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Increasing snow accumulation over the Antarctic Ice Sheet may mitigate future sea level rise. However, current estimates of mitigation potential are poorly constrained due to limited records of past variability. We present an annually resolved reconstruction of Antarctic snow accumulation from 1801 to 2000 CE, employing a paleoclimate data assimilation methodology to integrate ice core records with a multi‐model ensemble of climate simulations. Our reconstruction correlates well with instrumental reanalysis, and we find that Antarctic accumulation rates increased over the 20th‐century, resulting in a modest amount (∼1 mm) of sea level mitigation. Mitigation is primarily driven by an accelerating trend since around 1970. Our results contrast with previous mitigation estimates of ∼10–12 mm; this discrepancy is due to poorly constrained baseline estimates of 19th‐century accumulation in East Antarctica. Our reconstruction suggests that the uncertainty of future sea level mitigation from increasing Antarctic accumulation has been underestimated. Plain Language Summary: Ice loss from Antarctica causes sea level to rise, but Antarctica can also mitigate sea level rise if snowfall on the continent increases faster than the ice loss. There is not currently a consensus on whether Antarctica will contribute to or mitigate sea level rise in the coming century, due to a lack of Antarctic snowfall records. In this paper, we merge information from ice core records with climate models to reconstruct annual Antarctic snowfall from 1801 to 2000. We find that snowfall on Antarctica increased during the 20th‐century, but only had a modest counteracting effect of 1 mm on sea level rise in the 20th‐century, much less than a previous estimate of 10 mm. The large discrepancy is due to uncertainty in East Antarctic accumulation in the 19th‐century. The potential for Antarctica to mitigate sea level is uncertain, affecting projections for future sea level rise. Key Points: Antarctic accumulation reconstruction using paleoclimate data assimilation finds modest (∼1 mm) 20th‐century sea level mitigationWe find similar 20th‐century trends to previous work; lower sea level mitigation (1 vs. 10 mm) is due to the 19th‐century baselineUncertainty in past East Antarctic accumulation limits confidence in future projections of Antarctic sea level mitigation [ABSTRACT FROM AUTHOR]

The annular structure of Antarctic oscillation (AAO) is a research hotpot, but its asymmetry receives less attention. In this paper, the self‐organizing map method is employed to cluster the AAO patterns into symmetric and asymmetric modes in austral autumn. The asymmetry is mainly reflected in the Pacific‐Atlantic sector, and the AAO evolves toward asymmetric positive polarity, with the most pronounced asymmetry in May. Originating from near Australia, the asymmetry indicates a zonal wave train in the Pacific‐Atlantic sector. Both modeled and observed results demonstrate that the sea surface temperature anomaly in the equatorial western Pacific stimulates a local meridional circulation anomaly and induces anomalous Rossby wave sources near Australia subsequently. An anomalous wave train propagating toward the Antarctic Peninsula is formed, and the associated geopotential anomaly enhances the asymmetry of AAO. Asymmetric AAO is conducive to the Antarctic dipole, which modulates the air temperature and sea ice anomalies around Antarctica. Plain Language Summary: The zonal symmetry of the Antarctic oscillation (AAO; also named southern annular mode) is well known to researchers, while the zonal asymmetry has received less attention. We use a clustering method to cluster the AAO modes in austral autumn into symmetric modes and asymmetric modes. The asymmetry of AAO is mainly reflected in the Pacific‐Atlantic sector. The results show that the AAO mode evolves to a positive asymmetric mode, and the asymmetry is most obvious in May. Both simulations and observations suggest that this asymmetry stems from sea surface temperature anomaly in the equatorial western Pacific that stimulates an anomalous local meridional circulation near Australia, and in turn triggers an anomalous wave train propagating toward the Antarctic Peninsula. The associated geopotential anomalies enhance the asymmetry of AAO. The asymmetric AAO favors the Antarctic dipole, regulating the air temperature and sea ice anomalies around Antarctica. Key Points: Via a cluster method, we obtained the zonal asymmetric Antarctic oscillation (AAO) mode, and analyzed its trend and preferred monthThe source of its asymmetry is from the sea surface temperature anomaly in the tropical western PacificThe asymmetric AAO mode has a vital influence on the Antarctic dipole [ABSTRACT FROM AUTHOR]

Orbital precession has been linked to glacial cycles and the atmospheric carbon dioxide (CO2) concentration, yet the direct impact of precession on the carbon cycle is not well understood. We analyze output from an Earth system model configured under different orbital parameters to isolate the impact of precession on air‐sea CO2 flux in the Southern Ocean—a component of the global carbon cycle that is thought to play a key role on past atmospheric CO2 variations. Here, we demonstrate that periods of high precession are coincident with anomalous CO2 outgassing from the Southern Ocean. Under high precession, we find a poleward shift in the southern westerly winds, enhanced Southern Ocean meridional overturning, and an increase in the surface ocean partial pressure of CO2 along the core of the Antarctic Circumpolar Current. These results suggest that orbital precession may have played an important role in driving changes in atmospheric CO2. Plain Language Summary: Over the past one million years, Earth has experienced several glacial and interglacial periods. As a glacial period is ending, carbon in the atmosphere can rise by up to 50%. The cause for this change is currently unknown, but most theories suggest that this carbon is released from the deep ocean into the atmosphere. The Southern Ocean surrounding Antarctica is the location of a lot of carbon outgassing from the deep ocean into the atmosphere, so it could be responsible for some of this change in atmospheric carbon. One of Earth's orbital cycles, precession, has been shown to change circulation in the Southern Ocean, that can affect how much carbon is carried from the deep ocean to the surface and released into the atmosphere. This paper uses simulations of a climate model to show that high precession corresponds to a 20% increase in the release of carbon from the Southern Ocean into the atmosphere. These findings suggest that precession could have affected changes in past atmospheric carbon concentrations. Key Points: Increased insolation during austral summer due to orbital precession shifts the southern westerlies polewardPoleward shifted westerlies enhance CO2 outgassing due to increased turbulent exchange and vertical transport of carbon‐rich watersEnhanced transport of carbon‐rich waters is driven by a deepening of the overturning circulation in response to poleward shifted winds [ABSTRACT FROM AUTHOR]

Biological productivity in the Southern Ocean is modulated by iron availability. Every summer, a large phytoplankton bloom forms northwest of the Ross Sea, above the Antarctic Australian Ridge (AAR), due to a plume of iron‐rich waters. Here, we investigate the origin and trajectories of these iron‐rich waters by analyzing water mass observations and Lagrangian experiments. Output from the Southern Ocean State Estimate (SOSE) and in situ measurements reveal that iron‐rich AAR bloom waters share properties with Modified Circumpolar Deep Water (MCDW), which forms on the Antarctic shelf‐slope. The Lagrangian experiments are conducted using SOSE velocities. Bloom waters tracked with virtual Lagrangian particles highlight an along isopycnal pathway of MCDW from Antarctica's shelf‐slope to the AAR bloom site, illustrating advection of these waters by the Balleny Gyre. These results are supported by temperature‐salinity analyses, which show a correlation between waters advected northwards; MCDW properties; and high iron concentrations. Plain Language Summary: The biological productivity in the Southern Ocean is limited by the availability of iron and light. During the summer, when light is abundant, a current of iron‐rich waters allows the formation of a phytoplankton bloom on the Pacific sector of the Southern Ocean, above the Australian Antarctic Ridge (AAR). The AAR bloom has an average area of 266,000 km2 and has been observed by satellites for more than 20 years. The temporal and spatial consistency of this bloom has important ecological implications. For example, it is a persistent feeding ground for humpback whales. In this work, we use observations and model output to characterize the properties of these iron‐rich waters and their origin. Our analyses suggest that these iron‐rich waters form near the Antarctic shelf and are transported to the AAR bloom area by the local circulation. Quantifying the connectivity between iron sources and the surface is crucial to better monitor Southern Ocean ecosystems as well as their impact on carbon sequestration. Key Points: The Antarctic Australian Ridge phytoplankton bloom's cooler, iron‐rich waters share properties with Modified Circumpolar Deep WaterLagrangian simulations reveal pathways for iron supply from the Antarctic shelf‐slope to the bloom site following the Balleny GyreThe cooler, iron‐rich waters at the bloom site are correlated with northward Lagrangian displacements after a year of advection [ABSTRACT FROM AUTHOR]

Geothermal heat plays a vital role in Antarctic ice sheet stability. The continental geothermal heat flow distribution depends on lithospheric composition and ongoing tectonism. Heat‐producing elements are unevenly enriched in the crust over deep time by various geological processes. The contribution of crustal heat production to geothermal heat flow is widely recognized; however, in Antarctica, crustal geology is largely hidden, and its complexity has frequently been excluded in thermal studies due to limited observations and oversimplified assumptions. Li and Aitken (2024), https://doi.org/10.1029/2023GL106201 take a significant step forward, focusing on Antarctic crustal radiogenic heat. Utilizing gravity inversion and rock composition data, they show that the crustal heterogeneity introduces considerable variability to heat flow. However, modeling crustal heat production proves challenging because it lacks distinct associations with geophysical observables and has a narrow spatial association. Robust quantification of geothermal heat production and heat flow must incorporate explicit aspects of geology. Plain Language Summary: Even moderate amounts of geothermal heat, or the natural warmth from the Earth's interior, can cause the base of Antarctica's ice sheets to melt or change how the ice behaves as it flows slowly toward the coast. Geothermal heat is not evenly spread within continents. Instead, it's influenced by how plate tectonics has affected the types of rocks present. While scientists agree that the Earth's crust is a major contributor to geothermal heat generation, studies in Antarctica have often left out how rock type differences might affect heat distribution. The study by Li and Aitken (2024), https://doi.org/10.1029/2023gl106201 looks more closely at how the Earth's crust beneath Antarctica varies and how that affects the heat impacting its ice sheets from below. In this commentary, we highlight that although heat production is difficult to model, their findings are important for understanding the natural influences on ice sheets as we observe and predict the impact of ongoing climate change. However, to provide robust estimates, a detailed geological understanding is required. Key Points: Antarctic ice sheet stability is highly dependent on the crustal contribution of radiogenic heat production to geothermal heat flowContrasts in crustal density may indicate upper crustal radiogenic heat production distributionsRobust geothermal models need to factor in heterogeneity at the scales of intrusions, metamorphic facies, and sedimentary units [ABSTRACT FROM AUTHOR]

Antarctic ice shelves are losing mass at drastically different rates, primarily due to differing rates of oceanic heat supply to their bases. However, a generalized theory for the inflow of relatively warm water into ice shelf cavities is lacking. This study proposes such a theory based on a geostrophically constrained inflow, combined with a threshold bathymetric elevation, the Highest Unconnected isoBath (HUB), that obstructs warm water access to ice shelf grounding lines. This theory captures ∼ 90% of the variance in melt rates across a suite of idealized process‐oriented ocean/ice shelf simulations with quasi‐randomized geometries. Applied to observations of ice shelf geometries and offshore hydrography, the theory captures ∼80% of the variance in measured ice shelf melt rates. These findings provide a generalized theoretical framework for melt resulting from buoyancy‐driven warm water access to geometrically complex Antarctic ice shelf cavities. Plain Language Summary: The floating extensions of Antarctic glaciers ("ice shelves") are losing ice at drastically different rates. A large component of this ice loss is due to melting from below by relatively warm ocean waters, which typically lie hundreds of meters below the surface. Previous studies have attempted to predict ice shelf melt rates using knowledge of the interface between the ice and the ocean. However, these relationships struggle to capture the variations in melt rates around Antarctica, in part because they do not account for obstruction of warm water access by variations in the shae of the seafloor. In this study we introduce a theory for the rate at which warm waters access Antarctica's ice shelves, which indirectly predicts how much the ice shelf melts. This theory is grounded in the assumption that the ocean flow beneath cavities is dominated by the rotation of the earth, and utilizes a novel quantification of seafloor obstruction of warm water inflows. We show that this theory is successful at predicting melt in simulations of ice shelves of different shapes, and in observations of real ice shelves. This work provides a theoretical grounding for melt resulting from warm subsurface waters flowing underneath Antarctic ice shelves. Key Points: We introduce a new theoretical framework for inflow of warm water into ice shelf cavities based on geostrophically‐constrained circulationA new metric, the Highest Unconnected Isobath (HUB), quantifies bathymetric barriers to warm water access in complex geometriesOur HUB‐informed theoretical framework is able to accurately predict melt rates across a suite of idealized models and observational data [ABSTRACT FROM AUTHOR]

