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Monitoring the Antarctic sea‐ice is essential for improving our knowledge of the Southern Ocean. We used satellite sea‐ice concentration data for the 2002–2020 period to retrieve the sea‐ice extent (SIE) and analyze its variability in the Pacific sector of the Southern Ocean. Results provide observational evidence of the recurring formation of a sea‐ice protrusion that extends to 60°S at 150°W during the winter season. Furthermore, we discuss how the Pacific‐Antarctic Ridge (PAR) influences this phenomenon. Our findings show that the northward deflection of the southern Antarctic Circumpolar Current front is driven by the PAR and is associated with the enhanced sea‐ice advance. The PAR also constrains eddy trajectories, limiting their interaction with the sea‐ice edge. These factors, within the 160°W–135°W sector, cause an average SIE increase of 61,000 km2 and 46,293 km2 per year more than the upstream and downstream areas, respectively. Plain Language Summary: Monitoring the variability of marine and continental ice is essential for the study of climate change. The aim of this study is to investigate the processes that favor the formation or melting of sea‐ice in the Pacific sector of the Southern Ocean. To this end, we analyzed sea‐ice variability over the last two decades (2002–2020) and its interaction with the Antarctic Circumpolar Current (ACC) and the Pacific‐Antarctic Ridge (PAR), a topographic submarine feature that separates the Pacific and Antarctic plates. We used satellite data obtained through passive microwave sensors and altimetry for sea‐ice and ACC eddy analysis, respectively. Our results provide observational evidence of the periodic formation of a sea‐ice protrusion in the study area and show that the median extent of the sea‐ice faithfully follows the PAR. Moreover, our results show that the PAR acts as a barrier that deflects the ACC and its associated eddies northward, which limits their ability to melt the sea‐ice east of the PAR. These processes encourage the exceptional growth of the sea‐ice in this area. These outcomes have strong implications for our understanding of sea‐ice variability over the Southern Ocean. Key Points: The Pacific‐Antarctic Ridge diverts warm and cold core eddies northward, encouraging an expansion of Antarctic sea‐ice at 150°WThe Southern Ocean is characterized by the recurring formation of a sea‐ice protrusion that extends to 60°S between 150°W and 140°W [ABSTRACT FROM AUTHOR]

The predictive skill of summer sea ice concentration (SIC) in the Arctic presents a steep decline when initialized before June, which is the so‐called spring predictability barrier for Arctic sea ice. This study explores the potential influence of surface heat flux, cloud and water vapor anomalies on monthly to seasonal predictions of Arctic SIC anomalies. The results show an enhancement in skill predicting Arctic September SIC in the models that use surface fluxes, clouds, or water vapor in combination with SIC and surface sea temperature as predictors when initialized in boreal spring. This result shows the potential to reduce the spring barrier for Arctic SIC predictions by including the surface heat budget. The enhanced predictive skill can be very likely linked to the improved representation of the thermodynamics associated with water vapor and cloudiness anomalies in spring. Plain Language Summary: Under the influence of climate change, the summer sea ice extent in the Arctic presents significant variability and thus is crucial to human activities in the Arctic. However, the predictive skill for summer sea ice in the Arctic shows a steep decline when predictions are initiated before June, which is called the spring barrier. In this study, we show that adding surface heat fluxes (net surface heat flux, net surface long wave radiation and net surface shortwave radiation), clouds or water vapor into a statistical prediction model can better represent the interactive processes at the atmosphere and ocean interface. The enhancement improves the predictive skill of summer sea ice in the Arctic and reduces the spring barrier, especially in areas of the Beaufort Sea, Chukchi Sea, and Central Arctic. Key Points: The predictive skill of summer Arctic sea ice using statistical prediction models presents a steep decline initialized before JuneThe spring barrier can be reduced, but not eliminated, by using spring surface heat fluxes in combination with sea ice concentration and sea surface temperatureThe statistical models using surface heat fluxes help to improve representation of thermodynamics associated with water vapor and cloudiness [ABSTRACT FROM AUTHOR]

Accelerating ice loss from Thwaites Glacier is contributing approximately 5% of global sea‐level rise, and could add tens of centimeters to sea level over the coming centuries. We use an ocean model to calculate sub‐ice melting for a succession of Digital Elevation Models of the main trunk of Thwaites Glacier from 2011 to 2022. The ice evolution during this period induces a strong geometrical feedback onto melting. Ice thinning and retreat provides a larger melting area, thicker and better‐connected sub‐ice water column, and steeper ice base. This leads to stronger sub‐ice ocean currents, increasing melting by over 30% without any change in forcing from wider ocean conditions. This geometrical feedback over just 12 years is comparable to melting changes arising from plausible century‐scale changes in ocean conditions and subglacial meltwater inflow. These findings imply that ocean‐driven ice loss from Thwaites Glacier may only be weakly influenced by anthropogenic emissions mitigation. Plain Language Summary: The West Antarctic Ice Sheet is losing ice, making a substantial contribution to global sea‐level rise. This ice loss is known to be triggered by changes in ocean melting of the floating parts of the ice sheet. Computer predictions show that this ice loss could make a large contribution to global sea‐level over the coming centuries, but the future trajectory is very uncertain. In this study we simulated the ocean melting of Thwaites Glacier during 2011–2022, a period when the glacier rapidly thinned and retreated. We show that the geometrical evolution of the glacier during this period led to a substantial increase in ocean melting, caused by the exposure of more ice base to warm ocean waters, and changing ocean currents beneath the ice. This change in melting is similar to what might be expected from 100 years of ocean warming under anthropogenic climate change. These results imply that the future melting of such glaciers is strongly controlled by the geometrical evolution of the ice through internal ice and ocean feedbacks, and will therefore only weakly be influenced by reductions in the emissions of greenhouse gases. Key Points: Model simulations are used to investigate oceanic melting of the main trunk of Thwaites Glacier during its rapid retreat between 2011 and 2022The evolution of the ice geometry leads to an increase in melting by more than 30% without any change in ocean forcingThis strong feedback means the future ocean melting of Thwaites Glacier may only be weakly influenced by changes in anthropogenic forcing [ABSTRACT FROM AUTHOR]

Rapid retreat of Arctic sea ice extends the area of open ocean for new trans‐Arctic shipping routes. However, the projected routes may be too optimistic in terms of savings in shipping costs from shortened trans‐Arctic distances as they do not consider the increased sea fog frequency (SFF) over areas of the retreating sea ice. We show that delays due to sea fog can be 1–4 days, about 23%–27% along the Northwest Passage and 4%–11% along the Northern Sea Route than previous estimated. We design a route based on the projected sea‐ice extent and SFF. The new route can reduce the sailing time by 0.3–1 day by detouring the routes with lighter impacts of sea fog. More importantly the new route will lower the risk of catastrophic accidents compared to the shortest route and saves the additional costs due to unscheduled port calls. Plain Language Summary: Rapid loss of sea ice under global warming opens up the Arctic Ocean to shipping. The available trans‐Arctic shipping in the near future will strongly decrease sailing time and financial costs from the Far East to Northwest Europe. Previous designs of trans‐Arctic shipping routes mainly considered sea ice distribution, but ignored the important effect of frequent and high‐risk polar sea fog. Here, we project Arctic sea fog frequency in 21st century and evaluate its influence on trans‐Arctic shipping routes. The results highlight the necessity to redesign the routes by including sea fog effect for safer ship navigation. Key Points: About 20%–30% of Arctic shipping routes experience frequent sea fog frequency more than 20%Sea fog increases the shipping time by 23%–27% along the Northwest Passage and 4%–11% along the Northern Sea Route than previous estimationsShipping routes by minimizing the impacts of sea fog can save 0.3–1 days sailing time by detouring dense‐sea‐fog region [ABSTRACT FROM AUTHOR]

The Unified Forecast System (UFS) is the next generation modeling infrastructure under development for NOAA's operational numerical weather/climate predictions. This study is the first attempt with UFS for seasonal predictions application. In particular, 9‐month hindcasts are performed starting from every month during 2007–2020. The UFS performance in predicting Arctic sea ice was compared to hindcasts by (a) the current operational Climate Forecast System version 2 (CFSv2) and (b) an experimental sea ice prediction system (CFSm5) at NOAA. Evaluations indicate that UFS demonstrates consistently improved skills than both CFSv2 and CFSm5 in seasonal sea ice predictions, together with more realistic climatological sea ice distributions. Diagnostics suggest that the better climatological sea ice distributions in UFS is related to its atmospheric states simulated with FV3, the atmospheric component of UFS, and reinforced by associated ocean circulations. In addition, applications of the multi‐model ensemble strategy do not present skill improvements over the UFS forecasts alone. Plain Language Summary: Arctic sea‐ice cover illustrates significant year‐to‐year fluctuations superimposed on the long‐term decline trend. Seasonal forecasts of the ice interannual fluctuations could play an important role in providing planning information for shipping industries, improving management of ocean and coastal resources in the Arctic, and better serving Northern communities. In this study, we evaluated the capability of the Unified Forecast System (UFS) in predicting the Arctic sea ice coverage. The UFS is the next generation modeling infrastructure under development for NOAA's operational numerical weather/climate predictions, which has not been tried with seasonal predictions application before. Evaluations suggested that UFS demonstrates consistently improved skills in seasonal sea ice predictions than two systems currently in operations at NOAA, together with more realistic climatological sea ice distributions. Our further diagnostics suggest that the improvement is mainly related to the atmospheric states simulated with UFS, but is also reinforced by associated ocean circulations. Key Points: This work is the first attempt with the Unified Forecast System (UFS) for seasonal predictionsUFS presents better performance in seasonal prediction of Arctic sea ice than current operational systems at NOAABetter sea ice predictions with UFS are mainly related to its better representation of atmospheric states [ABSTRACT FROM AUTHOR]

Estimates of nitrate loading to the Arctic Ocean are limited by the lack of field observations within deltas partly due to logistical constraints. To overcome this limitation, we use a remote sensing framework to estimate retention of nitrate in Arctic deltas. We achieve this by coupling hydrological and biogeochemical process models at the network scale for five major Arctic deltas. Binary masks of delta channels were used to simulate flow direction and magnitude through networks. Models were parameterized using historical and seasonal observations. Simulated nitrate retention ranged from 2.9% to 15% of the incoming load. Retention rates were largest during winter but smallest during spring conditions when increased discharges export large nitrate masses to the coast. Under future climate scenarios, retention rates fall by ∼1%–10%. Arctic deltas have an important effect on the magnitude of nitrate entering Arctic seas and the inclusion of processing in deltas can improve flux estimates. Plain Language Summary: The Arctic Ocean is the smallest and shallowest of the world's oceans but receives the largest riverine input per basin volume. Thus, the flux of nitrate from land to sea through rivers in the circumpolar Arctic can have large impacts on coastal life and global biogeochemical cycles. To assess the impact of terrestrially derived nitrate on Arctic Ocean chemistry under current and future scenarios, accurate accounting of nitrate loading is needed. Deltas act as filters that reduce nitrate loads, but current estimates of flux exclude the effects of Arctic deltas on nitrate export. We use a novel approach based on satellite imagery to estimate the potential impact of deltas on nitrate retention on five Arctic deltas, representing a range of morphologies and sizes. Our analysis shows seasonal and morphological differences in nitrate processing rates in each delta with retention rates ranging from 2.9% to 15% of incoming load and most retention occurring during winter months. Under future climate scenarios, the efficiency of nitrate retention decreases by up to 10%. Our models suggest that Arctic deltas alter the magnitude of nitrate entering the ocean and future predictions of loading or current earth system models could be improved by incorporating their effect. Key Points: Remotely sensed data provide a useful tool for revealing patterns of nitrate retention in Arctic deltasChannel networks of Arctic deltas are most efficient at retaining nitrate during spring and winterHotspots of nitrate removal cluster according to network structure and discharge on Arctic deltas [ABSTRACT FROM AUTHOR]

