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Ice shelves fringing the Antarctic continent experience low or high basal melt rates depending on local shelf conditions, ocean circulation and intensity of ice‐sea‐air exchanges. Recent studies have uncovered potential cold‐to‐warm transitions in specific ice‐shelf cavities, which could lead to a dramatic increase in sea‐level rise. Here we demonstrate that brine rejection in coastal polynyas promotes bistable dynamics in ice‐shelf cavities, which would be otherwise monostable, for a broad diversity of Circumpolar Deep Water temperatures. We develop a generic low‐dimensional box model featuring warm and cold circulation modes and apply it to nine ice‐shelf cavities. We find that most ice‐shelf cavities are in a bistable regime and are therefore susceptible to irreversible abrupt transitions for a realistic range of sea‐ice formation rates. Bistability is robust to changes in cavity parameterization. However, the vertical mixing scheme at the ice‐shelf front can be tuned to make the transitions reversible.

Antarctic Bottom Water (AABW), formed from Dense Shelf Water (DSW) in Antarctic coastal polynyas, partly drives the global overturning circulation. Cape Darnley Polynya in East Antarctica, the most recently‐identified AABW formation site, remains poorly understood. We present simulations of the Cape Darnley region quantifying the main processes responsible for forming DSW and identify two key, opposing influences. Wintertime DSW export from Cape Darnley—mean of 0.28×106$0.28\\times 1{0}^{6}$ m3 s−1 (0.28 Sv)—is suppressed by basal melting of the Amery Ice Shelf, but enhanced by cold, saline preconditioning from Mackenzie Polynya. A doubling of Amery Ice Shelf melting reduces export to 0.26 Sv (7.0% decrease), while a shutdown of Mackenzie Polynya reduces DSW export to 0.18 Sv (a 36% decrease). Our findings reveal the sensitivity of DSW formation at Cape Darnley to oceanic and glaciological conditions, with implications for future AABW production, the global overturning circulation, and climate.

Open-ocean polynyas (OOPs) in the Southern Ocean are ice-free areas within the winter ice pack that are associated with deep convection, potentially contributing to the formation of Antarctic Bottom Water. To enhance the credibility of Earth system models (ESMs), their ability to simulate OOPs realistically is thus crucial. Here we investigate OOPs that emerge intermittently in a high-resolution (HR) preindustrial simulation with the Energy Exascale Earth System Model, version 0.1 (E3SMv0), an offspring of the Community Earth System Model (CESM). While low-resolution (LR) simulations with E3SMv0 show no signs of OOP formation, the preindustrial E3SMv0-HR simulation produces both large Weddell Sea polynyas (WSPs) as well as small Maud Rise polynyas (MRPs). The latter are associated with a prominent seamount in the eastern Weddell Sea, and their preconditioning and formation is the focus of this study. The steep flanks of the rugged topography in this region are in E3SMv0-HR sufficiently well resolved for the impinging flow to produce pronounced Taylor caps that precondition the region for convection. Aided by an accumulation of heat in the Weddell Deep Water layer, the ultimate trigger of convection that leads to MRPs is the advection of anomalously high upper-ocean-layer salinity. The crucial difference to WSP-producing LR ESM simulations is that in E3SMv0-HR, WSPs are realistically preceded by MRPs, which in turn are a result of the flow around bathymetry being represented with unprecedented detail.

Strong offshore wind events (SOWEs) occur frequently near the Antarctic coast during austral winter. These wind events are typically associated with passage of synoptic- or meso-scale cyclones, which interact with the katabatic wind field and affect sea ice and oceanic processes in coastal polynyas. Based on numerical simulations from the coupled Finite Element Sea-ice Ocean Model (FESOM) driven by the CORE-II forcing, two coastal polynyas along the East Antarctica coast––the Prydz Bay Polynya and the Shackleton Polynya are selected to examine the response of sea ice and oceanic properties to SOWEs. In these polynyas, the southern or western flanks of cyclones play a crucial role in increasing the offshore winds depending on the local topography. Case studies for both polynyas show that during SOWEs, when the wind speed is 2–3 times higher than normal values, the offshore component of sea ice velocity can increase by 3–4 times. Sea ice concentration can decrease by 20–40%, and sea ice production can increase up to two to four folds. SOWEs increase surface salinity variability and mixed layer depth, and such effects may persist for 5–10 days. Formation of high salinity shelf water (HSSW) is detected in the coastal regions from surface to 800 m after 10–15 days of the SOWEs, while the HSSW features in deep layers exhibit weak response on the synoptic time scale. HSSW formation averaged over winter is notably greater in years with longer duration of SOWEs.