In 2023, Antarctic sea‐ice extent (SIE) reached record lows, with winter SIE falling to 2.5Mkm2 below the satellite era average. With this multi‐model study, we investigate the occurrence of anomalies of this magnitude in latest‐generation global climate models. When these anomalies occur, SIE takes decades to recover: this indicates that SIE may transition to a new, lower, state over the next few decades. Under internal variability alone, models are extremely unlikely to simulate these anomalies, with return period >1000 years for most models. The only models with return period <1000 years for these anomalies have likely unrealistically large interannual variability. Based on extreme value theory, the return period is reduced from 2650 years under internal variability to 580 years under a strong climate change forcing scenario. Plain Language Summary: In 2023, the area of winter Antarctic sea ice fell to the lowest measured since satellite records began in late 1978. It is still under debate how far this low can be explained by natural variations, and how much can be explained by climate change. Global climate models are tools used to study past and predict future global change. We show that, without climate change, the latest generation of these models are extremely unlikely to simulate a sea‐ice reduction from the mean as large as observed in winter 2023. Including strong climate change quadruples the chance of such a reduction, but the chance is still very low. When these rare reductions are simulated, sea ice takes around 10 years to recover to a new, lower, area: this indicates that Antarctic sea ice may transition to a new, lower, state over the next few decades. Key Points: The latest generation of global climate models rarely simulate an Antarctic sea‐ice extent anomaly as negative as observed in winter 2023The return period for such an anomaly is 2650 years under internal variability, decreasing to 580 years under strong climate change forcingAfter the anomaly occurs, sea‐ice extent recovers within a decade to a new, lower state [ABSTRACT FROM AUTHOR]

The majority of ice mass loss from Antarctica flows through narrow, fast sliding regions of ice. The lateral boundaries of these regions, termed shear margins, are characterized by lateral shear strains in excess of ∼10−3 yr−1. Shear heating within these margins could warm ice significantly–even to the melting point–but other processes such as lateral advection of cold ice and fabric development compete with this effect. Radar observations can help constrain where temperate ice exists because englacial temperature increases electric conductivity which increases radar attenuation. We utilize the temperature‐dependent attenuation of ice to develop a novel method for constraining englacial temperature in shear margins by combining existing thermal models with very high frequency radar depth‐sounding data. We find evidence supporting temperate shear margins in 18 locations and find evidence for non‐temperate margins in 37 locations, notably in the Amundsen Sea Embayment. Plain Language Summary: Most of the ice lost from Antarctica to the ocean goes through narrow areas of fast sliding ice. At the sides of these fast sliding areas the fast ice abuts slow ice and heavily deforms. Deformation warms the ice–potentially to its melting temperature. This warming should be noticeable in radar data because ice at or near the melting point reduces the power of radar waves. We develop a method for using airborne radar measurements to look for the presence of this warm ice, and apply our method across the Antarctic continent. We find evidence supportive of warm ice in many locations, but we notably find little evidence of warm ice in the Amundsen Sea Embayment where the inflow of cold ice may offset warming. Key Points: We develop a novel method for identifying evidence for temperate ice in Antarctic shear margins using existing airborne radar data setsWe find evidence for temperate ice in 18, and evidence contrary in 37, of 106 radar lines over shear margins we consider across AntarcticaNeglecting heats sinks like advective cooling overpredicts the size of temperate zones, particularly in the Amundsen Sea Embayment [ABSTRACT FROM AUTHOR]

At marine‐terminating glaciers, both buoyant plumes and local currents energize turbulent exchanges that control ice melt. Because of challenges in making centimeter‐scale measurements at glaciers, these dynamics at near‐vertical ice‐ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically bolted to an underwater ice face, and use these to elucidate the interplay between buoyancy and externally forced currents in meltwater plumes. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near‐boundary stress. When external currents strengthen, the plume diffuses far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in‐situ observations to demonstrate how buoyant melt plumes energize near‐boundary turbulence, and why their dynamics are critical in predicting ice melt. Plain Language Summary: Melting glaciers are projected to produce 10s of centimeters of sea level rise over the next few decades. Despite this threat, the fundamental fluid dynamics which drive melt at tidewater glaciers remain poorly characterized. This is primarily attributed to challenges associated with measuring the temperature and velocity of ocean water at the submerged cliffs of actively calving glaciers. To this end, we have developed a robotically deployed instrument that can be bolted to a glacier's face. This instrument is capable of measuring temperature and kinetic energy of ocean waters within a few centimeters of the ice, representing the first measurements of their kind. Our observations demonstrate the ways in which meltwater at ice boundaries can accelerate melt. In particular, the meltwater tends to be less salty (and hence lighter) than the nearby ocean waters (which are salty, warm and heavy), so the meltwater rises along the ice face, creating an energetic, near boundary flow. With our new measurements, we show that these flows are as important as large‐scale currents in providing energy to the ice to fuel melt. We anticipate these data will help our community create more accurate models of ice melt needed to predict the advance or retreat of marine ice cliffs of Greenland, Alaska and Antarctica. Key Points: Robotic observations at a submerged near‐vertical iceberg face capture turbulent dynamics of buoyant melt plumes and background currentsBuoyant plumes extend 20–50 cm from the boundary and generate broad‐spectrum temperature and velocity fluctuations that drive horizontal turbulent transports of heatWhen ambient horizontal flows are weak, buoyant plumes are the dominant source of boundary layer turbulence that drives heat flux to the ice [ABSTRACT FROM AUTHOR]

Stable water isotopes in inland Antarctic ice cores are powerful paleoclimate proxies; however, their relationship with dynamical atmospheric circulations remains controversial. Using a water isotope climate model (MIROC5‐iso), we assessed the influence of the Last Glacial Maximum (LGM; ∼21,000 years ago) sea surface temperatures (SST) and sea ice (SIC) on Antarctic precipitation isotopes (δ18Op) through atmospheric circulation. The results revealed that the synoptic circulation mostly maintained southward moisture transport, reaching inland Antarctica. The steepened meridional SST gradient in the mid‐latitudes increased δ18Op in inland Antarctica with the enhanced baroclinic instability and synoptic moisture transport. In contrast, expanded SIC distribution decreased δ18Op over Antarctica by enhanced preferential removal of heavy isotopes during vapor transport due to the increased transport distance and enhanced surface cooling. These findings propose to use Antarctic ice cores to describe the southern hemisphere atmospheric circulation, represented by the westerly jets, during the LGM and other past climates. Plain Language Summary: Stable water isotopes are widely used to reconstruct past variations of Earth's climate, such as the temperature in Antarctica during the Last Glacial Maximum (LGM) ∼21,000 years ago. This is an essential period for the climate community, given the order of magnitude of the temperature change between the LGM and today, which is similar to that of today's warming. However, the relationship between stable water isotopes and temperature is still subject to debate because of the influence of other climatic factors. Using an isotope‐enabled climate model, we found that the isotopic composition of Antarctic precipitation was not simply associated with the high and low average Antarctic temperature, but was substantially influenced by changes in dynamical atmospheric circulation related to sea surface temperature and sea ice expansion. We also suggest that representation of the past atmospheric circulations, such as westerly jets, can be constrained using water isotopic signals in Antarctic ice cores. Key Points: Synoptic circulations sustained the moisture transport toward inland AntarcticaMeridional sea surface temperature gradient enhanced moisture and heavy isotope precipitations on Antarctica, unlike sea ice expansionAntarctic ice core isotopes may refine southern hemisphere atmospheric circulations during the last glacial maximum [ABSTRACT FROM AUTHOR]

Recent observations suggest that El Niño–Southern Oscillation (ENSO) impacts basal melting of West Antarctic ice shelves, yet sparse ocean observations limit our understanding of the associated processes. Here we investigate how ENSO events modulate subsurface West Antarctic shelf temperatures using high‐resolution global ocean‐sea ice model simulations. During El Niño, the subsurface shelf warming between 150 m and the shelf bottom can be up to 0.5°C in front of ice shelves. This warming arises from a weaker Amundsen Sea Low (ASL) and weaker coastal easterlies that reduce on‐shelf Ekman transport of cold surface waters, enabling enhanced transport of warm Circumpolar Deep Water (CDW) onto the shelf. A largely opposite response occurs during La Niña, with a stronger ASL and stronger Ekman transport that results in less cross‐shelf CDW transport and cooling in the subsurface. These findings have implications for interpreting basal melting on interannual to decadal time‐scales in West Antarctica. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) is the Earth's dominant year‐to‐year climate variation. The impacts of its two phases, El Niño and La Niña, extend from the tropics to Antarctica through atmospheric waves. Past studies have suggested that West Antarctic ice shelves melt more during El Niño because of warmer ocean waters at the ice shelf bases. However, oceanic changes during El Niño lead to warming on the shelf near the ice which is difficult to isolate. That is because ENSO is only one of many drivers that impact shelf water temperatures. In this work, we simulate isolated ENSO events using an ocean circulation model. We show that during El Niño, the on‐shelf flow of cold surface waters in West Antarctica, driven by coastal easterly winds, is reduced because the winds weaken. To balance out this mass deficit at the surface, more warm CDW flows onto the continental shelf below. During La Niña, we see a largely opposite response. Stronger coastal easterlies increase the on‐shelf flow of cold surface waters and less CDW is flowing onto the shelf. Our results show the link between ENSO and mass loss of the West Antarctic ice shelves and ice sheet. Key Points: Ocean‐sea ice model simulations of El Niño and La Niña events illustrate how they modulate West Antarctic shelf temperaturesEl Niño weakens coastal easterlies, reduces on‐shelf Ekman flow of cold waters, increasing cross‐shelf flow of warm Circumpolar Deep Water (CDW)The La Niña shelf circulation response is largely opposite and reduces cross‐shelf transport of warm CDW [ABSTRACT FROM AUTHOR]

Following the Hunga Tonga–Hunga Ha'apai (HTHH) eruption in January 2022, stratospheric ozone depletion was observed at Southern Hemisphere mid‐latitudes and over Antarctica during the 2022 austral wintertime and springtime, respectively. The eruption injected sulfur dioxide and unprecedented amounts of water vapor into the stratosphere. This work examines the chemistry contribution of the volcanic materials to ozone depletion using chemistry‐climate model simulations with nudged meteorology. Simulated 2022 ozone and nitrogen oxide (NOx = NO + NO2) anomalies show good agreement with satellite observations. We find that chemistry yields up to 4% ozone destruction at mid‐latitudes near ∼70 hPa in August and up to 20% ozone destruction over Antarctica near ∼80 hPa in October. Most of the ozone depletion is attributed to internal variability and dynamical changes forced by the eruption. Both the modeling and observations show a significant NOx reduction associated with the HTHH aerosol plume, indicating enhanced dinitrogen pentoxide hydrolysis on sulfate aerosol. Plain Language Summary: The January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano injected a large amount of water vapor (H2O) and moderate amounts of sulfur dioxide (SO2) into the stratosphere. Stratospheric ozone depletion was observed following the eruption at Southern Hemisphere (SH) mid‐latitudes and over Antarctica during the 2022 austral wintertime and springtime, respectively. The ozone layer in the stratosphere protects both people and the biosphere by absorbing harmful solar ultraviolet radiation. We use computer simulation to examine the impacts of chemical processes on the ozone layer from the volcanic materials. We find that chemistry results in 4% and 20% of the ozone loss at SH mid‐latitudes near 70 hPa in August and Antarctica around 80 hPa in October respectively. Most of the ozone changes are due to transport and dynamical processes from internal variability in the climate system and a forced response by the HTHH eruption. Key Points: Chemistry yields 4% and 20% ozone depletion in the lower stratosphere at mid‐latitudes and Antarctica in August and October, respectivelyThe majority of ozone depletion is ascribed to internal variability and dynamical changes induced by the eruptionHTHH aerosol plume is associated with notable NOx reduction, indicating enhanced dinitrogen pentoxide hydrolysis on sulfate aerosol [ABSTRACT FROM AUTHOR]