We evaluate Linear Inverse Models (LIMs) trained on last millennium model data to predict Arctic sea‐ice concentration, thickness, and other atmospheric and oceanic variables on monthly timescales. We find that more than 500 years of training data and 100 years of validation data are needed to reliably estimate LIM forecast skill. The best LIM has skill up to 8 months lead time and outperforms an autoregressive model of order one (AR1) forecast at all locations, with particularly large outperformance near the ice edge. However, for out‐of‐sample validation tests using data from various different model simulations and reanalysis products, they underperform an AR1 model due to differences in the location of the sea‐ice edge from the training data. We present a metric for predicting LIM forecast skill, based on the spatial correlation of the variance in the training and validation data sets. Plain Language Summary: Rapid changes in sea‐ice concentration in recent decades have produced new navigational challenges and hazards in the region, elevating the importance of seasonal sea‐ice forecasts. Arctic sea ice has been shown to contain inherent predictability on seasonal timescales, yet current predictions generally show poor skill. Here, we employ a statistical technique referred to as Linear Inverse Modeling, which uses linearized dynamical modes estimated from a training data set to predict sea‐ice conditions on monthly timescales. We find a Linear Inverse Model is able to outperform a baseline statistical model throughout the Arctic when initialized on the data derived from the same model. Key Points: A Linear Inverse Model is evaluated using last millennium model simulations for Arctic climate predictionThe Linear Inverse Model successfully predicts Arctic conditions when the same model simulation is used for training and validationLinear Inverse Model forecast skill is proportional to the spatial correlation of variance in the validation and training data [ABSTRACT FROM AUTHOR]

Understanding the future fate of the Greenland Ice Sheet (GIS) in the context of anthropogenic CO2 emissions is crucial to predict sea level rise. With the fully coupled Earth system model of intermediate complexity CLIMBER‐X, we study the stability of the GIS and its transient response to CO2 emissions over the next 10 Kyr. Bifurcation points exist at global temperature anomalies of 0.6 and 1.6 K relative to pre‐industrial. For system states in the vicinity of the equilibrium ice volumes corresponding to these temperature anomalies, mass loss rate and sensitivity of mass loss to cumulative CO2 emission peak. These critical ice volumes are crossed for cumulative emissions of 1,000 and 2,500 GtC, which would cause long‐term sea level rise by 1.8 and 6.9 m respectively. In summary, we find tipping of the GIS within the range of the temperature limits of the Paris agreement. Plain Language Summary: With ongoing carbon dioxide emissions from the burning of fossil fuels, the atmosphere heats up, which has dramatic consequences for the ice sheets on Earth. In this study, we focus on the Greenland ice sheet (GIS), which holds so much ice that a complete melting would cause the global sea level to rise by 7 m. However, future mass loss of the GIS is challenging to predict because it is a non‐linear function of temperature and occurs over long timescales. For this reason, we use CLIMBER‐X, which is a coupled model of the whole Earth system. We find that the GIS features two critical volume thresholds, whose crossing would imply extensive further mass loss so that it would be difficult for the ice to grow back, even in thousands of years. Near these critical ice volumes, the mass loss rates are particularly high, and differences in the total carbon dioxide emission have a large impact. In summary, if cumulative emissions larger than 1,000 Gt carbon are released into the atmosphere, the GIS will shrink below a critical threshold and mass loss will inevitably continue until a substantial part of the ice sheet has melted. Key Points: Bifurcation points exist at global mean temperature anomalies of 0.6 and 1.6 K relative to pre‐industrialMass loss rate and sensitivity to cumulative CO2 emission peak near the equilibrium ice volumes belonging to these temperature anomaliesSubstantial long‐term mass loss of the Greenland ice sheet for cumulative emissions larger than 1,000 Gt carbon [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]

Melt ponds forming on Arctic sea ice in summer significantly reduce the surface albedo and impact the heat and mass balance of the sea ice. Therefore, their areal coverage, which can undergo rapid change, is crucial to monitor. We present a revised method to extract melt pond fraction (MPF) from Sentinel‐2 satellite imagery, which is evaluated by MPF products from higher‐resolution satellite and helicopter‐borne imagery. The analysis of melt pond evolution during the MOSAiC campaign in summer 2020, shows a split of the Central Observatory (CO) into a level ice and a highly deformed ice part, the latter of which exhibits exceptional early melt pond formation compared to the vicinity. Average CO MPFs are 17% before and 23% after the major drainage. Arctic‐wide analysis of MPF for years 2017–2021 shows a consistent seasonal cycle in all regions and years. Plain Language Summary: In the Arctic summer, puddles of surface melt water, called melt ponds, form on the sea ice. These melt ponds reduce the ability of the surface to reflect the sunlight. Instead, they absorb more solar energy and pave the way into the ocean beneath where the energy is also absorbed. Thus, it is important to know where these melt ponds develop and what fraction of the surface they cover. To investigate this, we present a classification algorithm that is used to extract the areal fraction of melt ponds from satellite measurements. The special focus of this study is the MOSAiC campaign in summer 2020, where the research vessel Polarstern drifted with an ice floe for 1 year. We can see a separation of this floe into two parts. One of them shows melt pond formation much earlier than the other. This is because of different ice age and surface properties. Additionally, we use the classification algorithm to analyze the differences of melt pond fraction between different dates and regions in the Arctic. Key Points: Algorithm to extract melt pond and open water areas from Sentinel‐2 imagery with maximum uncertainty of 6%Exceptional early melt pond formation on MOSAiC Central Observatory, summer 2020, compared to broader vicinityHigh spatial and temporal variability of melt pond fraction on local and regional scales [ABSTRACT FROM AUTHOR]

To deal with the great challenges in simulating the highly non‐linear physics of Antarctic sea ice, a multivariate balanced atmospheric ensemble forcing is developed based on the high‐resolution component of ERA5, which considers the relationship between different variables and adjacent times. To validate the performance of this new forcing, experiments were conducted from 1 January 2016 to 28 February 2017. Compared to simulations forced with the ensemble component of ERA5, a more reasonable ensemble of simulations is produced by the atmospheric ensemble forcing developed in this study, which suppresses the sea‐ice concentration (SIC) simulation errors and produces a better estimation of SIC simulation uncertainties. Further sea‐ice thickness budget analysis reveals that this impact of atmospheric ensemble forcing on sea ice simulation is due to a modulation of atmosphere‐ocean and sea ice‐ocean thermodynamic processes. These results lay the foundation for further improvements in Antarctic sea ice data assimilation and probabilistic prediction. Plain Language Summary: Antarctic sea ice plays an important role in the Earth system. However, its accurate simulation still faces many challenges. Considering the highly non‐linear sea ice processes, adopting an ensemble simulation procedure is a way to improve Antarctic sea ice modeling skill. Perturbing the sea ice‐ocean coupled model indirectly through the atmospheric ensemble forcing can ensure the dynamic consistency of model states. In this study, we developed a method to perturb high‐frequency ERA5 reanalysis in a multivariate balanced way for the Southern Ocean coupled sea ice‐ocean model. Results show obvious improvements in the Antarctic sea ice simulation with the newly developed atmospheric ensemble forcing from both deterministic and probabilistic perspectives, which is achieved by modulation of atmosphere‐ocean and sea ice‐ocean thermodynamic processes. This is an important step toward the development of a comprehensive reanalysis for the sparsely observed Antarctic sea ice, which should benefit the scientific research and human activities in the Southern Ocean. Key Points: A multivariate balanced atmospheric ensemble forcing is developed for the Southern Ocean coupled sea ice‐ocean modelAdopting this new forcing can suppress model errors of sea‐ice concentration and produce better estimates of simulation uncertaintiesThe improvement is primarily caused by atmosphere‐ocean and sea ice‐ocean thermodynamic processes [ABSTRACT FROM AUTHOR]

While previous studies have suggested a substantial role of ozone‐depleting substances (ODSs) in historical climate change, their relative contribution to historical anthropogenic warming has not been quantified before. Analyzing all‐but‐one‐forcing, 20‐member ensembles of historical simulations with a state‐of‐the‐art Earth System Model, we find that over the 1955–2005 period ODSs are responsible for 30% of global warming, 37% of Arctic warming, and 33% of summertime Arctic sea ice loss. Effective Radiative Forcing (ERF) calculations reveal that the global warming response to ODSs per unit of ERF is about 20% larger than for CO2, which may be due to stronger feedbacks and the difference in temporal evolution with ODSs having leveled off and CO2 still increasing in 2005. While the response to both peaks in the Arctic, the ODS ERF opposes Arctic amplification more than the CO2 ERF. Our findings highlight the importance of the Montreal Protocol for mitigating future climate change. Plain Language Summary: Ozone‐depleting substances (ODSs) are chemicals developed in the 1920s and 1930s for use in spray cans, refrigerators and plastic foams. Their commercial use increased rapidly in the 1950s and 1960s, but their phase out is underway since the signing of the Montreal Protocol in the 1987, following the identification of their devastating impact on the stratospheric ozone layer. It is well known that ODSs are powerful greenhouse gases, with the second largest warming effect between 1955 and 2005. However, their relative contribution to past global warming has not been quantified previously using comprehensive climate models. Here we show that ODSs were responsible for roughly a third of late 20th Century global warming, Arctic warming and Arctic sea ice decline. In addition, we find that the impact of ODSs on global temperatures is about 20% larger than expected based on the impacts they have on the radiative balance. The impacts of ODSs peak in the Arctic, while their radiative forcing peaks in the tropics, and thus opposes Arctic warming amplification. These findings enhance our understanding of drivers of past climate change, and highlight the importance of the Montreal Protocol for future climate change mitigation. Key Points: Between 1955 and 2005, global warming from ozone‐depleting substances (ODSs) was more than half of that due to CO2, and about one third of total anthropogenic warmingODSs have been about 20% more effective than CO2 at causing global warming per unit radiative forcingODS warming peaks in the Arctic and is 66% of that due to CO2, while their Effective Radiative Forcing (ERF) opposes Arctic amplification more than the CO2 ERF [ABSTRACT FROM AUTHOR]