Polynyas, or ice-free regions within the sea ice pack, are a common occurrence around Antarctica. A recurrent and often large polynya is the Terra Nova Bay Polynya (TNBP), located on the western side of the Ross Sea just off Victoria Land. In this study, we investigate the atmospheric conditions leading to the occurrence of the TNBP and its spatial variability, as estimated using satellite-derived ice surface temperature and sea ice concentration data. A cluster analysis revealed that katabatic winds descending the Transantarctic Mountains, account for about 45% of the days when the TNBP exceeded its 2010–2017 mean extent plus one standard deviation. Warmer and more moist air intrusions from lower-latitudes from the Pacific Ocean, which are favoured in the negative phase of the Southern Annular Mode, play a role in its expansion in the remaining days. This is more frequent in the transition seasons, when such events are more likely to reach Antarctica and contribute to the occurrence and the widening of the polynya. In-situ weather data confirmed the effects of the mid-latitude air intrusions, while sea ice drifts of up to 25 km day −1 cleared the ice offshore and promoted the widening of the polynya starting from the coastal areas. Knowing the atmospheric factors involved in the occurrence of coastal polynyas around Antarctica is essential as it helps in improving their representation and predictability in climate models and hence advance the models’ capabilities in projecting Antarctic sea ice variability.

We present an unprecedented set of high‐resolution climate simulations, consisting of a 500‐year pre‐industrial control simulation and a 250‐year historical and future climate simulation from 1850 to 2100. A high‐resolution configuration of the Community Earth System Model version 1.3 (CESM1.3) is used for the simulations with a nominal horizontal resolution of 0.25° for the atmosphere and land models and 0.1° for the ocean and sea‐ice models. At these resolutions, the model permits tropical cyclones and ocean mesoscale eddies, allowing interactions between these synoptic and mesoscale phenomena with large‐scale circulations. An overview of the results from these simulations is provided with a focus on model drift, mean climate, internal modes of variability, representation of the historical and future climates, and extreme events. Comparisons are made to solutions from an identical set of simulations using the standard resolution (nominal 1°) CESM1.3 and to available observations for the historical period to address some key scientific questions concerning the impact and benefit of increasing model horizontal resolution in climate simulations. An emerging prominent feature of the high‐resolution pre‐industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation. Overall, high‐resolution simulations show significant improvements in representing global mean temperature changes, seasonal cycle of sea‐surface temperature and mixed layer depth, extreme events and in relationships between extreme events and climate modes. Plain Language Summary Although the current generation of climate models has demonstrated high fidelity in simulating and projecting global temperature change, these models show large uncertainties when it comes to questions concerning how rising global temperatures will impact local weather conditions. This is because the resolution (~100 km) at which the majority of climate models simulate the climate is not fine enough to resolve these small‐scale regional features. Conducting long‐term (multi‐centuries) high‐resolution (~10 km) climate simulations has been a great challenge for the research community due to the extremely high computational demands. Through international collaboration, we are able to address this challenge by delivering an unprecedented set of multi‐century high‐resolution climate simulations using the Community Earth System Model (CESM), capable of directly representing tropical cyclones and extreme rainfall events. In this paper, we give an overall assessment of the value and benefit of the high‐resolution CESM climate simulations by making a direct comparison to an identical set of low‐resolution CESM simulations. We showcase some of the major improvements of the high‐resolution CESM in simulating global mean temperature changes, seasonal cycle of sea‐surface temperature, and extreme events, such as tropical cyclones and relationships between tropical cyclones and El Niño‐Southern Oscillation. Key Points An unprecedented set of multi‐century high‐resolution Community Earth System Model (CESM) simulations is described High‐resolution CESM simulations reveal a potential role of Southern Ocean polynyas in multidecadal climate variability High‐resolution CESM exhibits significantly improved simulations of extreme events, such as tropical cyclones and atmospheric rivers