Permafrost is a widespread phenomenon in the cold regions of the globe and is under‐represented in global monitoring networks. This study presents a novel low‐cost, low‐power, and robust Autonomous Electrical Resistivity Tomography (A‐ERT) monitoring system and open‐source processing tools for permafrost monitoring. The processing workflow incorporates diagnostic and filtering tools and utilizes open‐source software, ResIPy, for data inversion. The workflow facilitates quick and efficient extraction of key information from large data sets. Field experiments conducted in Antarctica demonstrated the system's capability to operate in harsh and remote environments and provided high‐temporal‐resolution imaging of ground freezing and thawing dynamics. This data set and processing workflow allow for a detailed investigation of how meteorological conditions impact subsurface processes. The A‐ERT setup can complement existing monitoring networks on permafrost and is suitable for continuous monitoring in polar and mountainous regions, contributing to cryosphere research and gaining deeper insights into permafrost and active layer dynamics. Plain Language Summary: Permafrost, frozen ground in cold regions, has significant impacts on the global environment. Monitoring of permafrost is crucial because it influences the global carbon cycle, hydrology, contaminant movement, and ecosystem stability. However, current monitoring systems have limitations, particularly in remote regions like Antarctica. To tackle this challenge, a new monitoring system, Autonomous Electrical Resistivity Tomography (A‐ERT), was introduced. A‐ERT is a geophysical technique that employs electrical signals to study ground freezes and thaws with high precision over time. Alongside this, open‐source processing tools were developed to process obtained A‐ERT data and efficiently extract essential information from large data sets. The developed A‐ERT system is robust, low‐cost, low‐power, and designed to operate in harsh conditions. Tested in Antarctica, our findings show that A‐ERT data combined with processing pipelines offers a valuable tool for examining freezing and thawing processes in extreme environments. The proposed setup can contribute to a network of autonomous permafrost monitoring systems, important for cryosphere research and advancing our understanding of climate change's impact on permafrost dynamics. Key Points: We present a robust low‐cost Autonomous Electrical Resistivity Tomography system for permafrost monitoring in polar and mountainous regionsWe introduce an open‐source tool for processing and inverting large data sets, enabling quick and efficient extraction of key informationField experiments conducted in Antarctica show high‐temporal‐resolution imaging of ground freezing and thawing dynamics [ABSTRACT FROM AUTHOR]

We examine the relationships between the observed long‐term trends of the zonal wind in the mesopause regions at King Sejong Station (KSS), Antarctica, and wind trends in the Southern Hemisphere (SH) middle atmosphere using the 15‐year data set from KSS meteor radar, Aura MLS and MERRA‐2. During July, significant positive trends of zonal winds appear above z = 90 km and near the stratopause over the KSS, while negative trends exist between the two layers. In the SH winter, the observed mesopause winds correlate positively (negatively) with stratospheric (mesospheric) winds in the polar region, while they exhibit opposite correlations with the low‐latitude winds. The positive mesopause trends of zonal winds near KSS are connected, through the thermal wind relationship, to cooling (warming) trends induced by the upward (downward) trends of residual circulation over the high‐latitude mesosphere and low‐latitude stratosphere (high‐latitude stratosphere), which shows vertical coupling throughout the SH winter middle atmosphere. Plain Language Summary: Based on 15‐year (2007–2021) observations of horizontal winds obtained at mesopause altitudes of 80–100 km by a meteor radar at King Sejong Station (KSS; 62.22°S, 58.78°W), Antarctica, we find long‐term trends of eastward winds above 90 km in July. To investigate the relationship between the observed long‐term trends of the mesopause wind over the Southern Hemisphere (SH) polar region and winds in the SH middle atmosphere, we analyze Aura MLS satellite data and MERRA‐2 reanalysis data. In the SH winter, the mesopause zonal winds over polar regions are positively (negatively) correlated with zonal winds in the polar stratosphere (polar mesosphere), while the opposite correlations are found between the polar mesopause and low latitudes. We demonstrate that the trends of temperature induced by adiabatic warming or cooling associated with meridional circulation in the stratosphere and mesosphere are connected to the observed mesopause wind trends in the polar region (KSS), which accounts for the vertical coupling between the mesopause region and stratosphere across the mesosphere. Key Points: Trends of zonal winds in the Southern Hemisphere mesopause and stratosphere are investigated using meteor radar and satellite observationsSignificant correlations are found between the mesopause wind at high latitudes and the winds in the middle atmosphereThe observed wind trends in austral winter are coupled vertically between the mesopause region and stratosphere across the mesosphere [ABSTRACT FROM AUTHOR]

The Southern Ocean plays a major role in controlling the evolution of Antarctic glaciers and in turn their impact on sea level rise. We present the Southern Ocean high‐resolution (SOhi) simulation of the MITgcm ocean model to reproduce ice‐ocean interaction at 1/24° around Antarctica, including all ice shelf cavities and oceanic tides. We evaluate the model accuracy on the continental shelf using Marine Mammals Exploring the Oceans Pole to Pole data and compare the results with three other MITgcm ocean models (ECCO4, SOSE, and LLC4320) and the ISMIP6 temperature reconstruction. Below 400 m, all the models exhibit a warm bias on the continental shelf, but the bias is reduced in the high‐resolution simulations. We hypothesize some of the bias is due to an overestimation of sea ice cover, which reduces heat loss to the atmosphere. Both high‐resolution and accurate bathymetry are required to improve model accuracy around Antarctica. Plain Language Summary: Warm water from the Southern Ocean melts the glaciers and ice shelves around the Antarctic margin, leading to glacier de‐stabilization, and sea level rise. We present the Southern Ocean high‐resolution (SOhi) model to better represent ocean circulation and ice‐ocean interaction around Antarctica. We assess the accuracy of SOhi with in situ data from marine mammals and compare the results with three other ocean model simulations and to a baseline reference. All model results are slightly too warm on the continental shelf, but the higher‐resolution models yield colder waters in better agreement with observations. We attribute the warm bias to an overestimation of the sea ice cover in the ocean models. An improved bathymetry also significantly improves model accuracy in Antarctica. Key Points: High‐resolution ocean models with more complete physics are in better agreement with in situ ocean data than coarser resolution modelsAccurate bathymetry is essential to capture circulation pathways and warm water intrusions on the Antarctic continental shelfHigh‐resolution ocean models may remain too warm on the continental shelf because they over‐predict the sea ice cover [ABSTRACT FROM AUTHOR]

In some areas of Antarctica, blue‐colored bare ice is exposed at the surface. These blue ice areas (BIAs) can trap meteorites or old ice and are vital for understanding the climatic history. By combining multi‐sensor remote sensing data (MODIS, RADARSAT‐2, and TanDEM‐X) in a deep learning framework, we map blue ice across the continent at 200‐m resolution. We use a novel methodology for image segmentation with "noisy" labels to learn an underlying "clean" pattern with a neural network. In total, BIAs cover ca. 140,000 km2 (∼1%) of Antarctica, of which nearly 50% located within 20 km of the grounding line. There, the low albedo of blue ice enhances melt‐water production and its mapping is crucial for mass balance studies that determine the stability of the ice sheet. Moreover, the map provides input for fieldwork missions and can act as constraint for other geophysical mapping efforts. Plain Language Summary: While most of the continent of Antarctica is covered by snow, in some areas, ice is exposed at the surface, with a typical blue color. At lower elevations, blue ice enhances melt‐water production, which is important for studying the future of the ice sheet. Moreover, scientific teams frequently visit blue ice areas (BIAs) as they act as traps for meteorites and very old ice. In this study, we map the extent and the exact location of BIAs using various satellite observations. These diverse observations are efficiently combined in an artificial intelligence algorithm. We develop the algorithm so that it can learn to map blue ice even though existing training labels, which teach the algorithm what blue ice looks like, are imperfect. We quantify that the new map scores better on various performance metrics compared to the current most‐used blue ice map. Moreover, for the first time, we estimate uncertainties of the detection of blue ice. The map indicates that ca. 1% of the surface of Antarctica exposes blue ice and will be important for fieldwork missions and understanding surface processes leading to melt and potential sea level rise. Key Points: We map blue ice areas in Antarctica by combining multi‐sensor satellite observations in a convolutional neural networkBlue ice covers ca. 140,000 km2 (∼1%) of Antarctica, of which ca. 50% located in the grounding zoneOur map will improve mass balance estimates and studies on ice‐shelf stability, and will support searches for meteorites or old ice [ABSTRACT FROM AUTHOR]

The location and origin of Neoproterozoic‐Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic‐Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a one‐dimensional (1D) joint seismic‐MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6–8 km shows up in the stacked image of teleseismic P‐wave receiver function analysis; coinciding with the abrupt Moho offset, a near‐vertical channel with (a) low resistivity extending to the uppermost mantle depths, and (b) high crustal Poisson's ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it. Plain Language Summary: Our study seeks to unravel the history of a supercontinent called Gondwana. We do this by exploring ancient geological features known as sutures. These sutures are like stitches that hold the Earth's crust together, and they're crucial in understanding how continents were once connected. We specifically focused on a place in Antarctica called the Larsemann Hills, which is located near an important suture zone. This region hasn't received much attention from scientists until now. During the 36th Chinese National Antarctic Research Expedition, we made some exciting discoveries. We found a clear boundary in the Earth's crust, a bit like a seam in a piece of clothing. At the same time, we noticed a unique underground pathway. This pathway had special properties, suggesting that it reaches deep into the Earth's mantle. It's a bit like finding a hidden treasure beneath the Earth's surface. Our findings strongly suggest a connection between these underground discoveries and the ancient sutures in the Earth's crust. In other words, we're piecing together a puzzle that can help us learn more about the Earth's past and how continents have moved over millions of years. Key Points: A distinct Moho discontinuity offset of 6–8 km is found in the stacked image obtained from P‐wave receiver functionIn conjunction with the abrupt Moho offset, a nearly vertical conduit with low resistivity and high Poisson's ratio is identifiedThese geophysical results provide crucial evidence for determining the collisional nature and location of the Prydz orogenic belt [ABSTRACT FROM AUTHOR]

We simulated the seasonal temperature evolution in the atmosphere of Antarctica and the Arctic focusing on infrared processes. Contributions by other processes were parametrized and kept fixed throughout the simulations. The model was run for current CO2 and CH4 and for doubled concentrations. For doubling CH4 the warming in Antarctica is restricted to the lowest few hundred meters above the surface while in the Arctic we find a warming in the whole troposphere. We find that the amount of water is the main driver for the differences between both polar regions. When increasing both, CO2 and CH4 from pre‐industrial values to current concentrations, and averaged over the whole troposphere, we find a warming of 0.42 K for the Arctic and a slight cooling of 0.01 K for Antarctica. Our results contribute to the understanding of the lack of warming seen in Antarctica throughout the last decades. Plain Language Summary: In 2015 we have initiated a discussion on a fundamental property of the radiation in the atmosphere over Antarctica: The negative greenhouse effect (Schmithüsen et al., 2015, https://doi.org/10.1002/2015GL066749). A negative greenhouse effect means, the atmosphere emits more radiation to space than it receives from the surface. This results in a cooling somewhere in the Antarctic atmosphere during some months of the year, when increasing CO2. We now simulate how the Antarctic atmospheric temperature responds in all altitude levels to CO2 and CH4 increases, and show this is different from the temperature response in the Arctic. We show for example, that an increase in CH4 cools nearly the whole troposphere, although the response for CH4 is much lower in amplitude than for CO2. We find that the amount of water is the main driver for the differences between both polar regions. Since the amount of water vapor strongly depends on temperature, the colder Antarctic atmosphere responds differently to the Arctic when greenhouse gases increase. Our studies could be one important factor when understanding the lack of warming in Antarctica throughout the last decades. Key Points: We simulate the temperature development in both polar regions in the infrared and find that doubling CO2 and CH4 lead to opposing forcingsThe different amount of water vapor shows to be responsible for the differences in warming/cooling in both polar regionsIn Antarctica doubling CH4 leads to a cooling of almost the whole troposphere, a future increase in H2O could invert this [ABSTRACT FROM AUTHOR]