Previous studies have found inconsistent responses of the North Atlantic jet to Arctic sea‐ice loss. The response of wintertime atmospheric circulation and surface climate over the North Atlantic‐European region to future Arctic sea‐ice loss under 2°C global warming is analyzed, using model output from the Polar Amplification Model Intercomparison Project. The models agree that the North Atlantic jet shifts slightly southward in response to sea‐ice loss, but they disagree on the sign of the jet speed response. The jet response induces a dipole anomaly of precipitation and storm track activity over the North Atlantic‐European region. The changes in jet latitude and speed induce distinct regional surface climate responses, and together they strongly shape the North Atlantic‐European response to future Arctic sea‐ice loss. Constraining the North Atlantic jet response is important for reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Plain Language Summary: Variations in the North Atlantic jet affect temperature, precipitation, and storminess in Europe. It is not well understood how the jet and European climate will respond to future sea‐ice loss in the Arctic under human‐caused increases in global temperature. We study how atmospheric circulation and surface climate in winter over the North Atlantic‐European region would respond to future Arctic sea‐ice loss under 2°C global warming, using an unprecedented number of coordinated simulations from different climate models. In many models the North Atlantic jet stream in winter responds by shifting southward, but the response of its speed is less clear. These jet stream changes are found to strongly shape the responses in precipitation and storm track activity. More precise estimates of the jet stream response are key to reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Key Points: Projected Arctic sea‐ice loss causes a robust equatorward shift of the North Atlantic jet across models, but no robust change in jet speedChanges in jet position and speed strongly shape the European winter surface climate response to projected Arctic sea‐ice lossConstraining the jet response is important for reducing uncertainty in the European precipitation response to future Arctic sea‐ice loss [ABSTRACT FROM AUTHOR]

The most common source of information about sea ice conditions is remote sensing data, especially images obtained from synthetic aperture radar (SAR) and passive microwave radiometers (PMR). Here we introduce an adaptive fusion scheme based on Graph Laplacians that allows us to retrieve the most relevant information from satellite images. In a first test case, we explore the potential of sea ice classification employing SAR and PMR separately and simultaneously, in order to evaluate the complementarity of both sensors and to assess the result of a combined use. Our test case illustrates the flexibility and efficiency of the proposed scheme and indicates an advantage of combining AMSR‐2 89 GHz and Sentinel‐1 data for sea ice mapping. Plain Language Summary: The Earth's land and ocean surface is monitored from space using different sensors mounted on various satellite platforms. Each type of sensor has its advantages and limitations. Combining data from different sensors can potentially solve ambiguities in information retrievals associated with the use of only a single sensor. Here, we apply a multi‐sensor fusion scheme that can be used for various data combinations in order to extract relevant information. The main goal of this work is to explore the potential of simultaneously applying two sensors for sea ice mapping and monitoring, namely synthetic aperture radar and passive microwave radiometers, in order to improve the separation of sea ice types. Key Points: We propose an adaptive scheme for the fusion of passive microwave radiometers and synthetic aperture radar data for sea ice classificationWe demonstrate the flexibility and efficiency of the proposed scheme on a test case by evaluating the performance of various data scenariosWe illustrate the advantages and limitations of applying the data of each sensor simultaneously and separately [ABSTRACT FROM AUTHOR]

Anthropogenic‐induced variations of the Atlantic Meridional Overturning Circulation (AMOC) and the associated Deep‐Water Formation (DWF) are a major concern. Using measurements of triple oxygen isotopes in the deep North Atlantic, we present novel evidence for a dramatic decadal to centennial shift in ocean conditions at the source region of DWF. These measurements suggest a recent decrease in the percentage of photosynthetic O2 derived from the source regions of AMOC in the Nordic Seas compared to the Little Ice Age. 1‐D model simulations suggest that a reduction in photosynthetic O2 production can explain the observed decrease. Alternatively, it may indicate a substantial decrease in sea‐ice cover and thus increased air‐sea gas exchange, bringing the isotopic composition of O2 closer to equilibrium with the atmosphere. Our novel data can serve as a benchmark for climate models. Plain Language Summary: In this study, we use the isotopic signature of dissolved oxygen gas in the deep North Atlantic Ocean to demonstrate a recent dramatic change in the Nordic Seas. Water in the deep Atlantic at present was at the surface of the Nordic Seas several hundred years ago, during the Little Ice Age (LIA). The isotopic signature shows that the percentage of oxygen sourced from photosynthesis was greater during the LIA. This may indicate a greater rate of photosynthesis than at present. However, we know that when sea ice covers the ocean surface, it prevents the influx of atmospheric oxygen, while sunlight penetration through the ice keeps a high rate of photosynthesis and production of biological oxygen. Therefore, it is also possible that during the LIA, sea‐ice cover was much more extensive than today. Key Points: Oxygen isotopic evidence for a recent dramatic shift in the North Atlantic source region of deep water‐massesThe isotopic measurements suggest high extent of ice cover and/or higher primary production rates during the Little Ice AgeThese new observations can serve as a benchmark for climate models [ABSTRACT FROM AUTHOR]

Satellite observations of sea ice freeboard are integral to the estimation of sea ice thickness. It is commonly assumed that radar pulses from satellite‐mounted Ku‐band altimeters penetrate through the snow and reflect from the snow‐ice interface. We would therefore expect a negative correlation between snow accumulation and radar freeboard measurements, as increased snow loading weighs the ice floe down. In this study we produce daily resolution radar freeboard products from the CryoSat‐2 and Sentinel‐3 altimeters via a recently developed optimal interpolation scheme. We find statistically significant (p < 0.05) positive correlations between radar freeboard anomalies and modeled snow accumulation. This suggests that, in the period after snowfall, radar pulses are not scattering from the snow‐ice interface as commonly assumed. Our results offer satellite‐based evidence of winter Ku‐band radar scattering above the snow‐ice interface, violating a key assumption in sea ice thickness retrievals. Plain Language Summary: Arctic sea ice thickness is often estimated using radar pulses from satellite‐mounted Ku‐band altimeters, which retrieve the radar freeboard. This is a proxy for the height of the ice surface above the waterline. Ku‐band radar pulses are widely assumed to penetrate through the overlying snowpack and reflect from the top of the sea ice. This means that increased snow loading on a sea ice floe is expected to reduce its radar freeboard, as the snow weighs the sea ice down. We produce daily resolution pan‐Arctic radar freeboard data sets from CryoSat‐2 and Sentinel‐3 retrievals. Using these new products, we find that an increased snow load often increases the radar freeboard, suggesting that the radar pulses are not reflecting off the ice surface. This could explain why satellite‐based sea ice thickness estimates don't always match in situ observations. Key Points: We reveal synoptic‐scale positive correlations between snow accumulation and Ku‐band radar freeboard changeThese correlations indicate that the conventional assumption of full radar‐wave penetration of the snowpack does not always hold trueThis sensitivity in freeboard estimates to snow accumulation introduces synoptic variability into satellite estimates of sea ice thickness [ABSTRACT FROM AUTHOR]

Recent progress in understanding Beaufort Gyre (BG) dynamics reveals an important role of ice‐ocean stress in stabilizing BG freshwater content (FWC) over seasonal to interannual timescales. But how the BG's stratification and FWC respond to surface forcing over decadal timescales has not been fully explored. Using a global ocean‐sea ice model, we partition the BG into upper, middle (halocline), and lower (thermocline) layers and perform a volume budget analysis over 1948–2017. We find that the BG's asymmetric geometry (with steep and tight isohalines over continental slopes relative to the deep basin) is key in determining the mean volume transport balance. We further find that a net Ekman suction during 1983–1995 causes the upper and middle layers to deflate isopycnally, while an enhanced Ekman pumping during 1996–2017 causes these layers to inflate both isopycnally and diapycnally, the latter via anomalous flux from the upper to the middle layer. Plain Language Summary: The Beaufort Gyre (BG) has increased its liquid freshwater content (FWC) by 40% in the past two decades. If released and transported downstream to the subpolar North Atlantic Ocean, the excess water might affect the ocean circulation via suppression of deep‐water formation. However, which layer is responsible for BG freshwater accumulations and releases over decadal timescales and the corresponding physical processes remain unclear, hampering our attempts to make future predictions. Here we use an ocean‐sea ice model to explore such changes in its three characteristic layers (upper mixed‐layer water, Pacific Water in the middle layer, and Atlantic Water in the lower layer). We find that the asymmetry of the BG, which has been simplified as a symmetric bowl shape in most previous studies, is important in determining the BG's layered mean state. Over decadal timescales, changes in BG volume are controlled by annual‐mean Ekman pumping/suction resulting from combined wind and ice‐ocean stresses. This study emphasizes the role of asymmetric geometry in determining the BG mean volume balance. It also explores the role of mean flow across the gyre's lateral boundaries in regulating BG's volume and FWC over decadal timescales. Key Points: The Beaufort Gyre's asymmetric geometry is the key to explain a net mean lateral outflow in the upper layer, despite Ekman convergenceIn the mean, the Gyre is fed by a northeast inflow into its middle layer, with outflow to its southwest from both upper and middle layersDeflation/inflation processes are asymmetric in response to anomalous Ekman suction/pumping on decadal timescales [ABSTRACT FROM AUTHOR]

Mimas' small size, lack of geologic activity, and high eccentricity suggest a frozen, inactive history. However, Cassini libration measurements are best explained by a present‐day liquid ocean under an ice shell 24–31 km thick, a configuration that tidal heating can support. These unexpected findings have motivated further study of Mimas' surface geology to constrain its interior evolution. Here, we model the formation of Mimas' large impact basin, Herschel, using iSALE‐2D. Our goal is to determine whether the current estimated ice shell thickness is sufficient to withstand the impact and produce the observed basin morphology. We find that, if Mimas had an ocean at the time of the Herschel‐forming impact, the ice shell had to have been 10s of km thicker than today. These findings are consistent with inferences from minimal tectonic activity that Mimas must be a young ocean world, if it indeed has an ocean today. Plain Language Summary: Mimas, a small moon of Saturn, is heavily cratered and lacks the typical characteristics of an ocean‐bearing moon, such as the active surface of its neighboring moon, Enceladus. However, measurements of Mimas, made by the Cassini mission, are best explained by an ocean under a relatively thick ice shell. Here, we try to understand how this ice shell and ocean may have changed with time by modeling the formation of Mimas' largest impact basin, Herschel. When such large impacts occur, the shapes and depths of the resulting basin can be affected by the presence of the ocean. We find that Herschel could not have formed in an ice shell at the present‐day thickness without obliterating the ice shell at the impact site. Rather, we conclude that, if Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. Mimas as an emerging ocean world places important constraints on the formation, evolution, and habitability of all of the mid‐sized moons of Saturn. Key Points: We performed numerical simulations of the formation of the Herschel impact basin to explore Mimas' evolutionThe Herschel impact basin could not have formed in an ice shell of the thickness indicated by Mimas' present‐day librationsHerschel's morphology and the lack of tectonics on Mimas are compatible with a thinning ice shell and geologically young ocean [ABSTRACT FROM AUTHOR]

