalies in the satellite era. 2. **Identification of three distinct sea ice states:** Change point analysis reveals three statistically different periods in the 44-year sea ice record: November 1978 to August 2007, September 2007 to August 2016, and September 2016 to June 2023. The third period represents a new, low sea ice state. 3. **Subsurface Southern Ocean warming:** The study shows a strong link between subsurface Southern Ocean warming (below 100 m) and the transition to the new low sea ice state in 2016. The warming began approximately one year prior to the identified change point and persisted through 2023, exhibiting strong spatial agreement with circumpolar low sea ice anomalies. This suggests a primary role for subsurface ocean warming in driving the recent record lows. 4. **Changes in sea ice persistence:** The relationship between maximum spring sea ice extent and minimum summer sea ice extent, previously observed, is no longer evident after 2016. Instead, the minimum summer sea ice extent now shows a strong correlation with the following spring maximum, suggesting a shift in Antarctic sea ice behavior and underlining the influence of altered subsurface ocean conditions. 5. **Circumpolar nature of the low sea ice state and ocean warming:** Hovmöller diagrams and spatial maps show widespread circumpolar patterns of low sea ice extent and subsurface ocean warming, emphasizing the large-scale nature of this event and weakening the argument that isolated regional atmospheric events are the sole drivers. 6. **Limited influence of large-scale climate modes:** While large-scale climate modes do influence Antarctic sea ice variability, their phase cannot account for the circumpolar low sea ice seen in recent years, particularly in light of the subsurface ocean warming.

The findings strongly suggest that the recent record low Antarctic sea ice extent is not merely a manifestation of interannual variability but indicates a transition to a new, low sea ice state primarily influenced by sustained subsurface Southern Ocean warming. This warming, likely driven by anthropogenic greenhouse gas emissions, seems to have disrupted established relationships between sea ice and climate drivers, such as the Southern Annular Mode. The observed change in sea ice persistence characteristics further strengthens the case for a regime shift. This has broad implications for ocean stratification, circulation, ice-shelf melting, coastal stability, and the Antarctic ecosystem. While the role of Antarctic meltwater remains uncertain, the continued subsurface warming, combined with accelerating meltwater from the ice sheet and shelf, presents a complex scenario. The study's findings highlight the urgent need to reduce greenhouse gas emissions to mitigate the far-reaching consequences of Antarctic sea ice loss.

This research demonstrates a statistically significant shift towards a new, lower-extent state for Antarctic sea ice driven primarily by sustained subsurface Southern Ocean warming. The alteration of sea ice persistence characteristics underscores this regime shift. The findings emphasize the urgent need for further investigation and emphasize the critical role of anthropogenic climate change in this phenomenon. Future studies should focus on improving ocean observations beneath sea ice, refining process-based understanding of ocean-sea ice feedback, and expanding modelling efforts to project future sea ice states under varying emission scenarios.

The study acknowledges limitations arising from the relatively short duration of the current low sea ice state (seven years) and the scarcity of ocean observations from beneath the sea ice. These limitations make it challenging to definitively determine whether this state constitutes a permanent regime shift or part of longer-term multidecadal variability. The complexities of meltwater influences on sea ice are also noted, with uncertainties regarding its magnitude and its counteracting effect against warming influences.