Deception Island is one of the most active and best‐documented volcanoes in Antarctica. Since its last eruption in 1970, several geophysical surveys have targeted reconstructing its magmatic systems. However, geophysics fails to reconstruct the pathways magma and fluids follow from depth to erupt at the surface. Here, novel data selection strategies and multi‐frequency absorption inversions have been framed in a Geographical Information System, using all available geological (vents and faults distribution), geochemical and geophysical knowledge of the volcano. The result is the detection of these eruptive pathways. The model offers the first image of the magma and associated fluids pathways feed the 1967, 1969, and 1970 eruptions. Results suggest that future ascending paths might lead to active research bases and zones of planned helicopter rescue. The connection between seismic absorption, temperature, and fluid content makes it a promising attribute for detecting and monitoring eruptions at active calderas. Plain Language Summary: Deception Island is the gateway for tourists to Antarctica and a laboratory to understand ice‐capped volcanoes and their eruptions. While the Island has been the target of many geophysical studies, no clear tomographic model shows how deep eruptive pathways of its last eruptions have reached the surface in the 1960s and 1970s. This is a recurrent topic in volcano geophysics: dikes and fluid migrations develop across structures considered too small to be detected by tomographic techniques. This paper demonstrates that seismic absorption has sufficient sensitivity to temperature and fluid content to detect these pathways. Once integrated within a Geographical Information System with all the information we have on the volcano, the models resolve the feeding systems of these eruptions, from a tectonically deformed deep magma chamber to shallow cold dyke intrusions and fluid migrations still feeding the volcano today. The correlation between seismic absorption, temperature, and fluid content offers a new tool for detecting and monitoring shallow volcanic hazards. Key Points: High absorption detects deep eruptive pathways from the caldera center to its rimAbsorption imaging reconstructs shallow pathways of hazardous materialsSeismic absorption is sensitive to thermal anomalies at depth [ABSTRACT FROM AUTHOR]

Ice shelves regulate the stability of marine ice sheets. We track fractures on Pine Island Glacier, a quickly accelerating glacier in West Antarctica that contributes more to sea level rise than any other glacier. Using an on‐ice seismic network deployed from 2012 to 2014, we catalog icequakes that dominantly consist of flexural gravity waves. Icequakes occur near the rift tip and in two distinct areas of the shear margin, and TerraSAR‐X imagery shows significant fracture in each source region. Rift‐tip icequakes increase with ice speed, linking rift fracture to glaciological stresses and/or localized thinning. Using a simple flexural gravity wave model, we deconvolve wave propagation effects to estimate icequake source durations of 19.5–50.0 s and transient loads of 3.8–14.0 kPa corresponding to 4.3–15.9 m of crevasse growth per icequake. These long‐source durations suggest that water flow may limit the rate of crevasse opening. Plain Language Summary: Large shelves of floating ice strengthen glaciers in Antarctica, helping to protect against rapid sea level rise that can occur when glaciers flow into the ocean. Ice shelves can collapse through rapid cracking (synonym of fracturing), but it is difficult to directly observe cracking on ice shelves. In this paper, we track cracks on Pine Island Glacier, an ice shelf in Antarctica that is particularly vulnerable to collapse. We see cracks in pictures taken by satellites. Cracking causes the ice shelf to shake up and down, which we record using the same equipment that records earthquakes. We record shaking located at a set of cracks at the side of the ice shelf and at the tip of a single massive crack called a rift. Rift cracking seems related to the speed that the ice shelf is flowing. We also use a computer simulation of shaking to learn about the details of the crack process. Our simulation suggests that the crack process might be more complicated than a single crack opening evenly at a constant rate. Key Points: Fracture at Pine Island Glacier generates flexural gravity waves, a wave type related to interaction between a floating plate and supporting fluidRift‐tip seismicity rate increases with ice speed either due to changes in the underlying ice shelf stress state or localized thinningRecorded flexural gravity waves are consistent with a point load of ∼10 kPa applied over ∼30s, corresponding to ∼10m of vertical cracking [ABSTRACT FROM AUTHOR]

The additional water from the Antarctic ice sheet and ice shelves due to climate‐induced melt can impact ocean circulation and global climate. However, the major processes driving melt are not adequately represented in Coupled Model Intercomparison Project phase 6 (CMIP6) models. Here, we analyze a novel multi‐model ensemble of CMIP6 models with consistent meltwater addition to examine the robustness of the modeled response to meltwater, which has not been possible in previous single‐model studies. Antarctic meltwater addition induces a substantial weakening of open‐ocean deep convection. Additionally, Antarctic Bottom Water warms, its volume contracts, and the sea surface cools. However, the magnitude of the reduction varies greatly across models, with differing anomalies correlated with their respective mean‐state climatology, indicating the state‐dependency of the climate response to meltwater. A better representation of the Southern Ocean mean state is necessary for narrowing the inter‐model spread of response to Antarctic meltwater. Plain Language Summary: The melting of the Antarctic ice sheet and ice shelves can have significant impacts on ocean circulation and thermal structure, but current climate models do not fully capture these effects. In this study, we analyze seven climate models to understand how they respond to the addition of meltwater from Antarctica. We find that the presence of Antarctic meltwater leads to a significant weakening of deep convection in the open ocean. The meltwater also causes Antarctic Bottom Water to warm and its volume to decrease, while the sea surface cools and sea ice expands. However, the magnitude of the response to meltwater varies across models, suggesting that the mean‐state conditions of the Southern Ocean play a role. A better representation of the mean state and the inclusion of Antarctic meltwater in climate models will help reduce uncertainties and improve our understanding of the impact of Antarctic meltwater on climate. Key Points: Antarctic meltwater substantially reduces the strength of simulated Southern Ocean deep convection in climate modelsThe additional meltwater induces Antarctic Bottom Water warming and contraction, with dense water classes converting to lighter onesDifferences in the magnitude of these responses between models can be partly attributed to their different base states [ABSTRACT FROM AUTHOR]

From the Eocene (∼50 million years ago) to today, Southern Ocean circulation has evolved from the existence of two ocean gyres to the dominance of the Antarctic Circumpolar Current (ACC). It has generally been thought that the opening of Southern Ocean gateways in the late Eocene, in addition to the alignment of westerly winds with these gateways or the presence of the Antarctic ice sheet, was a sufficient requirement for the transition to an ACC of similar strength to its modern equivalent. Nevertheless, models representing these changes produce a much weaker ACC. Here we show, using an eddying ocean model, that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. This deep convection is caused by cold temperatures and high salinities due to sea‐ice production around the Antarctic continent, leading to both the formation of Antarctic Bottom Water and a modern‐strength ACC. Plain Language Summary: The evolution of ocean circulation from the early Southern Ocean around 50 million years ago to today has seen much debate over the past decades. The main characteristic of the modern Southern Ocean is the prevalence of the Antarctic Circumpolar Current (ACC), the world's strongest current. In the past it has been thought that the deepening of ocean gateways, and changes in the strength and location of winds, led to an ACC of similar strength to its modern equivalent. Nevertheless, ocean models simulating these changes typically reproduce an ACC with less than a third of the modern ACC's strength. Here we show that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. Deep convection is due to a combination of cooling and increase in salinity by sea ice formation which allows for water at the ocean surface to become denser than the water below, leading to the mixing of the water column to great depths. This deep convection allows for the ocean to be energized, leading to a modern‐strength Antarctic Circumpolar Current. A cool climate around the Antarctic continent is therefore crucial for the development of a modern‐strength Antarctic Circumpolar Current. Key Points: Deep convection is able to drive a strong Antarctic Circumpolar Current (ACC)Surface cooling and sea ice formation are crucial drivers of this deep convectionACC similar to a rim current around convective sites, energizing ocean circulation [ABSTRACT FROM AUTHOR]

Recent mass loss from ice sheets and ice shelves is now persistent and prolonged enough that it impacts downstream oceanographic conditions. To demonstrate this, we use an ensemble of coupled GISS‐E2.1‐G simulations forced with historical estimates of anomalous freshwater, in addition to other climate forcings, from 1990 through 2019. There are detectable differences in zonal‐mean sea surface temperatures (SST) and sea ice in the Southern Ocean, and in regional sea level around Antarctica and in the western North Atlantic. These impacts mostly improve the model's representation of historical changes, including reversing the forced trends in Antarctic sea ice. The changes in SST may have implications for estimates of the SST pattern effect on climate sensitivity and for cloud feedbacks. We conclude that the changes are sufficiently large that model groups should strive to include more accurate estimates of these drivers in all‐forcing historical simulations in future coupled model intercomparisons. Plain Language Summary: Simulations of recent historical periods are a key test of climate model reliability and skill. These model simulations require an accounting of all the drivers of climate change. We show that the impact of historical changes in freshwater fluxes from ice sheets and ice shelves on the ocean (through changes in salinity and stratification) are detectable in sea surface temperature and sea ice trends, and help improve the match between the modeled climate changes and observations. We recommend that more accurate estimates of these drivers be included in all climate simulations that do not explicitly model ice sheets and ice shelves. Key Points: The response to anomalous meltwater from ice sheets and shelves is large enough for it to be a forcing in historical climate simulationsWhen the GISS model includes these drivers, Southern Ocean SST and sea ice trends better match observationsSteric and dynamic impacts on regional sea level in parts of the North Atlantic and coastal Antarctica are significant [ABSTRACT FROM AUTHOR]

Multidecadal satellite observations indicate that the Antarctic Ice Sheet (AIS) is losing mass at an accelerating rate, which has the potential to impact many aspects of the climate system. While previous studies demonstrated the importance of AIS freshwater (FW) discharge for regional and global climate processes using climate model experiments, many have applied unrealistic FW forcings. Here, we explore potential Southern Ocean (SO) impacts of realistic AIS mass loss over the 21st century in the Community Earth System Model version 2 by applying observation‐based historical and ice sheet model‐based future AIS FW forcing. The added FW reduces wintertime deep convective area by 72% while retaining 83% more sea ice. Congruent with other studies, we find the increased FW discharge extensively impacts local and remote SO surface and subsurface temperature and stratification. These results demonstrate the necessity of accounting for AIS mass loss in global climate models for projecting future climate. Plain Language Summary: We know from several decades of satellite observations that the Antarctic Ice Sheet is losing mass at an accelerating rate. This accelerated mass loss (solid and liquid FW) is often poorly represented—if at all—in global climate models (GCM) and previous studies indicate that even a simple representation of mass loss can have profound climate impacts. Here, we apply a more realistic (in space and time) representation of Antarctic mass loss to a GCM. We find that the added FW leads to more retention of Antarctic sea ice, a decline in wintertime convection of surface waters to the interior ocean, a cooler surface but warmer interior ocean, and a more strongly stratified upper ocean. As such, we argue that properly accounting for Antarctic mass loss in GCMs is imperative for accurately projecting long‐term future climate change. Key Points: We explore impacts of realistic Antarctic Ice Sheet (AIS) freshwater discharge on the Southern Ocean (SO) using a state‐of‐the‐art climate modelAIS discharge drives drastic changes in SO stratification, winter deep convection, surface and interior temperature, and sea iceOur results suggest that regional AIS discharge can have far‐reaching impacts on the SO that can feedback on the climate system [ABSTRACT FROM AUTHOR]