Seismic instruments such as broadband seismometers and distributed acoustic sensors (DAS) have a demonstrated potential for wide‐scale and continuous in situ monitoring of near‐surface environmental and anthropogenic processes. DAS is attractive for development as a multi‐geophysical observatory due to the prevalence of existing fiber infrastructure in regions with environmental, cultural, or strategic significance. Here we present results from a multi‐seasonal acquisition of DAS data on a seafloor cable in the Beaufort Sea, Alaska. During a November 2021 data collect we captured the rapid transition of ambient noise characteristics from an "ice‐free" state to an "ice‐bound" state. A sea ice formation front was plainly visible on the DAS record and was observed to propagate 20 km seaward over a period of 8 hr. Satellite‐based instrumentation were unable to record this event due to cloud cover, low light conditions, and orbital frequency. Plain Language Summary: Distributed acoustic sensing (DAS) is an emerging technology that uses fiber optic cable to mimic a high resolution array of one dimensional seismometers. Distributed acoustic sensors is compatible with any single‐mode fiber optic cable, including seafloor telecommunications cables. Previous recordings of DAS data on seafloor cables have demonstrated that this technology is able to detect and track individual ocean waves, surface vessels, and whales. In this first‐ever acquisition of DAS data from a seafloor beneath a frozen ocean, we show that DAS is also capable of mapping sea ice coverage with unprecedented spatial resolution and can track sea ice formation and breakup up in near real time. Key Points: Seafloor distributed acoustic sensing can distinguish between sea ice and open water on the basis of endemic wave modesThis technology is capable of high resolution, near real time imaging of sea ice dynamics, such as rapid freezing of marginal iceThis can complement large scale, low resolution satellite‐based imaging in regions of high scientific or operational importance [ABSTRACT FROM AUTHOR]

Impacts into icy bodies often generate near‐surface melt chambers and thermal perturbations that soften the ice. We explore the post‐impact evolution of non‐penetrating impacts into Europa's ice shell. Simulations of viscous ice deformation show that dense impact melts founder before refreezing. If the transient cavity depth exceeds half the ice shell thickness, over 40% of the impact melt drains into the underlying ocean. Drainage of impact melts from the near‐surface to the ocean occurs on timescales of 103–104 years. The drainage of melts to the ocean occurs for all plausible ice shell thicknesses and ice viscosities, suggesting that melt foundering is a natural consequence of impacts on icy worlds. Post‐impact viscous deformation is an important process on icy worlds that affects cryovolcanism, likely modifies crater morphology, creates porous columns through the ice for surface‐to‐ocean exchange, and may supply the oxidants required for habitability to subsurface oceans. Plain Language Summary: Europa is an ocean world with an outer ice shell in our Solar System. Outer ice shells on ocean worlds serve as both a barrier and facilitator for internal ocean habitability and the detectability of its potential inhabitants. A prominent feature we observe on the surface of ice shells is highly variable craters from impacts that indent but do not penetrate the ice shell. Here, we find that many of the impacts into Europa's ice shell would have generated large near‐surface melt chambers, which rapidly drain into the underlying ocean and form a continuous surface‐to‐ocean melt column. This melt drainage occurs with any entrained surface materials and provides a consistent means of exchange between the surface and ocean, which likely increases the habitability of internal oceans. Furthermore, this drainage displaces large volumes of water from below impact craters, on the order of 10's of cubic kilometers. This large scale drainage may help to explain the wide range of collapsed, enigmatic crater morphologies observed on Europa, and other ocean worlds. Key Points: Impacts into ice shells generate melt chambers that viciously deform and sink through the ice shell, likely modifying crater morphologiesIf transient cavity depth exceeds half the ice thickness, impact melts drain to the ocean and may deliver oxidants needed for habitabilityDraining impact melts form a stable porous channel that connects the ocean to near‐surface and may allow for the escape of ocean fluids [ABSTRACT FROM AUTHOR]

The East Asian winter monsoon (EAWM) is associated with northerly surface winds affecting crop and livestock productivity and social and economic activities across East Asia. However, the relationship between EAWM dynamics and marine biota remains poorly understood. In this study, we analyzed fossil ostracods from sediment cores collected in the northwestern Pacific Ocean to detect the appearance/disappearance of circumpolar species during the late Quaternary. We compared our ostracod records, including dating results, with ice‐rafted debris records from the Sea of Japan, and identified two southward migration events of Arctic ostracods at 120–100 ka (Marine Isotope Stage [MIS] 5d stadial) and 30–15 ka (MIS 3 and 2, peak Last Glacial), which corresponded to a strengthened EAWM system. Our results indicate that an intensified EAWM influenced the Yellow Sea Bottom Cold Water formation, lowered the winter water temperatures, and allowed Arctic ostracods to migrate southward into the Yellow Sea. Plain Language Summary: The impacts of climatic conditions on marine ecosystems and the distribution of marine species are major concerns due to ongoing anthropogenic climate change. Fossil records can provide direct evidence of past climate impacts, and the Yellow Sea (YS) is an important location to track changes in cold‐adapted marine species, as it is located at the southern end of the distribution of Arctic circumpolar species. In this study, we found that circumpolar ostracod species dominated the YS during distinct stadial and peak glacial periods—120 to 100 and 30 to 15 thousand years ago—intervals that are consistent with the periods of a strengthened East Asian winter monsoon (EAWM) during the Last Glacial Period. We suggest that the stronger EAWM during these periods enhanced the formation of Yellow Sea Bottom Cold Water and/or lowered the winter temperatures of the YS, allowing Arctic ostracods to inhabit more southern regions of the YS. Key Points: Arctic ostracods inhabited the Yellow Sea between 120 and 100 ka (Marine Isotope Stage [MIS] 5d stadial) and 30–15 ka (MIS 3–2, peak Last Glacial)The presence of Arctic ostracods indicates the enhancement of Yellow Sea Bottom Cold Water formationThe East Asian winter monsoon controlled the southward and northward migrations of Arctic marine species [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]

Though thermohaline staircases exist in a large fraction of the Arctic halocline, the interactions of such staircases with shear and turbulence are still largely unexplored. We perform a series of three‐dimensional Direct Numerical Simulations with and without shear and demonstrate the capacity of shear to disrupt Arctic staircases. We force an oscillating shear throughout the domain in order to represent the effects of passing internal waves of wavelengths much larger than the simulated domain. Shear is shown to disrupt staircases when the Richardson number falls below 1/4 for small layer heights (0.5 m thick), but larger layers (1 m thick) are more resistant. We determine the presence and strength of the layers using measures of steppiness from the literature, finding that a metric based on the rms temperature gradient proves most accurate. This work demonstrates that double‐diffusive interfaces can be surprisingly robust, sometimes even maintaining their structure in strong, dynamically unstable shears. Plain Language Summary: In the Arctic, there exists a large mass of relatively warm water that originates from the Atlantic Ocean. Near the top of this watermass, where the temperature decreases rapidly, stacks of layers are present, each about 1–3 m tall, within which temperature and salinity are uniform. These peculiar structures are known as "staircases," and they tend to form in regions that are deep enough that these layers are shielded from the near‐surface ocean turbulence. Because they exist above the warmer water of Atlantic origin and below the cooler water below the sea ice, the heat fluxes through them affect the rate at which heat from the Atlantic water propagates upward. Once this heat passes through these staircases, it inevitably reaches the sea ice and promotes sea‐ice melt. Thus, staircases serve as a gatekeeper protecting the sea ice from the warm water of Atlantic origin. We investigate the sensitivity of these structures to destruction by common wave‐induced motions. Key Points: We establish conditions for the disruption of staircases by strong vertical shear associated with internal wavesThin layers are more easily disrupted by shear than thick onesWe evaluate the efficacy of previous steppiness metrics, finding that the rms temperature gradient yields accurate results [ABSTRACT FROM AUTHOR]

Atlantic Meridional Overturning Circulation (AMOC) exhibits interdecadal to multidecadal variability, yet the role of surface freshwater flux (FWF) variability in this AMOC variability remains unclear. This study isolates the contribution of FWF variability in modulating AMOC through a partially coupled experiment, in which the effect of the interactive FWF is disabled. It is demonstrated that the impact of the coupled FWF variability enhances the persistence of density and deep convection anomalies in the Labrador Sea (LS), thus lengthening the period of the AMOC oscillation on multidecadal timescale and suppressing its ∼30‐year periodicity. Further lead‐lag regressions illuminate that the more persistent LS density anomalies are maintained by two mechanisms: (a) The local temperature‐salinity coupling through the evaporation and (b) a downstream propagation along the East Greenland Current of the extra salinity anomaly due to the sea ice melting changes associated with an atmosphere forcing over the southern Greenland tip. Plain Language Summary: The long‐term variability of Atlantic Meridional Overturning Circulation (AMOC) has a profound impact on the Earth's climate. However, the extent to which they are regulated by variability in surface freshwater fluxes (FWF) remains largely elusive. Here, we use a partial‐coupling technique to isolate the contribution of the FWF variability to the low‐frequency AMOC variability. In the absence of interactive FWF, the density anomalies important to the deep convection in the Labrador Sea (LS) are dominated by the temperature anomalies being advected around the subpolar gyre, with the salinity anomalies working to partially compensate the temperature effect. With interactive FWF, however, convection anomalies in the LS initialized by temperature changes are extended and reinforced by salinity changes and persist over a much longer time, thus extending the periodicity of the AMOC variability on multidecadal timescale. The more persistent deep convection in the LS is found to be achieved through a local and a remote mechanism. First, the local density anomalies initialized by temperature changes are extended and reinforced by evaporation‐induced salinity changes. In addition, salinity anomalies in the upstream Irminger Sea due to sea ice melting anomalies can propagate into LS to affect LS density anomalies. Key Points: The surface freshwater flux (FWF) variability lengthens the period of the Atlantic Meridional Overturning Circulation (AMOC) variabilityThe contribution from salinity in generating deep convection is enhanced with the active FWF effectBoth local and remote effects of FWF are important in modulating AMOC low‐frequency variability [ABSTRACT FROM AUTHOR]