This study investigates winter polynyas in the southern Ross Sea, Antarctica where several polynyas are known to form. Coastal polynyas are areas of lower sea ice concentration and/or thickness along the coast that are otherwise surrounded by more extensive, thicker sea ice pack. Polynyas are also locations where organisms can exploit both the ice substrate and pelagic resources. Using a self organizing map algorithm, we identify polynya events in the Community Earth System Model Version 2 Large Ensemble (CESM2-LE). The neural network algorithm is able to identify polynya events without imposing an ice concentration or thickness threshold, as is often done when identifying polynyas. The CESM2-LE produces a wintertime polynya feature comparable in size and location to the Ross Sea polynya, and during polynya events there are large turbulent heat fluxes and export of sea ice from the Ross Sea. In the CESM2-LE polynya event frequency is projected to decrease sharply in the later twentyfirst century, leading to increasing sea ice concentrations and thicknesses in the region. The drivers of the polynya frequency decline are likely both large scale circulation changes and local atmosphere and ocean feedbacks. If declines in wintertime polynya frequency over the twentyfirst century do occur they may impact Antarctic Bottom Water formation and local net primary productivity. Thus, better understanding potential local and unexpected sea ice changes in the Ross Sea is important for both assessing climate system impacts and ecological impacts on the Ross Sea ecosystem, which is currently protected by an internationally recognized marine protected area.

Polynyas facilitate air–sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project Phase 6 (CMIP6) and two satellite-based sea ice products. Only half of the 27 models form open-water polynyas (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future.

Since the mid-2010s, a sharp rebound in dense shelf water (DSW) salinity has been observed in the Ross Sea, coinciding with a persistent decline in sea ice around Antarctica. Despite extensive scientific study and significant attention given to these two phenomena due to their potential influence on Earth’s changing climate, a physical link between them has yet to be established. Here, by examining atmospheric circulation in response to shifts in major climate variability modes before and after these dramatic changes, we showed that a combination of large-scale circulation patterns drove both the decline in Antarctic sea ice and the increase in Ross Sea DSW salinity. After the mid-2010s, climate-driven atmospheric circulation generated anomalous northerly winds across much of the high-latitude Southern Ocean. However, the Ross Sea sector uniquely experienced strengthened southerly flow of cold continental air. Consequently, this process led to enhanced sea ice loss around Antarctica through warmer air advection from the north, yet simultaneously, expanded the Ross Sea polynyas, enhancing sea ice formation and contributing to increased Ross Sea DSW salinity. Our findings, therefore, establish a robust linkage between these two Southern Ocean transitions, highlighting their interdependence driven by large-scale climate modes.

Polynyas along the Antarctic coastline are essential for sea ice production and the formation of Antarctic Bottom Water (AABW). They are formed and maintained by strong and persistent katabatic winds that push the ice away from the coast. Satellite data, in-situ meteorological observations and Reanalysis datasets all indicate a decline in sea ice concentration (SIC) over the Terra Nova Bay Polynya (TNBP) from 2013 to 2022, consistent with the occurrence of an increasing trend of strong katabatic wind events (SKWEs). On the interannual timescale, significant correlations between the TNBP area and the duration of SKWEs were observed in austral summer, autumn and winter, and also in April and October during 2003–2022. Sea ice volume budget analysis shows that SKWEs drive rapid sea-ice removal through advection and divergence, whereas thermodynamic processes dominate sea ice formation on an annual basis. The increase in SKWEs was associated with a deepened and southwestward-displaced Amundsen Sea Low (ASL) and an enhanced pressure gradient between the interior plateau and the coast, both potentially linked to a more positive Southern Annular Mode (SAM). These findings provide a mechanistic understanding of long-term polynya variability, with implications for regional sea-ice changes and AABW.