The expansion of Antarctic sea ice since 1979 in the presence of increasing greenhouse gases remains one of the most puzzling features of current climate change. Some studies have proposed that the formation of the ozone hole, via the Southern Annular Mode, might explain that expansion, and a recent paper highlighted a robust causal link between summertime Southern Annular Mode (SAM) anomalies and sea ice anomalies in the subsequent autumn. Here we show that many models are able to capture this relationship between the SAM and sea ice, but also emphasize that the SAM only explains a small fraction of the year‐to‐year variability. Finally, examining multidecadal trends, in models and in observations, we confirm the findings of several previous studies and conclude that the SAM–and thus the ozone hole–are not the primary drivers of the sea ice expansion around Antarctica in recent decades. Plain Language Summary: Unlike its Arctic counterpart, sea ice around Antarctica has been growing since 1979, even as the levels of carbon dioxide in the atmosphere have increased. Given that the ozone hole formed over the South Pole around the same time, one is led to ask whether the ozone hole may be responsible for the growth of Antarctic sea ice (recall that there is no ozone hole over the North Pole). In this study, looking at both models and observations, we show that the ozone hole is capable of affecting the surface winds and these, in turn, can make sea ice expand. However, the magnitude of this effect is small. Also since the ozone hole started healing after the year 2000, while Antarctic sea ice kept expanding, we conclude that ozone depletion is not the main reason for the expansion of Antarctic sea ice in recent decades. Key Points: Many CMIP5 models are able to capture the observed seasonal correlation between summertime Southern Annular Mode (SAM) and Antarctic sea ice extentThe SAM, however, only explains 15 of the year‐to‐year sea ice extent variability in the fall, in both models and observationsSAM trends, and ozone depletion, are not the primary drivers of the observed Antarctic sea ice expansion in the last four decades [ABSTRACT FROM AUTHOR]

Ice cores provide detailed records of past climate change. Water stable isotopes are the most commonly used ice core climate proxy, but their quantitative interpretation remains challenging. Here, I argue that the gas age‐ice age difference (Δage) is a powerful proxy for past surface temperature. An analytical framework is derived that directly links past temperature to firn properties that can be reliably reconstructed (Δage, lock‐in depth). The framework is calibrated using both present‐day spatial patterns and last glacial maximum temperatures reconstructed via borehole thermometry. The usefulness of the method is demonstrated using three case studies from Greenland and Antarctic ice cores. The calibration suggests that several firn densification models, with the possible exception of the Herron‐Langway model, have insufficient sensitivity to accumulation rates. This low sensitivity, in combination with large amplitude temperature forcing, can explain historical difficulties of densification models in simulating ice age firn thickness in East Antarctica. Plain Language Summary: It is important for scientists to understand past natural climate change. Ice cores drilled in the polar regions contain ancient ice up to 800,000 years old and can be used to reconstruct past climate. Ice from the polar regions contains air bubbles that are trapped at the bottom of the thick (50–120 m) perennial snow pack called the firn. The air in these bubbles is younger than the ice that surrounds it, and this age difference is called Δage. This paper develops a new method to estimate past temperatures of the ice sheet surface using our knowledge of changes in ice core Δage. The method is very simple and fast and agrees very well with independent temperature reconstructions where available. Key Points: The ice core gas age‐ice age difference (Δage) is a powerful proxy for past surface temperatureA simple analytical framework allows for reliable past temperature estimation using empirical estimates of ΔageLow firn model sensitivity to accumulation rate contributes to published model‐data mismatch of ice age firn thickness in East Antarctica [ABSTRACT FROM AUTHOR]

Iceberg A‐68 separated from the Larsen C Ice Shelf in July 2017 and the impact of this event on the local ocean circulation has yet to be assessed. Here, we conduct numerical simulations of ocean dynamics near and below the ice shelf pre‐ and post‐calving. Results agree with in situ and remote observations of the area as they indicate that basal melt is primarily controlled by wintertime sea‐ice formation, which in turn produces High Salinity Shelf Water (HSSW). After the calving event, we simulate a 50% increase in HSSW intrusion under the ice shelf, enhancing ocean heat delivery by 30%. This results in doubling of the melt rate under Gipps Ice Rise, suggesting a positive feedback for further retreat that could destabilize the Larsen C Ice Shelf. Assessing the impact of ice‐front retreat on the heat delivery under the ice is crucial to better understand ice‐shelf dynamics in a warming environment. Plain Language Summary: In July 2017, the Larsen C Ice Shelf, Antarctica, calved one of the largest icebergs ever observed: A‐68. The impact of this event on the ocean circulation near and below the ice remains unknown. Our simulations from an ocean model suggest that the iceberg separation may have led to increased ocean heat delivery under the ice, yielding more vigorous bottom melt. Key Points: We present ocean simulations under Larsen C in good agreement with local ocean, basal melt and sea‐ice observationsThe removal of iceberg A‐68 yields heat intrusion to increase by 30% causing melt around Gipps Ice Rise to doubleOcean models need to consider ice‐front retreat events and realistic ice‐shelf geometry to accurately project sub‐shelf ocean circulation [ABSTRACT FROM AUTHOR]

We investigate feedbacks between subsurface continental shelf ocean temperatures and Antarctic glacial melt using a coupled climate model. The model was forced with SSP5‐8.5 and an uncoupled projection of basal melt and calving fluxes. SSP5‐8.5 forcing with fixed pre‐industrial glacial melt warms all continental shelves, such that historically "cool" and "fresh" shelves transition to "warm." Additional glacial melt, added at depth, cools the Eastern Ross, Amundsen, and Bellingshausen seas, suggesting a negative feedback on basal melt—a novel result for a coarse resolution coupled model. From the Weddell Sea, along East Antarctica, and into the western Ross Sea—where continental shelves transition to a "warm" state—additional glacial melt increases temperatures at the continental shelf sea floor, suggesting a positive feedback. The sign of the glacial melt–subsurface temperature feedback is critically dependent on continental shelf properties, climate state, and the vertical distribution of glacial melt inputs. Plain Language Summary: Antarctic ice shelves lose mass as freshwater—primarily from melting at their base and when icebergs break off. We know that the rate at which Antarctic ice shelves are losing mass is increasing and we expect this freshwater input into the ocean to increase further in a warmer world. Most climate models do not include a representation of this changing freshwater input, and its climatic impacts are uncertain. One particularly important climate impact is how the additional freshwater changes the subsurface ocean temperature. If freshwater warms/cools the coastal ocean it could increase/decrease melt in a positive/negative feedback loop. But existing studies disagree about this effect and even about the sign of the feedback. We added a timeseries of freshwater input to a climate model and ran experiments with strong climate warming to investigate this feedback. The sign of the feedback varies by region. Where climate change drives a cold coastal ocean to transition to a new warmer state, we find that freshwater causes a strong additional warming (a positive feedback). Where the coastal ocean is already warm in the present day and warms in future, freshwater reduces this warming (a negative feedback). To achieve the negative feedback freshwater must be added at depth. Key Points: A glacial melt projection is added to a 1° coupled climate model under SSP5‐8.5 to investigate ocean temperature feedbacksAdditional glacial melt cools the Eastern Ross, Amundsen, and Bellingshausen seas at depth, but warms elsewhereThe addition of glacial melt at depth is necessary to generate the cooling response [ABSTRACT FROM AUTHOR]

Air‐sea exchange of carbon dioxide (CO2) in the Southern Ocean plays an important role in the global carbon budget. Previous studies have suggested that flow around topographic features of the Southern Ocean enhances the upward supply of carbon from the deep to the surface, influencing air‐sea CO2 exchange. Here, we investigate the role of seafloor topography on the transport of carbon and associated air‐sea CO2 flux in an idealized channel model. We find elevated CO2 outgassing upstream of a seafloor ridge, driven by anomalous advection of dissolved inorganic carbon. Argo‐like Lagrangian particles in our channel model sample heterogeneously in the vicinity of the seafloor ridge, which could impact float‐based estimates of CO2 flux. Plain Language Summary: The Southern Ocean, the ocean surrounding Antarctica, contributes significantly to carbon exchange between the global ocean and the atmosphere, which in turn matters for climate change. Here, we use a simplified model of the Southern Ocean to see how mountain ranges on the sea floor influence the carbon exchange at the ocean‐atmosphere interface. We find that the seafloor mountain ranges lead to more carbon exchange. Floating carbon sensors in our model ocean may under or over sample the water near the mountains and this can affect the carbon exchange that they report. Key Points: We examine the localized patterns of air‐sea carbon dioxide (CO2) fluxes in an idealized Southern Ocean‐like model with simple biogeochemistryWe find intense sea‐air CO2 fluxes upstream of seafloor topography driven by anomalous advection of inorganic carbonDue to the topography, uncertainty in the flux is highly sensitive to sampling network design [ABSTRACT FROM AUTHOR]

We have discovered that the peak phase time of predawn thermosphere‐ionosphere Na (TINa) layers (∼110–150 km altitude) undergoes clear annual variations with the earliest occurrence in summer and latest in winter over Boulder (40.13°N, 105.24°W), which are closely correlated to annual phase variations of sunrise and tidal winds. Such discoveries were enabled by the first characterization of 12 monthly composites of TINa layers from January through December using 7 years of lidar observations (2011–2017). Despite their tenuous densities, the predawn TINa layers have nearly 100% occurrence rate (160 out of 164 nights of observations). Monthly composites show downward‐phase‐progression TINa descending at similar phase speeds as Climatological Tidal Model of the Thermosphere tidal winds. These TINa layers occur in ion convergence but neutral divergence regions, modeled using tidal winds. These results support the formation mechanism (neutralization of converged TINa+ forming TINa) proposed previously and suggest that migrating tidal winds experience annual phase variations. Plain Language Summary: With tons of cosmic dust falling on Earth every day, metallic atoms and ions (e.g., Fe, Na, K, and Ca+) are released via meteor ablation and sputtering into the upper atmosphere, forming permanent metal layers (∼75–105 km) that have been known for nearly a century. What was unknown until 2011 was the existence of thermosphere‐ionosphere metal (TIMt) layers that were discovered with high‐sensitivity lidar observations first from Antarctica and then were observed globally. These neutral TIMt layers locate above the permanent layers with an upper reach to ∼200 km and exhibit intermittent occurrence. In 2021 surprising regular occurrence of TIMt layers in Na species (TINa) was reported for the first time from lidar observations over Boulder, Colorado, where TINa layers occur before dawn and after dusk nearly every night. By analyzing 7 years of lidar data, we have further discovered that the Boulder predawn TINa layers occur in earlier hours in summer than in winter. Such annual phase variations are correlated with sunrise and solar‐heating‐driven tidal winds. These TIMt layers are of great scientific interest as they provide unique tracers for making direct measurements in the least understood but crucially important "thermospheric gap" region of 100–200 km. Key Points: First characterization of 12 monthly composites of Na mixing ratios reveals annual phase variations of predawn thermosphere‐ionosphere Na (TINa) layers (110–150 km)Predawn TINa occur ∼2.5 hr earlier in summer than in winter, closely correlating to annual phase variations of sunrise and Climatological Tidal Model of the Thermosphere semidiurnal westward‐propagating tidal windsTINa layers descend at similar rates as and in tidal‐wind‐modeled ion convergence regions, supporting neutralization of TINa+ forming TINa [ABSTRACT FROM AUTHOR]

Giant icebergs release cold, fresh meltwater as they drift, perturbing the physical conditions of the surface ocean. This study uses satellite‐derived sea surface salinity and temperature measurements to explore the physical impact of supergiant iceberg A68A between September 2020 and June 2021. During A68A's drift through the Scotia Sea in austral spring, gradual but persistent edge‐wasting contributed to a freshening of several psu extending hundreds of kilometers ahead of the iceberg, whilst the cooling signal was more pronounced in the iceberg's wake. The magnitude of the physical perturbation intensified during A68A's breakup near South Georgia. Several large meltwater lenses surrounding the descendant icebergs displayed temperature anomalies of up to −4.5°C, whilst the salinity measurements indicated a surface (skin‐depth) anomaly regularly exceeding order −10 psu. The perturbations stretched at times >1,000 km and persisted for >2 months following A68A's melt in April 2021. Plain Language Summary: Giant icebergs release cold, fresh meltwater as they drift, altering the conditions of the ocean. This study uses sea surface salinity and temperature data collected by satellites to explore these impacts for iceberg A68 between September 2020 and June 2021. During A68A's drift through the Scotia Sea, a freshening of several psu extended hundreds of kilometers ahead of the iceberg, and a pronounced cooling followed behind. The magnitude of the salinity and temperature anomalies intensified when A68A fragmented near South Georgia. The surface ocean displayed temperature anomalies of up to −4.5°C, and salinity anomalies regularly exceeding −10 psu. The perturbation stretched at times >1,000 km and persisted for >2 months following A68A's melt in April 2021. Key Points: Meltwater released from giant iceberg A68A caused large scale freshening and cooling of the surrounding surface oceanIntense temperature and salinity anomalies up to −4.5°C and exceeding −10 psu were recorded during the breakup near South GeorgiaThe freshening and cooling signals stretched at times over 1,000 km and persisted for over 2 months following the iceberg's melt [ABSTRACT FROM AUTHOR]