Observations from the RAPID array at 26.5°N suggest a weakening in the Atlantic meridional overturning circulation between 2004 and 2020. The shortness of this time series raises the question of whether this weakening should be thought of as natural variability or as related to climate change. To estimate natural variability, preindustrial control runs of the Coupled Model Intercomparison Project Phase 6 are used and the observed weakening is found to fall within the distribution of naturally occurring trends. These trends result from model variability on periods longer than the observational window, and variation between models at these frequencies is especially pronounced. Finally, adding noise to inflate model variability also inflates model trends; however, the effect on trends is sensitive only to the low frequency portion of the noise spectrum. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is a major ocean circulation system that transports heat northward in the Atlantic basin thus contributing to the regulation of the climate system. Fluctuations in the AMOC have been linked with changes in Arctic sea ice and to variations in the climate of Europe and North America. The longest direct observational record of the AMOC spans only 15.8 years and has shown a weakening of the circulation. Whether this weakening is due to natural oscillations in the climate system or is a response to anthropogenic greenhouse gases in the atmosphere is unclear. To assess whether the ocean circulation produces spontaneously weakening signals of such intensity, long simulations of climate models run under a preindustrial scenario are used to estimate the natural variability of the AMOC. These models suggest that the current weakening of the AMOC is consistent with natural variability. An accurate representation of slow timescale fluctuations in the AMOC is what matters most to the detection of an anthropogenically driven AMOC trend. This variability, however, differs markedly across models. Key Points: The 2004–2020 observed Atlantic meridional overturning circulation (AMOC) trend is not significantly relative to Coupled Model Intercomparison Project Phase 6 (CMIP6) model natural variabilityCMIP6 models largely disagree on AMOC variability, including at decadal and longer periodsIt is periods longer than the observational window that set AMOC trend detectability thresholds [ABSTRACT FROM AUTHOR]

A regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) and its causes were investigated using a 2,000‐year‐long pre‐industrial control simulation of a fully coupled atmosphere‐ocean‐sea ice model. The volume budget analysis revealed that the air‐sea flux and ocean dynamics (OD) were the two primary driving mechanisms for NPSTMW formation, but their relative importance has periodically alternated in multidecadal timescales of approximately 50–70 years. The regime shift of the NPSTMW formation was closely related to the meridional (50 years) and zonal (70 years) movements of the Aleutian Low (AL). When AL shifted to the south or east, it induces the sea surface height anomalies propagating westward from the central North Pacific and preconditions the NPSTMW formation, thus the OD become relatively more important. Plain Language Summary: In the western North Pacific, a thick subsurface layer with low potential vorticity and relatively uniform potential density called the North Pacific subtropical mode water (NPSTMW) is found between depths of 100 and 400 m. Using a coupled atmosphere‐ocean‐sea ice model integrated over 2,000 years with constant CO2 concentration at the pre‐industrial level, we studied the multidecadal regime shift of the NPSTMW formation. It was found that the major driving mechanisms of NPSTMW formation periodically shifted from local atmospheric forcing to ocean dynamics (OD) (and vice versa) in approximately 50–70 years. It was also found that the multidecadal regime shifts of the NPSTMW formation are associated with the latitudinal (50 years) and longitudinal (70 years) shifts of the Aleutian Low (AL) position, that is, large (small) southward and eastward extent of the AL during the epoch when the OD (air‐sea flux [ASF]) dominates. The position shift of the AL affects the variability of the local ASF and remotely driven OD via the Ekman pumping/suction. Key Points: Driving mechanisms for the North Pacific subtropical mode water formation exhibit a regime shift with a periodicity of about 50–70 yearsMultidecadal regime shifts are associated with meridional and zonal shifts in the Aleutian Low (AL)Position shift of the AL affects the variability of the local air‐sea flux and remotely driven oceanic dynamics [ABSTRACT FROM AUTHOR]

The sensitivity of sea ice to the contrasting seasonal and perennial snow properties in the southeastern and northwestern Weddell Sea is not yet considered in sea ice model and satellite remote sensing applications. However, the analysis of physical snowpack properties in late summer in recent years reveals a high fraction of melt‐freeze forms resulting in significant higher snow densities in the northwestern than in the eastern Weddell Sea. The resulting lower thermal conductivity of the snowpack, which is only half of what has been previously assumed in models in the eastern Weddell Sea, reduces the sea ice bottom growth by 18 cm during winter. In the northwest, however, the potentially formed snow ice thickness of 22 cm at the snow/ice interface contributes to additional 7 cm of thermodynamic ice growth at the bottom. This sensitivity study emphasizes the enormous impact of unappreciated regional differences in snowpack properties on the thermodynamic ice growth. Plain Language Summary: The sea ice cover in the Weddell Sea shows different ice age classes and can therefore be considered as a representative basin of the ice‐covered Southern Ocean: while seasonal sea ice is found in the eastern Weddell Sea, it tends to be perennial in the west. Due to the year‐round Antarctic snow cover, this age classification also applies for the snow column. However, the associated regional differences in snow properties and snow‐to‐ice conversion processes at the snow/ice interface are not yet considered in both sea ice model and satellite remote sensing applications when retrieving, for example, sea ice thickness. Based on recent snow observations in the region, regionally adjusted values for snow density and thermal conductivity could be determined. While the adjusted parameters result in attenuated ice growth in the eastern Weddell Sea, significant snow ice formation causes additional thermodynamic ice growth in the western Weddell Sea. Key Points: High fraction of melt‐freeze forms causes significant higher snow densities in the northwestern than in the eastern Weddell SeaLower thermal conductivity of snow in the eastern Weddell Sea than previously assumed results in significantly reduced potential bottom sea ice growthIn the northwestern Weddell Sea, the substantial amount of snow ice leads to additional thermodynamic ice growth at the ice bottom [ABSTRACT FROM AUTHOR]

Systematic biases in General Circulation Model (GCM) simulations require some adjustment before their use in change assessment and adaptation management studies. GCM simulations of the Coupled Model Intercomparison Project 6, although outperform the previous generations of GCMs, exhibit persistent biases in magnitude, variability, and frequency across a range of variables of interest. Here, we propose a novel continuous wavelet‐based bias correction (CWBC) approach to address such biases in the time‐frequency domain. The correction focuses on the magnitude and frequency of the modeled time series, as interpreted via the time‐varying spectrum ascertained using the continuous wavelet transform. The approach is applied to correct systematic biases in the time series of Niño 3.4 sea surface temperature and Arctic sea‐ice extent. The application of CWBC successfully reproduces observed attributes in the bias‐corrected time series of both climate variables for the current climate simulation along with providing a sensible projection for the future. Plain Language Summary: The latest generation of General Circulation Models outperforms older versions in simulating the current and future climate. However, biases in magnitude, frequency, variability, and persistence related characteristics still remain in model simulations. The nested time based and other frequency analysis‐based bias correction approaches, for example, a fast Fourier transform or a discrete wavelet‐based bias correction, offer a good alternative to correct the biases. However, they cannot capture the underlying components of climate variables in the non‐dyadic spectrum. Keeping this in mind, we present here a continuous wavelet transform based bias correction approach to obtain the underlying information of climate variables at a finer spectrum resolution. The applications of continuous wavelet‐based bias correction algorithm show remarkable improvement in the amplitude and frequency of two example datasets, namely, a Coupled Model Intercomparison Project 6 (CMIP6) sea surface temperature simulation and the ensemble mean of CMIP5 Arctic sea‐ice extent simulations, for the current climate and more sensible correction for future climate simulations. Key Points: Systematic biases exist in the magnitude and frequency of climate events simulated by the modelsA continuous wavelet transform‐based method for identifying and correcting biases in the time‐frequency domain is presentedTime series of Niño 3.4 sea surface temperature and Arctic sea‐ice extent is used to demonstrate the usefulness of the approach [ABSTRACT FROM AUTHOR]

The Ice, Cloud, and land Elevation Satellite 2 (ICESat‐2) is a revolutionary remote sensing tool designed to provide high‐resolution height estimates of the Earth's surface. The photon counting LiDAR instrument onboard collects data over various surfaces and provides detailed information on regions where present observations are sparse or nonexistent. ICESat‐2 is initiating a wave of new science, and this study examines a methodology for deriving ocean surface wave and wind characteristics. Using a modified version of the ICESat‐2 land surface algorithm, individual waves are mapped along the ICESat‐2 track. Significant wave features, associated ocean wind speed, and wave energy spectra are determined from the ocean data. Validation against trustworthy data sources indicate the root mean squared error of significant wave heights is 0.3 m and is 2.4 m s−1 for wind speed. Wave spectra are more variable but peak swell and wind seas generally agree within ±8–13% of buoy estimates. Plain Language Summary: Large changes occurring in the North and South Poles due to global warming is consistently in the news. One problem we have with really understanding the changes in those places is we do not have the right tools to study them in detail or over large areas. But now we do! There is a new satellite orbiting the Earth called ICESat‐2 that is gathering information about the changes in the ice and snow heights at the poles. ICESat‐2 is also able to make measurements over the oceans and land. The oceans have a wide influence on weather and climate, and ICESat‐2 provides a new way for advancing ocean science. This paper talks about how ICESat‐2 can see individual ocean waves and determine important information about the ocean and air above the ocean. We use this information from the waves to compare with other data and confirm that useful ocean information is available over much smaller distances with fewer assumptions about the ocean condition. The new results can lead to other studies that are related to sea ice changes in high latitudes or changes in sea level, with added potential along coastal areas around the world. Key Points: ICESat‐2 provides high‐resolution surface elevation along‐track data for many unique applicationsIndividual ocean waves are observed by the satellite across the region of interest and inform on important ocean characteristicsValidation confirms and supports the significant wave height and wind speed results and highlights the ICESat‐2 contribution to marine meteorology and oceanography [ABSTRACT FROM AUTHOR]

Understanding influence of vegetation on past temperature changes in the Arctic region would help isolate uncertainty and build understanding of its broader climate system, with implications for paleoclimate reconstructions and future climate change. Using an Earth system model EC‐Earth, we conduct a series of simulations to investigate the impact of vegetation‐climate feedback on the Arctic climate during the mid‐Holocene. Results show Arctic greening induced by the warming resulting from stronger orbital forcing, further amplifies the Arctic warming. The increased vegetation contributes 0.33°C of Arctic warming and 0.35 × 106 km2 of Arctic sea ice loss. Increased Arctic vegetation leads to reduced land surface albedo and increased evapotranspiration, both of which cause local warming in spring and summer. The resultant sea ice loss causes warming in the following seasons, with atmospheric circulation anomalies further amplifying the warming. Our results highlight the significant contribution of vegetation‐climate feedback to Arctic climate under natural conditions. Plain Language Summary: Vegetation‐climate feedback is an essential process between the terrestrial biosphere and the atmosphere. Greening of the Arctic can be invoked by increased solar radiation, for example, during the mid‐Holocene (6,000 years ago), or increased greenhouse gas, for example, ongoing global warming. Here we perform simulations of vegetation and climate interactions during the mid‐Holocene and demonstrate the impact of vegetation‐climate feedback on the Arctic climate. We show that compared to the pre‐industrial period (1850 CE), the Arctic region experiences an expansion of vegetation under a warm climate due to increased solar radiation during the mid‐Holocene. The greening leads to reduced land albedo and enhanced evapotranspiration. These changes and resultant sea ice loss, in combination with associated atmospheric circulation changes, further amplify the Arctic warming. These changes alone are responsible for 0.33°C increase in surface air temperature and 0.35 × 106 km2 increase in sea ice loss. Given the facts of observed ongoing Arctic greening in recent decades, our modeling results suggest that the effect of the vegetation‐climate feedback will amplify the future Arctic warming and sea ice loss even more. Key Points: The Earth system model EC‐Earth reproduced the expansion of vegetation in northern high latitudes during the mid‐Holocene compared to the PIThe mid‐Holocene Arctic greening contributes 0.33°C increase in annual Arctic warming and 0.35 × 106 km2 of sea ice lossThe increased evapotranspiration and decreased surface albedo due to the Arctic greening amplify the Arctic warming in the growing season [ABSTRACT FROM AUTHOR]

