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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 km2 in 1979–2006 to 0.76 million km2 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. [ABSTRACT FROM AUTHOR]

Antarctic sea ice is a critical component of the climate system and a vital habitat for Southern Ocean ecosystems. Understanding the underlying physical processes and improving Antarctic sea ice prediction is of broad interest. Using the model data, we investigate sea ice and upper ocean predictability at interannual timescales in the Weddell Sea region. We find that oceanic predictability is largely confined to the Winter Water layer and responds to seasonal modifications of the water column, mainly driven by sea ice processes. Predictability depends not only on the depth of the Winter Water layer, but also on how strongly stratified its base is. Predictability is lost when warm Circumpolar Deep Water with no sea ice‐related memory entrains into the mixed layer. We show the strong dependence of sea ice predictability on the local upper ocean vertical structure, which suggests that both are likely to change in a warming climate. Plain Language Summary: Antarctic sea ice is a critical component of climate and a vital habitat for Southern Ocean ecosystems. Therefore, understanding the drivers and physical processes influencing Antarctic sea ice, and being able to predict Antarctic sea ice is of broad interest. We assess the predictability of sea ice and underlying upper ocean in the Weddell Sea region of the Southern Ocean using the model data. We find that sea ice processes influence the upper ocean temperature, and these thermal signatures linger in the ocean producing sea ice predictability over multiple months. Here, we show that the oceanic memory in the upper ocean is largely found within the Winter Water layer (i.e., cold water layer formed during sea ice formation). Oceanic memory and sea ice predictability are suddenly lost when warm deep waters from the ocean interior entrain into the surface mixed layer in mid‐winter. This limit to sea ice predictability has not been explored before, and it shows the strong dependency of sea ice predictability in a region to its local vertical structure of oceanic properties and their seasonal evolution. This implies that changes to upper ocean properties in a warming climate can likely alter the sea ice predictability patterns in the future. Key Points: Sea ice predictability in the Weddell Sea is strongly determined by temperature and salinity profiles of the underlying upper oceanEvery winter, the timing of the loss of sea ice predictability is defined when deep water is entrained into the mixed layerSea ice predictability depends not only on the depth of the Winter Water layer but also on how strongly stratified its base is [ABSTRACT FROM AUTHOR]

Warm Atlantic water (AW) that flows northward along the Svalbard west coast is thought to transport enough heat to melt regional Arctic sea ice effectively. Despite this common assumption, quantitative requirements necessary for AW to directly melt sea ice fast enough under realistic winter conditions are still poorly constrained. Here we use meteorological data, satellite observations of sea ice concentration and drift, and model output to demonstrate that most of the sea ice entering the area over the Yermak Plateau melts within a few weeks. Simulations using the Los Alamos Sea Ice Model (CICE) in a 1‐D vertically resolved configuration under a relatively wide range of in situ observed atmospheric and ocean forcing show a good fit to observations. Simulations require high‐frequency atmospheric forcing data to accurately reproduce vertical heat fluxes between the ice or snow and the atmosphere. Moreover, we switched off hydrostatic equilibrium to properly reproduce ice and snow thickness when observations showed that ice had a negative freeboard, without surface flooding and snow‐ice formation. This modeling shows that realistic melt rates require a combination of warm near‐surface AW and storm‐induced ocean mixing. However, if AW is warmer than usual (>5°C), then lower mixing rates are sufficient. Our results suggest that increased winter storm frequency and increased heat content of the AW may work together in reducing future sea ice cover in the Eurasian basin. Plain Language Summary: Northwest of Svalbard, north of Norway, an area known as Whalers Bay stays ice‐free in winter despite the negative air temperatures. It has been assumed that this open water is maintained by inflow of warm Atlantic water (AW) along Svalbard's west coast; however, this mechanism has never been demonstrated quantitatively. We combine observations and results from modeling to calculate the rate of ice melting necessary to keep Whalers Bay ice‐free and the amount of heat that must be transferred from the ocean to the ice to sustain such melting. We conclude that the presence of AW combined with the occurrence of storms releases the amount of heat necessary to keep the area ice‐free. When the AW is close to the surface and its temperature is above about 5°C, storms are no longer necessary to enhance heat transfer and produce the required melting. Since the amount of heat transported by the AW and the storm frequency have been increasing over several decades, we expect that the ice‐free area will increase in the future, affecting air‐sea‐ice fluxes, water mass transformation, marine ecology, sea ice cover, and commercial activity including transportation and fishing. Key Points: Most of the sea ice entering the area known as Whalers Bay, in the NW of Svalbard, melts in less than a month, keeping it almost ice‐freeMelting rates larger than 1.5 m month−1 result from a combination of the presence of warm Atlantic water and storm‐enhanced mixingIn the absence of storms, estimated high sea ice melting rates require ocean surface temperature above 5°C [ABSTRACT FROM AUTHOR]

Southern Ocean organisms are uniquely adapted to the extreme environmental conditions that characterise this region, making them especially vulnerable to climate change. Alterations to the physical environment have already been linked to alterations in the structure and functioning of entire ecosystems, and ecological disruptions are expected to continue to occur. Although our understanding of the physical processes driving ecological change in the Southern Ocean has improved in recent years, significant knowledge gaps remain largely as a result of insufficient observational data being available. High resolution ocean models are an important tool that can help us overcome data scarcity. However, models generally contain biases that may affect their ability to accurately represent environmental conditions in the region of interest. Thus, their outputs must be evaluated to understand if and how model outputs can be used to answer questions about ecological impacts. Here, we examined the suitability of ACCESS-OM2-01, a high-resolution coupled ocean–sea ice model, for ecological applications. Our results highlight the heterogeneous nature of the mean state of the environmental variables examined and their trends across the Southern Ocean. Our assessment shows that the ACCESS-OM2-01 model simulated well the observed seasonal cycle and broad baseline climatological conditions of the mixed layer depth and sea ice variables for the Southern Ocean over the past 50 years. However, the model performance varies across regions and at seasonal time scales: the model simulated much deeper winter mixed layer depth in the Weddell Sea and within the Antarctic Circumpolar Current, and the climatological sea-ice concentration for summer was lower in the model compared to available observations. Finally, we provide detailed Python-based scripts with all the code and steps for this analysis, which can be used as reference material to reapply elsewhere and evaluate the suitability of other model outputs for ecological applications. • ACCESS-OM2-01 offers realistic SIC, but somewhat biased MIZ and sea ice seasonality. • Modelled sea ice duration negatively biased, up to 3 months shorter than observations. • Seasonality and depth of modelled mixed layer is broadly consistent with observations. • Large bias in modelled winter and spring mixed layer depth in the Weddell Sea region. [ABSTRACT FROM AUTHOR]