The Weddell Gyre mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate. This region also features large and highly variable freshwater fluxes due to seasonal sea ice, net precipitation, and glacial melt; however, the impact of these freshwater fluxes on the regional carbon cycle has not been fully appreciated. Using a novel budget analysis of dissolved inorganic carbon (DIC) mass in the Biogeochemical Southern Ocean State Estimate, we highlight two freshwater‐driven transports. Where freshwater with minimal DIC enters the ocean, it displaces DIC‐rich seawater outwards, driving a lateral transport of 75 ± 5 Tg DIC/year. Additionally, sea ice export requires a compensating import of seawater, which carries 48 ± 11 Tg DIC/year into the gyre. Though often overlooked, these freshwater displacement effects are of leading order in the Weddell Gyre carbon budget in the state estimate and in regrouped box‐inversion estimates, with implications for evaluating basin‐scale carbon transport. Plain Language Summary: The ocean surrounding Antarctica plays a key role in the global carbon cycle because it is one of the few places where carbon‐enriched abyssal waters can exchange with the atmosphere. Here we focus on the Weddell Gyre that covers the Atlantic sector poleward of 60°S; this region features enormous exchanges of freshwater through the surface, via sea ice that covers the region each winter, frequent precipitation, and land ice melting into the ocean. Using a model that assimilates available observations, we explore how this freshwater affects the region's ability to transport carbon between the atmosphere and deep ocean. Whereas previous studies have focused on how springtime sea ice melt dilutes surface ocean carbon concentrations, we emphasize that throughout the year, adding water with minimal carbon (from rain, snow, or land ice) to the ocean displaces carbon‐rich seawater. Second, sea ice melt and formation alone do not displace seawater, but when sea ice flows out of the region, carbon‐rich seawater responds by flowing in the opposite direction. These freshwater displacement effects are leading order terms of the regional carbon budget in our model analysis and in our reconsideration of past observational estimates. Key Points: Displacement of seawater by net precipitation and glacial melt is a leading order term in Weddell Gyre dissolved inorganic carbon (DIC) budgetEvidence is provided by quantifying the DIC mass budget in Biogeochemical Southern Ocean State Estimate (B‐SOSE) and regrouping published box inversion resultsIn the B‐SOSE mean, freshwater displaces 75 Tg DIC/yr, whereas sea ice export drives a 48 Tg DIC/yr lateral import [ABSTRACT FROM AUTHOR]

The recent discovery of warm ocean water near the Totten Ice Shelf (TIS) has increased attention to the Sabrina Coast in East Antarctica. We report the result of 6‐day helicopter‐based observations conducted during the 61st Japanese Antarctic Research Expedition (JARE61), revealing warm ocean water (0.5–1°C) occupying a large previously unsampled area of the Sabrina Coast (116.5°E−120°E) below 550–600 m. Along the TIS front, we observe modified Circumpolar Deep Water (mCDW) well above freezing (∼−0.7°C), consistent with previous work. We identify glacial meltwater outflow from the TIS cavity west of 116°E. No signs of mCDW intrusions toward the Moscow University Ice Shelf cavity are observed; however, those observations were limited to only two shallow (∼330 m) profiles. We also highlight the advantages of helicopter‐based observations for accessibility, speed, maneuverability, and cost‐efficiency. The combination of ship‐ and helicopter‐based observations using the JARE61 approach will increase the potential of future polar oceanographic observations. Plain Language Summary: Totten Glacier hosts the most rapidly thinning ice in East Antarctica. This record of thinning is due to rapid melt along the grounding line of the Totten Ice Shelf (TIS), where warm ocean water from the open ocean flows into ice shelf cavities. To understand the pathways and mechanisms of warm water inflow, ocean observations for entire continental shelf regions are necessary. This has historically not been possible due to intense sea ice and icebergs in the region. To overcome this problem, we conducted a 6‐day helicopter‐based oceanographic campaign during the 61st Japanese Antarctic Research Expedition. Our measurements show that warm ocean water occupies a large area of the continental shelf off the TIS. We identify glacial meltwater outflow from the TIS cavity west of 116°E, which provides direct evidence of TIS melting. We also show that helicopter‐based measurements are better for the following aspects; (a) helicopter operations are insensitive to sea ice conditions, (b) helicopter travels faster than a ship, (c) helicopter can conduct measurement in a small ice crack, and (d) helicopter is cheaper to operate than an ice breaker. By combining ship‐based and helicopter‐based approaches, we may be able to enhance the efficiency of future polar oceanographic observations. Key Points: Six‐day helicopter‐based observations retrieved 67 temperature profiles covering the entire continental shelf region off the Totten Ice Shelf (TIS)Between 116.5°E and 120°E, modified Circumpolar Deep Water can be found uniformly over the continental shelf region below 550–600 mGlacial meltwater outflow from the TIS cavity is identified west of 116°E [ABSTRACT FROM AUTHOR]

The Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current. Plain Language Summary: The Antarctic Slope Current is a narrow ocean current that travels around Antarctica following the continental slope. It separates the shallow and cold continental shelf from much warmer waters in the open ocean. Intrusions of the relatively warm water across the continental slope impacts melting of Antarctic ice shelves and global sea level rise. Understanding what controls the strength and variability of the Antarctic Slope Current is therefore important. The Antarctic Slope Current usually has the largest velocities near the surface, but there are regions where there is also a strong bottom flow. We use a coupled ocean‐sea ice model and show that the bottom flow is controlled by the export of very dense shelf water that flows down the continental slope in a few locations around the continent. However, the bottom and surface flow does not vary together on interannual time scales which tells us that the two components of the Antarctic Slope Current are largely independent. The result is important as the formation of dense shelf water is expected to reduce in the future which will impact the deep flow, but not the surface component. Key Points: Simulations reveal a close spatial relationship between the bottom‐intensified Antarctic Slope Current and Dense Shelf Water exportInterannual variability of Dense Shelf Water is reflected in the bottom speed of the bottom‐intensified Antarctic Slope CurrentThe surface component varies independently from the bottom‐intensified flow implying two distinct, co‐located currents [ABSTRACT FROM AUTHOR]

The year when total column ozone (TCO) returns to 1980 levels is commonly used to quantify recovery from disturbance caused by ozone‐depleting substances. This date is reasonable but somewhat arbitrary. Here we borrow an indicator from climate change research, the signal‐to‐noise (S/N) metric to investigate how TCO might return to statistically similar levels to those of the pre‐ozone hole era (1960–1970). We calculate S/N for ozone projections from the Chemistry‐Climate Model Initiative. In most regions, a return to 1980 levels doesn't represent TCO recovery to pre‐disturbance conditions because it does not account for internal variability or reflect when statistically significant TCO losses occurred. Future projections show that, in most regions, TCO "de‐emerges" (i.e., S/N becomes less than one standard deviation from the pre‐disturbance mean) before it returns to its 1980 value. We conclude that S/N is an appropriate, perhaps complementary metric for determining when TCO returns to a pre‐disturbance state. Plain Language Summary: The stratospheric ozone layer is expected to recover from the damaging effects of chlorofluorocarbons (CFCs) and other halogen‐containing source gases this century. Ozone recovery is traditionally but somewhat arbitrarily defined as the year when total column ozone (TCO) abundances return to 1980 levels. Here, we use a metric commonly used in the climate change emergence literature—signal‐to‐noise (S/N)—to quantify statistically significant changes in TCO in models and observations relative to a period before CFC‐induced ozone depletion began. The S/N framing ties recovery to unperturbed environmental conditions, rather than to the environment in a specific year. We argue that tying recovery to unperturbed local environmental conditions is at least as reasonable as tying it to the same global date. Our calculations, which account for natural variability, show that 1980 does not reflect the onset of statistically significant ozone loss. Using a return of TCO to 1980 levels underestimates the time of recovery in Antarctica, where losses have been most significant, and overestimates the time to recovery in other regions. We conclude that signal‐to‐noise is a useful metric to assess the return of TCO to its baseline state and evaluate the effectiveness of the Montreal Protocol. Key Points: We present signal‐to‐noise calculations of stratospheric ozone depletion in chemistry‐climate models relative to 1960–1970Statistically significant ozone depletion does not emerge in 1980, which is traditionally used as a benchmark for ozone recoveryIn most regions de‐emergence occurs before ozone returns to 1980 levels except in Antarctica where de‐emergence occurs 25 years later [ABSTRACT FROM AUTHOR]

The Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes. Plain Language Summary: Algae in the ocean surface take up carbon dioxide from the atmosphere through photosynthesis and transfer it to the deep ocean when they die and sink. This process is key to maintaining a habitable planet and is known as the biological carbon pump (BCP). The seasonally ice‐covered ocean around Antarctica is one of the most active areas for algal growth, but also a region of rapid climate change. Because of the difficulty in taking measurements in this remote region, the physical and biological processes that control the growth and sinking of algae and its response to changing sea‐ice remain uncertain. In this study, we use a combination of satellites and autonomous robots to elucidate the role of sea‐ice variability on the BCP. We find that sea‐ice impacts algal growth by its influence on both the light and nutrient conditions needed for photosynthesis. Predicting the amount of algae that subsequently sinks to depth as carbon flux, although influenced by sea‐ice conditions, is more complex and linked to the greater marine ecosystem. Evidence suggests that the species of algae, zooplankton grazing, and the rate at which dead algae breaks down and sinks are important and should be a focus point for further research. Key Points: High‐resolution in‐situ observations are used to characterize multi‐year phytoplankton bloom phenology and amplitude in the Antarctic Marginal Ice ZoneYears with greater sea‐ice volume drive deeper mixing that tend to support higher magnitude blooms in the northeast Weddell SeaCarbon export efficiency is affected by bloom magnitude, community composition and water column stratification [ABSTRACT FROM AUTHOR]

Recent ice loss on the western Antarctic Peninsula has been driven by warming ocean waters on the continental shelf. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. High‐resolution ocean model simulations show that the temperature variability along the western Antarctic Peninsula is controlled by the rate of dense water formation in the Weddell Sea. Passive tracer advection reveals connectivity between the Weddell Sea and the coastline of the western Antarctic Peninsula and Bellingshausen Sea. During multi‐year periods of weak Weddell dense water formation, dense overflow transport in the Weddell Sea decreases, while the transport of cold water around the tip of the Antarctic Peninsula strengthens, driving a temperature decrease of 0.4°C along the western Antarctic Peninsula. This mechanism implies that western Antarctic Peninsula coastal ocean temperature may cool in the future if Weddell Dense Shelf Water production slows down. Plain Language Summary: Melting of the ice sheet along the western Antarctic Peninsula has been driven by warming ocean waters that are in contact with the underside of the ice. It is therefore important that we understand what processes drive variation in ocean temperature. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. Using a high‐resolution ocean model, we identify a new mechanism that controls the ocean temperature variability along the western Antarctic Peninsula that is linked to the formation of dense water to the east in the Weddell Sea. During years when dense water formation is weak in the Weddell Sea, there is an increased transport of cold waters westward around the tip of the Antarctic Peninsula that flood the coast of the western Antarctic Peninsula with cold waters. Conversely, when dense water formation is strong in the Weddell Sea, there is decreased inflow of cold waters and the ocean along the coast of the western Antarctic Peninsula warms. This mechanism implies that the ocean along the western Antarctic Peninsula may temporarily cool in the future if dense water formation in the Weddell Sea slows down. Key Points: A high resolution ocean model reveals connectivity from the Weddell Sea to the western Antarctic Peninsula and Bellingshausen SeaWhen Weddell Sea dense water formation is weak, transport of cold water along the coastline of the western Antarctic Peninsula increasesThis mechanism controls the simulated decadal variability along the western Antarctic Peninsula, and cools coastal waters by 0.4°C [ABSTRACT FROM AUTHOR]