This study exams the main sources of inter‐model spread in Arctic amplification of surface warming simulated in the abrupt‐4 × CO2 experiments of 18 CMIP6 models. It is found that the same seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean in summer due to sea‐ice melting is temporally stored in ocean in summer and is released in cold months, is responsible for the Arctic amplification in each of the 18 simulations. The models with more (less) ice melting and heat storing in the ocean in summer have the stronger (weaker) ocean heat release in cold season. Associated with more (less) heat release in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface sensible and latent heat fluxes. This explains why the Arctic surface warming is strongest in the cold months and so is its inter‐model spread. Plain Language Summary: The seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean during sea‐ice melting season is temporally stored in ocean, which in turn is released in cold months, is recognized as the primary mechanism accounting for the Arctic amplification of surface warming. The same seasonal energy transfer mechanism is responsible for the Arctic amplification in each of the 18 CIMP6 abrupt CO2 quadrupling climate simulations. The models that have more (less) ice melting and heat storing in the ocean during the early melting season (April‐May‐June) would have the stronger (weaker) ocean heat release in January‐February‐March, contributing directly to stronger (weaker) surface warming in cold months. Associated with more (less) heat release from Arctic Ocean in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface turbulent sensible and latent heat fluxes, which further increases the inter‐model spread of the winter warming, and thereby the annual mean warming as well. Therefore, the inter‐model spread of the sea‐ice melting in April‐May‐June accounts for the major portion (more than 80%) of the inter‐model spread in both the winter warming and the annual mean warming. Key Points: The surface warming in the Arctic is strongest in the cold months and so is its uncertainty, especially in the months of January‐February‐MarchContributions to the inter‐model spread of seasonal pattern of Arctic warming are quantified via a linearized radiative transfer modelThe inter‐model spread in sea ice melting in early summer is relayed to and gets amplified in cold months by oceanic heat storage/release [ABSTRACT FROM AUTHOR]

The structural integrity of the arctic sea ice cover is under threat owing largely to the combination of thinning and larger waves. Another contributor may be thermal cracking. In concentrating stress, thermal cracks may weaken the cover. Of interest, therefore, is the strength of thermally damaged ice. To that end, new experiments were performed on sea ice and on lab‐grown saline and salt‐free ice that had been cracked by thermal shocking. As expected, the cracks weakened the materials in accord with fracture mechanics. However, within tens to hundreds of seconds of shocking, the strength recovered completely, for the ice had healed. Healing is attributed to thermally activated sintering related to surface diffusion, assisted possibly by the formation of a quasi‐liquid layer on crack faces. Whether behavior on the small scale is indicative of behavior on the large scale remains to be determined. Plain Language Summary: The combination of thinning ice cover and increasing wave amplitude (owing to increasing oceanic fetch) threatens the structural integrity of the ice cover on the Arctic Ocean. Another contributor may be thermal cracking. In concentrating stress, thermal cracks may weaken the cover. Of interest, therefore, is the strength of thermally damaged ice. With that in mind, thermal cracks were introduced into sea ice and into lab‐grown saline ice and freshwater ice that was subsequently bent to failure in the laboratory. The cracks weakened the materials, in accord with fracture mechanics. However, within a short period of time, the strength completely recovered to non‐cracked levels. The cracks had healed. Healing is attributed to sintering upon warming, assisted possibly by the presence of a thin liquid layer on opposing crack faces. Whether behavior on the small scale is indicative of behavior on the large scale remains to be determined. Key Points: Thermally induced cracks within salty and salt‐free ice heal within tens to hundreds of secondsHealing completely restores flexural strength [ABSTRACT FROM AUTHOR]

Among mechanisms accounting for atmospheric pCO2 drawdown during glacial periods, processes operating in the North Atlantic (NA) and Southern Ocean (SO) have been proposed to be critical. Their individual and synergic effects during a course of glaciation, however, remain enigmatic. We conducted simulations to examine these effects at idealized glacial stages. Under early‐glacial‐like conditions, cooling in the SO can trigger an initial pCO2 drawdown while the associated sea ice expansion has little impact on air‐sea gas exchange. Under later glacial‐like conditions, further cooling in the NA enhances ocean carbon uptake due to a stronger solubility pump, and the SO‐induced stronger deep stratification prevents carbon exchange between the deep and upper ocean. Meanwhile, strengthened dust deposition increases the SO contribution to the global biological pump, and CO2 outgassing is suppressed by fully extended sea ice cover. More carbon is then stored in the deep Pacific, acting as a passive reservoir. Plain Language Summary: CO2 is one of the most important "greenhouse" gases that drive global climate changes. Tens of thousands of years ago, during the glacial time (known as the "ice age"), atmospheric CO2 was much lower than today. Research has shown that processes in different ocean regions, such as the Southern Ocean (SO) and North Atlantic (NA), made essential contributions to the glacial CO2 drawdown. However, the interplay between these processes remains unclear. Here, using an ocean general circulation model coupled with an atmospheric box that includes active air‐sea CO2 exchange, we examined the key processes at idealized stages of a glacial cycle. We found that: at an early stage, surface cooling in the SO triggers the CO2 decrease by shortening the time for air‐sea CO2 exchange; during a late stage, the SO and NA cooling act collaboratively to store more carbon in the deep ocean, where the SO plays a more determining role. Pacific seems to be a passive carbon pool during glaciation. Sea ice expansion hinders CO2 outgassing increasingly over the course of glaciation. Further CO2 decrease is achieved by a larger contribution of the SO to the global biological carbon pump. Key Points: The stepwise glacial atmospheric pCO2 decrease is linked to a Southern Ocean (SO)‐controlled synergy of interhemispheric mechanismsSea ice prevents CO2 outgassing increasingly from the early‐ to later‐glacial timeThe SO contribution to the global biological carbon pump might increase from interglacial to glacial periods [ABSTRACT FROM AUTHOR]

Surface winds around the Antarctic continent control coupled ocean‐ice processes that influence the climate system, including bottom water production, heat transport onto the continental shelf and sea ice coverage. However, few studies have examined projected changes in these winds, even though it would aid in the interpretation and understanding of the ocean's response to climate change. Using Coupled Model Intercomparison Project Phase 6 and reanalysis data, we show a significant reduction in the near‐Antarctic surface winds throughout the historical period that continues until the end of the twenty‐first century, amounting to 23% and 7% for the easterly and southerly wind components respectively under the high emission scenario. The most intense weakening happens during the summer season. We find that the weakening is coherent with the trend toward a positive Southern Annular Mode and a reduction of the pole‐to‐coast meridional pressure gradient, which we term Antarctic Annular Index. Plain Language Summary: Surface winds over the ocean around the Antarctic continent influence several aspects of the oceanic circulation and sea ice in the region that become relevant in the context of climate change. For example, Antarctic coastal surface winds have been found to drive the warming experienced in some regions that subsequently triggers increased ice shelf melt. However, there is little understanding regarding how this wind regime is expected to change in the future, with most research focusing on the mid‐latitude westerlies. In this work, we use Coupled Model Intercomparison Project Phase 6 models to quantify projected changes in these winds to the end of the twenty‐first century, hoping that it will aid in the interpretation of the ocean's response to climate change. Under the high emission scenario, we find a significant weakening of 23% for the easterly wind component and 7% for the meridional wind component. This weakening can be partly explained by a large‐scale pattern of change in sea level pressure that reflects in an increase of the atmospheric mode of variability known as the Southern Annular Mode, and a decrease of the pole‐to‐coast surface pressure gradient, which we term Antarctic Annular Index. Key Points: Coupled Model Intercomparison Project Phase 6 models show a weakening of the near‐Antarctic surface winds during the period 1979–2015Future projections in CMIP6 models show that the weakening trend continues until the end of the twenty‐first centuryWeakened winds are associated with a more positive Southern Annular Mode and a reduction in the pole‐to‐coast meridional pressure gradient [ABSTRACT FROM AUTHOR]

Antarctic geothermal heat flow (GHF) affects the thermal regime of ice sheets and simulations of ice and subglacial meltwater discharge to the ocean, but remains poorly constrained. We use an ice sheet model to investigate the impact of GHF anomalies on subglacial meltwater production in the Aurora Subglacial Basin, East Antarctica. We find that spatially‐variable GHF fields produce more meltwater than a constant GHF with the same background mean, and meltwater production increases as the resolution of GHF anomalies increases. Our results suggest that model simulations of this region systematically underestimate meltwater production using current GHF models. We determine the minimum basal heating required to bring the basal ice temperature to the pressure melting point, which should be taken together with the scale‐length of likely local variability in targeting in‐situ GHF field campaigns. Plain Language Summary: Geothermal heat flow (GHF) is important in controlling both the ice temperature and the production of meltwater at the base of the Antarctic Ice Sheet, which impacts how rapidly ice flows. However, GHF estimates are generally low resolution and highly uncertain. This uncertainty in GHF impacts the reliability of model simulations of the flow of ice and meltwater into the ocean, which in turn impacts estimates of the contribution of ice sheets to future sea level rise. We use an ice sheet model to investigate how spatial variations in GHF impact meltwater production in the Aurora Subglacial Basin (ASB), East Antarctica. GHF fields with spatial variations lead to consistently higher melt rates at the ice sheet base than a constant GHF field with the same mean value. We determine the minimum heat at the ice sheet base in the ASB required to cause the ice to melt, and highlight regions where small variations in GHF will have greater impacts on ice melting. These results show where measurements to constrain GHF should be prioritized to improve ice sheet model simulations. Key Points: We present simulations of subglacial melt in the Aurora Subglacial Basin using various geothermal heat flow (GHF) fieldsSpatially‐variable GHF enhances simulated subglacial melt compared with constant GHFWe highlight where GHF measurements should be prioritized for improved estimates of subglacial melt [ABSTRACT FROM AUTHOR]