Ice sheet evolution depends on subglacial conditions, with the ice-bed interface's strength exerting an outsized role on the ice dynamics. Along fast-flowing glaciers, this strength is often controlled by the deformation of subglacial till, making quantification of spatial variations of till strength essential for understanding ice-sheet contribution to sea-level. This task remains challenging due to a lack of in situ observations. We analyze continuous seismic data from the Whillans Ice Plain (WIP), West Antarctica, to uncover spatio-temporal patterns in subglacial conditions. We exploit tidally modulated stick-slip events as a natural source of sliding variability. We observe a significant reduction of the till seismic wave-speed between the WIP sticky-spots. These observations are consistent with a poroelastic model where the bed experiences relative porosity and effective pressure increases of >11% during stick-slips. We conclude that dilatant strengthening appears to be an essential mechanism in stabilizing the rapid motion of fastflowing ice streams. Plain Language Summary The rate of ice-mass loss from the Antarctic ice-sheet, and hence sea-level rise, is governed by the rate of ice flow. The frictional force of the glacier bed plays a large role in ice flow speed. For fast glaciers, this force is often controlled by the deformation of subglacial sediments. Therefore, understanding their mechanical properties is crucial to understanding glaciers' contribution to sea-level rise. This task remains challenging due to the lack of direct, in-situ observations. This paper analyzes seismic data recorded on the Whillans Ice Plain (WIP) in West Antarctica to reveal spatiotemporal phenomena in subglacial conditions. We exploit glacier stick-slip events as natural sources of slip variability. We observe a significant reduction in seismic wave-speed within the till between the WIP's sticky-spots. On the contrary, the sticky-spots themselves show no change. These observations are consistent with a poroelastic model. In between the sticky-spots, the bed undergoes an increase in effective pressure and porosity due to the sediments' plowing during the slip. These till variations slow down the glacier slip and prevent it from accelerating catastrophically. This phenomenon of strengthening by dilation of bed sediments appears to be an essential mechanism for stabilizing the rapid movement of fast-flowing ice-streams. [ABSTRACT FROM AUTHOR]

Antarctic ice‐shelf stability is threatened by surface melt, which has been implicated in several ice‐shelf collapse events over recent decades. Here, we first analyze cumulative days of wet snow/ice status ("melt days") for melt seasons from 1980 to 2021 over Antarctica's ice shelves using passive and active microwave satellite observations. As these observations do not directly reveal meltwater volumes, we calculate these using the physics‐based multi‐layer snow model SNOWPACK, driven by the global climate‐reanalysis model Modern‐Era Retrospective analysis for Research and Applications Version 2. We find a strong non‐linear relationship between melt days and meltwater production volume. SNOWPACK's calculation of melt days shows agreement with observations of both cumulative days, and spatial and interannual variability. Highest melt rates are found on the Peninsula ice shelves, particularly in the 1992/1993 and 1994/1995 austral summers. Over all ice shelves, SNOWPACK calculates a small, but significant, decreasing trend in both annual melt days and meltwater production volume over the 41 years. Plain Language Summary: Antarctic ice shelves, which are floating glacier ice extending from the continent into the ocean, are important for the ice sheet's overall stability. However, they are threatened by surface melting, which has been linked to several ice‐shelf collapse events in recent decades. Here, we analyze the duration and amount of surface melting on Antarctica's ice shelves from 1980 to 2021. We first analyze microwave satellite data to identify the number of "melt days" (days when meltwater is present). Second, as meltwater volumes cannot be quantified from microwave data, we calculate these using the snow model SNOWPACK. Our modeled melt days show good agreement with the satellite observations of both total days and the variability from year‐to‐year, giving us confidence in the model's ability to also simulate realistic temporal and spatial variations in meltwater volumes. We find a strong non‐linear relationship between the number of melt days and meltwater volumes each summer. We also find that the highest meltwater volumes are produced on the Peninsula, reaching a peak in the 1992/1993 and 1994/1995 austral summers. Across all ice shelves, SNOWPACK calculates a small, but significant, decreasing trend in both annual melt days and meltwater production volume over the 41 years. Key Points: SNOWPACK's performance in calculating melt days shows agreement with the microwave observations in terms of total melt days and variabilityThe relationship between cumulative melt days and modeled meltwater volume each melt season is strongly non‐linearSNOWPACK‐calculated annual melt days and meltwater production volume decrease at a small, but significant, rate from 1980 to 2021 [ABSTRACT FROM AUTHOR]

The density stratification in the ocean is directly related to the diapycnal mixing, which drives the abyssal cell of the Meridional Overturning Circulation (MOC). It is important to understand how stratification has been changing in the world's deep and abyssal oceans under climate change. Using repeat hydrographic data obtained since the 1990s, we find a decreasing stratification associated with changes in the source Antarctica Bottom Water (AABW) properties in its formation basins as well as in basins along its dispersal pathways. Averaged south of 60°S, the squared buoyancy frequency N2 shows a negative trend of −6% per decade in waters deeper than 4,000 m. The observed decadal reduction in stratification is associated with large spatial variability, especially in the Southern Ocean basins with multiple AABW sources. Additionally, there are also significant differences between neighboring basins that are related to the blocking effect of topography. Plain Language Summary: Cold and dense Antarctica Bottom Water (AABW) forms near the coastline of Antarctica, sinks to the ocean bottom, moves northward in deep branches of the Meridional Overturning Circulation, becomes lighter through mixing, and eventually upwells to shallower depths. The AABW properties change under the changing climate. Repeat hydrography sections occupied approximately once per decade since the 1990s have revealed continuously warming and freshening of the AABW. In many regions of the ocean, the warming and freshening trends are strongest at the bottom. This causes a decrease in the vertical gradient of density, namely stratification. As the level of mixing is strongly related to stratification, it is important to quantify its trend and understand its spatial structure. We quantify the trend in stratification globally and find a reduction in stratification in the Southern Ocean and along the spreading pathways of the AABW at lower latitudes. Key Points: Three decades of repeat hydrographic sections reveal decreasing density stratification in the deep and abyssal layers of the Southern OceanSouth of 60°S, N2 has reduced by 6% per decade in the ocean below 4,000 m, with a peak reduction rate of 15% per decade at 4,800 mThe decreasing stratification is found along the pathway of Antarctica Bottom Water and its vertical structure depends on the topography of basin boundaries [ABSTRACT FROM AUTHOR]

Totten Glacier is a fast‐moving East Antarctic outlet with the potential for significant future sea‐level contributions. We deployed four autonomous phase‐sensitive radars on its ice shelf to monitor ice‐ocean interactions near its grounding zone and made active source seismic observations to constrain gravity‐derived bathymetry models. We observe an asymmetry in basal melting with mean melt rates along the grounding zone differing by up to 20 m/a. Our new bathymetry model reveals that this melt rate asymmetry coincides with an asymmetry in water column thickness and that the low‐melting ice‐shelf portion is shielded from the main cavity circulation. A 2‐year record yields year‐to‐year melt rate variability of 7–9 m/a with no seasonal cycle. Our results highlight the key role of bathymetry near grounding lines for accurate modeling of ice‐shelf melt, and the importance of sustained multi‐year monitoring, especially at ice‐shelf cavities where the dominant melt rate drivers vary primarily inter‐annually. Plain Language Summary: The point were the Antarctic Ice Sheet goes afloat on the ocean represents a critical region, where minor variations in melt rates can impact glacier flow and influence the rate of sea‐level rise. East Antarctica's Totten Glacier holds the potential to raise global sea level by several meters. Therefore, to understand the conditions it is exposed to, we measured melt rates for 2 years in several key locations near the point where the ice first touches the ocean. Our new measurements of the shape of the Totten Ice Shelf cavity help explain an observed spatial pattern of basal melting and together with local melt rate data resolve a disagreement between existing melt rate estimates from remote‐sensing methods. Key Points: Totten Glacier melt rates vary spatially between 0 and over 20 m/a; differences are explained by water column thickness variations from updated bathymetryTemporal melt rate variability is primarily inter‐annual; melt rates differ by 7–9 m/a over two observed years and there is no clear seasonal cycleContrary to previous findings, we find no topographic barriers to the intrusion of warm water to the Totten Glacier grounding zone [ABSTRACT FROM AUTHOR]

Numerical ice sheet models use sliding laws to connect basal shear stress and ice velocity to simulate ice sliding. A sliding‐law parameter β2 is used to control Weertman's sliding law in numerical ice sheet models. Basal reflectivity derived from ice‐penetrating radar also provides information about frozen or thawed conditions underneath glaciers. To assess whether basal reflectivity can be used to constrain β2, we carry out statistical experiments between two recently published datasets: β2 inferred from three numerical ice sheet models (ISSM, Úa and STREAMICE) and airborne radar‐derived relative basal reflectivity from the AGASEA‐BBAS mission over Thwaites Glacier (TG). Our results show no robust correlation between the β2–relative reflectivity pair. Pearson's correlation coefficient, a test of linearity, ranges from −0.26 to −0.38. Spearman's correlation coefficient, which does not require a linear assumption, is also modest (∼−0.35). We conclude that β2 and relative basal reflectivity underneath TG do not infer similar basal conditions. Plain Language Summary: Ice sheet models use equations called sliding laws to simulate how fast or slow ice can flow over the bed of a glacier. One such sliding law is Weertman's sliding law, which is widely used by numerical ice sheet models. A parameter called β2 is used in Weertman's sliding law to match the observed velocity. Another parameter called basal reflectivity, derived from airborne ice penetrating radar data, is widely used to identify whether the bed underneath a glacier is frozen or thawed. Frozen or thawed conditions impact how fast a glacier can slide over its bed. Here, we try to find statistical correlations between β2 and basal reflectivity over Thwaites Glacier (TG), West Antarctica. Our results show that the correlation values are not robust, which suggests that β2 and basal reflectivity do not identify similar conditions at the bed underneath TG. Key Points: Correlation experiments are carried out between Weertman's sliding‐law parameter β2 and radar‐derived relative basal reflectivityThe correlations between β2 and relative basal reflectivity are low, suggesting that they do not identify similar conditions at the bedThe correlations do not improve with any value of the sliding exponent m between 3 and 20 [ABSTRACT FROM AUTHOR]

East Antarctica is the least well‐exposed Precambrian shield on Earth. What is known, or surmised, of its geological structure comes from extrapolation over large distances and from geophysics. Here, we present a map of effective elastic thickness, Te, computed from Antarctic bedrock topography and both terrestrial and satellite gravity data. Te is directly related to lithospheric strength and can distinguish domains of differing tectonic history. Our map reveals a broad region of high Te in Wilkes Land, interpreted as a craton, while elsewhere a corridor of low Te meanders between areas of higher Te. The low‐Te belt follows the trend of prominent linear magnetic anomalies identified in the Gamburtsev and Dronning Maud Land provinces, two of which have previously been identified as possible Grenville or Pan‐African age sutures. Plain Language Summary: This paper presents a new, high‐resolution map of the effective elastic thickness of the lithosphere of Antarctica. It is interpreted as indicating the tectonic structure of East Antarctica in the form of cratons and orogenic belts. Key Points: New map of effective elastic thickness (Te) for AntarcticaKnown and suspected cratonic areas have high TeA low‐Te corridor meanders between these, joining the Lambert Rift, Gamburtsev Subglacial Mountains, and Dronning Maud Land passive margin escarpment [ABSTRACT FROM AUTHOR]

We use United Kingdom Earth System Model simulations from the Coupled Model Intercomparison Project 6 to analyze the Ross Gyre (RG) dynamics during the historical 1850–2014 period and under two contrasting future climate‐change scenarios. The modeled RG is relatively stable, with an extent and strength that agree with observations. The projections exhibit an eastward gyre expansion into the Amundsen‐Bellingshausen Seas that starts during the 2040s. The associated cyclonic ocean circulation enhances the onshore transport of warm Circumpolar Deep Water into the inner regional shelf, a regime change that increases the local subsurface shelf temperatures by up to 1.2°C and is independent of future forcing scenario. The RG expansion is generated by a regional ocean surface stress curl intensification associated with anthropogenic sea ice loss. If realised in reality, such a warming would strongly influence the future stability of the West Antarctic Ice Sheet. Plain Language Summary: We use a climate model to analyze ocean changes around West Antarctica. Our results reveal a human‐driven ocean warming that increases the continental shelf temperature in the Amundsen‐Bellingshausen Seas by more than 1°C in only ∼30 years. This rapid warming is caused by the expansion of the Ross Gyre (RG), a large oceanic circulation in the region. The West Antarctic Ice Sheet is losing mass, causing sea‐level rise, with the most rapid ice losses occurring in the Amundsen‐Bellingshausen Seas. Our results suggest that an expansion of the RG could provide a mechanism whereby melt rates increase far beyond the current range. This could have an important influence on the sea‐level rise caused by this region, with global impacts. Key Points: The UK Earth System Model produces a fairly realistic depiction of ocean conditions in West AntarcticaFuture projections suggest a rapid warming of the Amundsen Sea induced by a Ross Gyre (RG) expansion that is independent of forcing scenarioThe RG expansion is primarily caused by a surface stress curl intensification induced by anthropogenic trends in Antarctic sea ice [ABSTRACT FROM AUTHOR]

The Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic events of the 21st century so far. According to satellite‐based measurements, 0.4 Tg of sulfur dioxide (SO2) was injected into the stratosphere during the eruption. By using observations and model simulations, here we investigate changes in the chemical compositions of the stratosphere 1 year after the HTHH eruption and examine the key physical and chemical processes that influence the ozone (O3) concentrations. Injected SO2 was oxidized into sulfate during the first 2 months, and transported from the tropics to the Antarctic by the Brewer‐Dobson circulation within 1 year. In mid‐to‐low latitudes, enhanced sulfate aerosol increased O3 concentrations in the middle stratosphere but declined in the lower stratosphere. In addition to the chemical processes, sulfate aerosols also reduced polar low‐stratospheric O3 concentrations through enhanced Antarctic upwelling anomalies. Plain Language Summary: The Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic eruptions of the 21st century and has attracted global attention. Volcanic ash and gases entering the atmosphere could affect weather and climate processes. Recent studies have largely explored the effects on global warming of HTHH eruption, and have founded that its climate impact is not very strong. However, its impacts on ozone (O3) remains unclear. We used observations and models to analyze how HTHH eruption could influence O3. It confirms that stratospheric O3 can be affected when volcano‐induced aerosols are transported. We suggest that physical and chemical processes combine together to influence stratospheric O3 after HTHH eruption. Moreover, the effect on O3 of HTHH eruption is probably one of the reasons for the recent discovery of a larger O3 hole in Antarctica. Key Points: Volcano‐induced stratospheric sulfate aerosols are transported toward the South Pole and downwards by the Brewer‐Dobson circulationCatalytic nitrogen oxide ozone loss cycles and sulfate aerosols' radiative effects cause extra‐polar stratospheric ozone anomaliesVolcanic aerosol‐induced heterogeneous chemistry and enhanced upward transport causes polar stratospheric ozone anomalies [ABSTRACT FROM AUTHOR]

Several large‐scale components of the climate system may undergo a rapid transition as critical conditions are exceeded. These tipping elements are also dynamically coupled, allowing for a global domino effect under global warming. Here we focus on such cascading events involving the Greenland Ice Sheet (GIS), the West Antarctica Ice Sheet (WAIS) and the Atlantic Meridional Overturning Circulation (AMOC). Using a conceptual model, we study the combined tipping behavior due to three dominant feedbacks: the marine ice sheet instability for the WAIS, the height‐surface mass balance feedback for the GIS and the salt‐advection feedback for the AMOC. We show that, in a realistic parameter range of the model, a tipping of the WAIS can inhibit cascading events by preserving the AMOC stability. Plain Language Summary: In the climate system, the interaction of specific components known as tipping elements are thought to be able to induce a global domino effect, or cascading tipping. In this study, we present a conceptual model containing the most strongly interacting components, namely the Atlantic Meridional Overturning Circulation (AMOC), the Greenland Ice Sheet and the West Antarctica Ice Sheet. We find that the stability of this system as a whole is strongly modified when interactions are included. Especially, while a Greenland Ice Sheet collapse destabilizes the AMOC, the model shows that a collapse of the West Antarctica Ice Sheet might prevent a global cascading event by stabilizing the AMOC. Key Points: A conceptual model of interacting Atlantic Meridional Overturning Circulation (AMOC), Greenland Ice Sheet and West Antarctica Ice Sheet (WAIS) is presentedInteractions between these tipping elements strongly modifies the stability of the whole systemA collapse of the WAIS can prevent tipping of the AMOC [ABSTRACT FROM AUTHOR]

Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north‐south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres. Plain Language Summary: Occasionally, the aurora suddenly brightens in local areas of the Northern and Southern Hemispheres, causing explosive and widespread phenomena. At this time, it is known that a part of the aurora travels toward east or west from the point of the explosion; however, the formation mechanism of this traveling aurora is unknown. Recently, this traveling aurora has been reproduced by computer simulation, and it has been shown that the direction of its traveling is determined by the state of the ionosphere rather than by the projected motion of the source region in the magnetosphere. In this paper, we observed traveling auroras with two identical high‐speed cameras installed in the Northern and Southern Hemispheres simultaneously and clarified for the first time that the state of the ionosphere determines the direction of the traveling aurora. Understanding the mechanism of such connection between the space and the Earth is important for predicting the space environment required for various human activities and technologies using the space. Key Points: An auroral breakup was captured simultaneously by ground‐based all‐sky cameras at magnetically conjugate stations in both hemispheresThe auroral surge evolved asymmetrically regarding the longitudinal direction in both hemispheresThe surge evolved in the direction of the ionospheric Hall current, which is consistent with the expectation from a global simulation [ABSTRACT FROM AUTHOR]

The englacial stratigraphic architecture of internal reflection horizons (IRHs) as imaged by ice‐penetrating radar (IPR) across ice sheets reflects the cumulative effects of surface mass balance, basal melt, and ice flow. IRHs, considered isochrones, have typically been traced in interior, slow‐flowing regions. Here, we identify three distinctive IRHs spanning the Institute and Möller catchments that cover 50% of West Antarctica's Weddell Sea Sector and are characterized by a complex system of ice stream tributaries. We place age constraints on IRHs through their intersections with previous geophysical surveys tied to Byrd Ice Core and by age‐depth modeling. We further show where the oldest ice likely exists within the region and that Holocene ice‐dynamic changes were limited to the catchment's lower reaches. The traced IRHs from this study have clear potential to nucleate a wider continental‐scale IRH database for validating ice sheet models. Plain Language Summary: Ice‐penetrating radar is widely used to measure the thickness of ice sheets, critical to assessments of global sea level rise potential. This technique also captures reflections from chemical contrasts within the ice sheet, caused by the atmospheric deposition of conductive impurities, known as "internal reflection horizons" (IRHs) that can be traced over large distances. As these deposits are laid down in distinct events, most IRHs are isochronous age tracers and contain valuable information on past ice sheet processes. In this paper we trace and place age constraints on stratigraphic horizons across a large portion of the West Antarctic Ice Sheet, including regions where fast ice flow has disrupted the ice sheet stratigraphy. The resulting data set allows us to identify where the oldest ice is buried in the study region and provides evidence that flow of the ice sheet interior has been stable during the Holocene. Our results can be used to test the performance of ice sheet models, which seek to simulate the response of ice sheets to long‐term environmental change. Key Points: We measure and date individual isochronal radar internal reflection horizons across the Weddell Sea sector of the West Antarctic Ice SheetHorizons dated to 1.9–3.2, 3.5–6.0, and 4.6–8.1 ka are widespread and linked to previous radar surveys of the Ross and Amundsen Sea sectorsThese form the basis for a wider database of ice sheet architecture for validating and calibrating ice sheet models of West Antarctica [ABSTRACT FROM AUTHOR]

Ice motion and boundaries are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent‐wide, maps of Antarctic‐wide ice motion and boundaries for the time period 1995–2022. The results reveal no change in the interior region of East Antarctica, iceberg detachments at ice shelf fronts, and widespread glacier speedup that propagates 100 km's inland in West Antarctica and the Antarctic Peninsula. Speedup affects the entire drainage of the Amundsen Sea Embayment sector; the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf; the east coast down to Larsen C Ice Shelf; Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis, Mertz and Denman glaciers, glaciers in Porpoise and Vincennes Bay; and Robert, Wilma and Rayner glaciers in Enderby Land, in East Antarctica. We attribute the observed glacier changes to increased melting by warmer ocean waters. Plain Language Summary: Ice velocity is a fundamental variable of glacier evolution that governs the transfer of mass from the ice sheets to the ocean. It is best measured from satellite platforms but long time series require combination of multiple satellite data of various performance levels and incomplete spatial coverage. We assembled the first set of continent‐wide mosaics using 2–5 years of data in each period to maximize spatial coverage and detect changes in ice dynamics over the entire continent. The results reveal no signal over most of the interior but important changes at the coast concentrated along channels occupied by outlet glaciers. In the Amundsen Sea Embayment sector of West Antarctica, changes are pervasive over the entire drainage, which indicates that the coastal forcing from the ocean affects the entire ice sheet. The products reveal precise areas of rapid change in the West and East coasts of the Antarctica Peninsula, and in West and East Antarctica. These observations will help constrain ice sheet models in charge of reconstructing ice sheet past changes and projecting its evolution in a changing climate. Key Points: We present historical, continent‐wide, mosaics of Antarctic ice motion using a suite of satellite sensors spanning 28 yearsWe report areas of accelerated flow stretching over 100 km's from the coast and the entire drainage of the Amundsen Sea Embayment sectorThe observed, ongoing, detailed glacier changes will help constrain ice sheet numerical models in charge of reconstruction and projection [ABSTRACT FROM AUTHOR]

Antarctic Ice Sheet projections show the highest sensitivity to increased basal melting in the Amundsen Sea. However, little is known about the processes that control future increase in melt rates. We build an ensemble of three ocean–sea‐ice–ice‐shelf simulations for both the recent decades and the late 21st century, constrained by regional atmosphere simulations and the multi‐model mean climate change of the fifth Climate Model Intercomparison Project under the RCP8.5 scenario. The ice‐shelf melt rates are typically multiplied by 1.4–2.2 from present day to future, for a total basal mass loss increased by 346 Gt yr−1 on average. This is equally explained by advection of warmer water from remote locations and regional changes in Ekman downwelling and in the ice‐shelf melt‐induced circulation, while increased iceberg melt plays no significant role. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been significantly overestimated. Plain Language Summary: Future sea level rise highly depends on how fast the ocean will melt the floating ice shelves in Antarctica, which modulates the ice flow from the ice sheet into the ocean. This is particularly true for the Amundsen Sea sector where the ice flow into the ocean is very sensitive to ocean‐induced melting. Here, we use a numerical model that represents the evolution of the Amundsen Sea, including under the floating ice shelves. Under a high‐end greenhouse‐gases concentration pathway, our simulations indicate that melt rates beneath the ice shelves may increase by 40%–120%. This is explained by both warmer seawater coming from distant regions and changes in the local wind stress. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been overestimated. Key Points: We present 1/12° ocean‐sea‐ice‐ice‐shelf projections at the end of the 21st century under the RCP8.5 scenarioIce shelf melt rates in the Amundsen Sea are typically multiplied by 1.4–2.2 from 1989–2009 to 2080–2100Advection of warmer water from remote locations and reduced Ekman downwelling are the main drivers of changes in ice shelf melt rates [ABSTRACT FROM AUTHOR]