Multidecadal variability of Atlantic Meridional Overturning Circulation (AMOC) has been reconstructed by various proxies, simulated in climate models, and linked to multidecadal Arctic salinity variability. Here we construct a simple conceptual model to understand the two‐way interactions of the Arctic with multidecadal AMOC variability through a delayed oscillator mechanism. We revise Stommel's Two‐Box Model by including an advective time delay for the Arctic density/salinity anomalies to reach the subpolar North Atlantic and a coupled negative feedback between the AMOC and the freshwater flux entering the Arctic through atmosphere and/or sea ice responses. Self‐sustained multidecadal AMOC oscillations exist in the revised Stommel's Two‐Box model if the oceanic advective time delay is longer than the oscillation threshold, and the periods of the AMOC delayed oscillator depend crucially on this advective time delay. The coupled freshwater feedback provides additional delayed negative feedback and reduces the advective time delay threshold required for the oscillations. Plain Language Summary: Stommel's Two‐Box Model provides a pioneering and powerful theoretical framework to study the mechanisms of steady Atlantic Meridional Overturning Circulation (AMOC) states and abrupt AMOC changes. However, Stommel's Two‐Box Model itself does not include a self‐sustained multidecadal AMOC oscillation solution. In our study, we revise Stommel's Two‐Box model by considering the delayed oceanic advective time lag for the Arctic density/salinity anomalies to reach the subpolar North Atlantic and the dependence of the freshwater flux entering the Arctic on the AMOC strength. The revised Stommel's Two‐Box Model is able to obtain the AMOC delayed oscillator at multidecadal timescales and suggests the important role of the Arctic salinity anomalies and associated delayed negative feedback in the multidecadal AMOC variability. The regimes and multidecadal periods of the AMOC delayed oscillator depend crucially on the advective time scale of the Arctic signal reaching the subpolar North Atlantic. The AMOC‐related coupled freshwater feedback provides additional delayed negative feedback and reduces the threshold of the advective time delay needed for the multidecadal AMOC oscillations. Key Points: Multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability exists in revised Stommel's Two‐Box Model with advective time delay and coupled freshwater feedback includedRegimes/periods of the AMOC delayed oscillator depend on the advective time lag of Arctic salinity signal and freshwater feedback strengthThe model suggests an important role of the Arctic salinity and delayed negative freshwater feedback in the multidecadal AMOC variability [ABSTRACT FROM AUTHOR]

Mixed‐phase clouds (MPCs), which consist of both supercooled cloud droplets and ice crystals, play an important role in the Earth's radiative energy budget and hydrological cycle. In particular, the fraction of ice crystals in MPCs determines their radiative effects, precipitation formation and lifetime. In order for ice crystals to form in MPCs, ice‐nucleating particles (INPs) are required. However, a large‐scale relationship between INPs and ice initiation in clouds has yet to be observed. By analyzing satellite observations of the typical transition temperature (T*) where MPCs become more frequent than liquid clouds, we constrain the importance of INPs in MPC formation. We find that over the Arctic and Southern Ocean, snow and sea ice cover significantly reduces T*. This indicates that the availability of INPs is essential in controlling cloud phase evolution and that local sources of INPs in the high‐latitudes play a key role in the formation of MPCs. Plain Language Summary: Mixed‐phase clouds (MPCs), which consist of both liquid droplets and ice crystals, play an important role for the Earth's climate system. For example, the number of ice crystals in MPCs determines how much sunlight is reflected by the cloud and how efficiently the cloud can form precipitation. The formation of ice crystals in MPCs requires a special subset of aerosol particles called ice‐nucleating particles (INPs). INPs are required for liquid cloud droplets to freeze at temperatures warmer than −36°C. However, a large‐scale relationship between INPs and ice formation in clouds has yet to be observed. Using satellite observations, we determine the transition temperature (T*) where MPCs become more frequent than liquid clouds and find that T* is strongly dependent on snow and sea ice cover over the Arctic and Southern Ocean. This indicates that sea ice and snow cover act as a lid that inhibits the emission of INPs from the ocean. In a warming world with retreating sea ice and snow cover, our results suggest that clouds in these regions will contain ice crystals at warmer temperatures than previously estimated and, thus, have potential implications for future warming predictions. Key Points: Ice‐nucleating particles (INPs) control ice formation in high‐latitude cloudsSea ice and snow cover inhibit the local emission of INPs, which directly influences cloud phase in the Arctic and Southern OceanThis has implications for the predicted negative cloud phase feedback with future warming and the associated sea ice and snow cover loss [ABSTRACT FROM AUTHOR]

While the annual mean Arctic Amplification (AA) index varied between two and three during the 1970–2000 period, it reached values exceeding four during the first two decades of the 21st century. The AA did not change in a continuous fashion but rather in two sharp increases around 1986 and 1999. During those steps the mean global surface air temperature trend remained almost constant, while the Arctic trend increased. Although the "best" CMIP6 models reproduce the increasing trend of the AA in 1980s they do not capture the sharply increasing trend of the AA after 1999 including its rapid step‐like increase. We propose that the first sharp AA increase around 1986 is due to external forcing, while the second step close to 1999 is due to internal climate variability, which models cannot reproduce in the observed time. Plain Language Summary: The rate of Arctic warming varied in time, while the rate of global warming was nearly constant over the period 1960–2020. This led to a variable Arctic Amplification (AA) defined as a ratio of the Arctic warming trend to mean global warming trend. In addition, the Arctic experienced sudden changes in the rate of warming with step‐like changes in annual mean Arctic Amplification around the years 1986 and 1999. Climate models do not capture the second observed sharp increase of the AA. We suggest that the first AA increase around 1986 is primarily due to external forcing (increasing concentration of carbon dioxide) while the second sharp increase around 1999 is dominated by internal climate variability which current models cannot capture accurately within the time scale of the physical phenomena. Key Points: Annual mean Arctic Amplification (AA) within the period 1970–2020 changed in steep steps around 1986 and 1999. It reached values over 4.0Even those CMIP6 models best at reproducing the AA did not reproduce the second sudden increase near 1999We conjecture that the first AA sharp increase is due to external forcing, while the second is due to internal climate variability [ABSTRACT FROM AUTHOR]

During late February and early March 2018, an unusual polynya was observed off the north coast of Greenland. This period was also notable for the occurrence of a sudden stratospheric warming. Here we use satellite and in situ data, a reanalysis and an ice‐ocean model to document the evolution of the polynya and its synoptic forcing. We show that its magnitude was unprecedented and that it was associated with the transient response to the sudden stratospheric warming leading to anomalous warm southerly flow in north Greenland. Indeed, regional wind speeds and temperatures were the highest during February going back to the 1960s. There is evidence that the thinning sea ice has increased its wind‐driven mobility. However, we show that the polynya would have developed under thicker ice conditions representative of the late 1970s and that even with the predicted trend toward thinner sea ice, it will only open during enhanced southerly flow. Plain Language Summary: In late February 2018, satellite imagery revealed the presence of a large polynya (a region of reduced sea ice cover within the pack), in the Wandel Sea off the north coast of Greenland. Since this region is not known for the development of polynyas, this discovery generated interest among Arctic observers and in the science community, raising questions about the nature and cause of this unusual event. In this paper, we show that its opening coincided with a period of sustained and unusually warm winds from the south, with above‐freezing temperatures and wind speeds in excess of 25 m/s reported at local weather stations. February 2018 was also notable for a sudden stratospheric warming event, in which an abrupt warming of the atmosphere between 10‐ and 50‐km altitudes occurred in conjunction with a reversal of the stratospheric winds. We show that this event was responsible for the polynya. We also use a computer model to confirm the dominant role of the winds in creating the polynya. Finally, we show that even with future thinning of sea ice due to climate change, extreme winds will remain necessary to create a polynya in this region over the next few decades. Key Points: During late February and early March 2018 a polynya opened off of north Greenland in a region not previously known for polynya formationThis event occurred after a sudden stratospheric warming that we show was responsible for the conditions that resulted in its developmentWe show that the high winds during the event played a dominant role in its formation and sea ice thickness played a less important role [ABSTRACT FROM AUTHOR]

The thick multiyear sea ice that once covered large parts of the Arctic is increasingly being replaced by thinner and weaker first‐year ice, making it more vulnerable to breakup by winds. We use the neXtSIM sea ice model to investigate the driving mechanisms behind a large breakup event in the Beaufort Sea during winter 2013. Our simulations are the first to successfully reproduce the timing, location, and propagation of sea ice leads associated with wind‐driven breakup and highlight the importance of accuracy of the atmospheric forcing, sea ice rheology, and changes in sea ice thickness. We found that the breakup resulted in enhanced export of multiyear ice from the Beaufort Sea. Overall, this leads to a relatively thinner and weaker simulated ice cover that potentially preconditions earlier breakup in spring and accelerates sea ice loss. Finally, our simulations indicate that large breakup events could become more frequent as Arctic sea ice continues to thin. Plain Language Summary: The loss of thick multiyear sea ice in the Arctic leads to weaker sea ice that is more easily broken up by strong winds. As a consequence, extreme sea ice breakup events may become more frequent, even during the middle of winter when the sea ice cover is frozen solid. This can lead to an earlier onset of the melt season and potentially accelerate Arctic sea ice loss. Such extreme breakup events are generally not captured by climate models, potentially limiting our confidence in projections of Arctic sea ice. We investigated the driving forces behind sea ice breakup events during winter and how they change in a future climate. Our sea ice model is the first to reproduce such breakup events and reveals that the combination of strong winds and thin sea ice is a key factor for these breakups. We found that winter breakups have a large effect on local heat and moisture transfer and cause enhanced sea ice production, but also increase the overall movement of the sea ice cover, making it more vulnerable. Finally, we show that if the Arctic sea ice continues to thin, these extreme breakup events could become even more frequent. Key Points: An extreme winter breakup event in the Beaufort Sea is accurately simulated with the neXtSIM sea ice modelHigh‐resolution and skillful atmospheric forcing, as well as appropriate sea ice rheology, are necessary to simulate such breakupsIn the model, thinning Beaufort Sea ice is more vulnerable to breakup, which may accelerate loss of multiyear ice [ABSTRACT FROM AUTHOR]

On 25 February 2022 Antarctic sea ice extent dropped to a satellite‐era record low level of 1.92 × 106 km2, 0.92 × 106 km2 below the long‐term mean. The area of sea ice was also at a record low level of 1.24 × 106 km2. Although no individual sector was at a record low, at the minimum there were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross (contributing 46%) and Weddell Seas (26%). The Amundsen Sea Low had a record low depth in October/November 2021, with a series of very deep depressions giving strong offshore winds. These accelerated ice loss during the melt season, creating a 1.00 × 106 km2 coastal polynya in the Ross Sea. In the northern Weddell Sea, westerly winds of record strength led to ice export from the region. Plain Language Summary: Sea ice is a critical part of the Antarctic climate system that is an important breeding habitat for seals and other marine biota, as well as playing an important part in the global ocean circulation. Unlike the Arctic, where sea ice extent has decreased over recent decades, sea ice around the Antarctic actually increased in extent for a large part of the record starting in the late 1970s. However, on 25 February 2022 the extent dropped to a record low level of 1.924 million square kilometers, 1 million square kilometers below the long‐term average. The origins of the sea ice loss can be traced back to extremely deep storms in the Ross Sea in October and November 2021, and the very strong southerly winds that the storms had on their western flank. These moved sea ice away from the Antarctic coast, exposing the ocean and allowing solar heating to warm the ocean and give further sea ice melt. An additional factor was the very strong westerly winds north of the Weddell Sea that moved sea ice out of this basin and toward the east. Key Points: Antarctic sea ice extent dropped to a record low level of 1.92 × 106 km2 on 25 February 2022There were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross and Weddell SeasDeep storms in October/November 2021 led to low sea ice concentration and a large coastal polynya that accelerated sea ice loss [ABSTRACT FROM AUTHOR]

The influence of atmospheric composition on the climates of present‐day and early Earth has been studied extensively, but the role of ocean composition has received less attention. We use the ROCKE‐3D ocean‐atmosphere general circulation model to investigate the response of Earth's present‐day and Archean climate system to low versus high ocean salinity. We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 to 50 g/kg results in a 71% reduction in sea ice cover in our present‐day Earth scenario. This same salinity change also halves the pCO2 threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO2 and CH4, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving ∼20% less energy from the Sun. Plain Language Summary: The composition of the atmosphere, especially the abundance of greenhouse gases, famously influences Earth's climate system. We use a climate model to show that the composition of the ocean can also have a major impact on surface temperature and ice cover. We focus specifically on the amount of salt dissolved in seawater, and we find that saltier oceans tend to result in warmer climates. These effects are modest today, but salt may be a key ingredient for early Earth habitability in the distant past when the Sun was less bright. Key Points: Saltier oceans result in warmer climates with less sea iceWarming with increasing salinity is strongly affected by changing ocean dynamicsA saltier ocean may have helped keep early Earth warm when the Sun was less luminous [ABSTRACT FROM AUTHOR]

There is low confidence in projections of Antarctic sea ice area (SIA), due to deficiencies in climate model sea ice processes. Ensemble regression techniques can help to reduce this uncertainty. We investigate relationships between SIA climatology and 21st century change in the Coupled Model Intercomparison Project, phase 6 (CMIP6) multi‐model ensemble. In summer, under a strong forcing scenario, each model loses the majority of its sea ice. Therefore, models with greater historical SIA exhibit greater reductions, so the observed climatology of SIA strongly constrains projections. Ensemble spread in historical summer SIA is smaller than in CMIP phase 5 (CMIP5), and CMIP6 gives a more robust constraint on future SIA. In winter, by 2100 under a strong forcing scenario, 40% of SIA disappears on average, and ensemble spread in historical mean SIA explains approximately half the spread in projected change. A greater winter ice loss in CMIP6 than CMIP5 is explained by the higher climate sensitivities of some CMIP6 models. Plain Language Summary: Antarctic sea ice area is important for ecosystems, human activity, and for the dynamics of the atmosphere and ocean. However, major climate change assessments such as the Intergovernmental Panel on Climate Change Sixth Assessment Report have placed little confidence in projections of Antarctic sea ice area. This is because climate models struggle to simulate many aspects of sea ice area as observed by satellites. However, not all models should be treated equally. Models with relatively more sea ice area in the recent past project greater losses in the future, especially in summer. This suggests that models with historical sea ice area closest to what we have observed may be more reliable in their future simulations. We use this information in a simple statistical model to show that, firstly, the newest climate models largely show near‐total sea ice loss in summer by the end of the 21st century, which was not universally true for older climate models. Secondly, newer climate models lose more winter sea ice than their predecessors in the same time period, which is related to the known greater global warming response to given greenhouse gas concentrations in the newer models. Key Points: We compare Coupled Model Intercomparison Project (CMIP), phase 5 and the CMIP, phase 6 (CMIP6) projections of Antarctic sea ice loss, assessing drivers of uncertainty and the robustness of projectionsUnder strong forcing, CMIP6 models lose most Antarctic summer sea ice by 2100, so projections are constrained by historical climatologyIncreased winter sea ice loss in CMIP6 is related to increased equilibrium climate sensitivity [ABSTRACT FROM AUTHOR]

Arctic sea ice has substantially changed over the past four decades, with a large decrease in sea‐ice area and volume. The exact causes of these changes are not entirely known. In our study, we make use of the Swedish Meteorological and Hydrological Institute Large Ensemble. This ensemble consists of 50 members realized with the EC‐Earth3 global climate model and covers the period 1970‐2100. We apply for the first time the Liang‐Kleeman information flow method to analyze the cause‐effect relationships between Arctic sea ice and its potential drivers. We show that recent and future changes in Arctic sea ice are mainly driven by air and sea‐surface temperatures and ocean heat transport. Conversely, changes in Arctic sea ice also considerably impact temperature and ocean heat transport. Finally, we find a progressive decrease in the influence of sea‐ice area and volume on air temperature and ocean heat transport through the twenty‐first century. Plain Language Summary: The Arctic has been warming at a larger rate than the rest of the world, resulting in a substantial loss of sea ice since the late 1970s. This has had and will continue to have an impact on our climate and societies. The exact causes of the ongoing sea‐ice loss are not entirely known, and understanding them is important in order to better prepare our societies for future climate changes. In our study, we apply a relatively novel approach that quantifies the cause‐effect relationships between Arctic sea ice and its potential drivers. We make use of a large range of model simulations performed with the EC‐Earth3 global climate model covering the period 1970‐2100. We find that air temperature, sea‐surface temperature, and the transport of heat by the ocean are important drivers of the ongoing and future retreat of Arctic sea ice. Conversely, changes in Arctic sea ice also affect the three former quantities. Our study demonstrates the performance of causal inference methods in the quest for a better understanding of relationships between climate variables. The geophysical and climate communities could greatly benefit from using these methods more intensively. Key Points: The Liang‐Kleeman rate of information transfer allows quantification of the directional dependence between Arctic sea ice and its driversRecent and future changes in Arctic sea ice are mainly driven by air and sea‐surface temperatures and ocean heat transportThe influence of Arctic sea ice on air temperature and ocean heat transport progressively decreases through the twenty‐first century [ABSTRACT FROM AUTHOR]

Historically, multiyear sea ice (MYI) covered a majority of the Arctic and circulated through the Beaufort Gyre for years. However, increased ice melt in the Beaufort Sea during the early 2000s was proposed to have severed this circulation. Constructing a regional MYI budget from 1997 to 2021 reveals that MYI import into the Beaufort Sea has increased year‐round, yet less MYI now survives through summer and is transported onwards in the Gyre. Annual average MYI loss quadrupled over the study period and increased from ∼7% to ∼33% of annual Fram Strait MYI export, while the peak in 2018 (385,000 km2) was similar in magnitude to Fram Strait MYI export. The ice‐albedo feedback coupled with the transition toward younger thinner MYI is responsible for the increased MYI loss. MYI transport through the Beaufort Gyre has not been severed, but it has been reduced so severely to prevent it from being redistributed throughout the Arctic Ocean. Plain Language Summary: Historically, sea ice grew thicker and aged into multiyear sea ice (MYI) as it was transported clockwise around the Beaufort Gyre for up to and beyond 10 years. This pattern facilitated the pan‐Arctic distribution of MYI that was typical of the 1980s and 1990s. However, warming temperatures and greater sea ice melt in the Beaufort Sea since the early 2000s have significantly increased the annual area of MYI lost to melt and was proposed to have severed MYI transport through the Beaufort Gyre. Here, we use a regional MYI budget to show that an increasing area of MYI is lost annually in the Beaufort Sea and that this has considerably altered and interrupted MYI transport through the Gyre for prolonged periods during recent years. This change has implications regionally for wildlife, shipping, and local communities, while also having an effect on the resiliency of the pan‐Arctic ice pack. Key Points: Multiyear sea ice (MYI) area loss in the Beaufort Sea quadrupled from 46,000 km2/yr−1 in 1997–2001 to 183,000 km2/yr−1 in 2017–2021MYI area loss peaked at 385,000 km2 in 2018, which is close to the annual MYI area export through Fram StraitThe Beaufort Sea has become a MYI export pathway rivaling the Fram Strait, encouraging the pan‐Arctic transition to a seasonal ice cover [ABSTRACT FROM AUTHOR]

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

Maud Rise is a seamount in the eastern Weddell Sea and the location of polynyas and a persistent halo of reduced sea ice. We present novel in situ data from two profiling floats with up to daily resolved hydrographic profiles in this region. The water properties below the mixed layer of the Maud Rise region are significantly correlated with bathymetric depth; thus, the Maud Rise flank defines the front between the Warm Deep Water of the abyssal ocean and the colder Taylor cap over Maud Rise. We analyze the spiciness curvature in density space to quantify the observed frequency or magnitude of intrusions, which are substantially increased along the flanks of Maud Rise. These intrusions are indicative of enhanced lateral and vertical mixing along heavily sloping isopycnals, creating favorable conditions for thermobaric and double diffusive convection and likely facilitating the formation of the Maud Rise halo and polynyas. Plain Language Summary: The cold, dense water atop the Maud Rise seamount in the Southern Ocean is separated from the warmer mean flow of the Weddell Gyre by an effect called a Taylor column. At the northern flank of Maud Rise, we observe a front between these water masses. Due to the water masses' difference in temperature, salinity, and density, across‐front intrusions are accompanied by vertical transports and mixing processes. These can possibly explain previously observed local melting of the sea ice along the flanks of Maud Rise. Key Points: High frequency (1–3 days) float profiles were collected over both Maud Rise and its vicinity over several seasonal cyclesTemperature and salinity below the mixed layer are strongly bathymetry‐dependent at Maud RiseEnhanced spiciness variability indicates intrusions and mixing between Maud Rise Deep Water and the surrounding Warm Deep Water [ABSTRACT FROM AUTHOR]

Arctic sea ice has declined strikingly over the past 40 years under climate change. The physical processes driving these changes and the relative roles of external forcing versus natural variability remain under debate, partly due to lack of confidence in the simulation of Arctic sea ice by coupled climate models. Here, we nudge a widely‐used CMIP5‐class model (CESM1) toward observed winds above the boundary layer. These simulations reproduce observed Arctic trend patterns and Arctic‐average interannual fluctuations in near‐surface temperature. We find that atmospheric circulation plays a crucial role in September Arctic sea ice loss and sea ice interannual variability. Trends in the observed atmospheric circulation that are not part of the modeled forced response cause approximately 20%–25% of the modeled September sea ice decline, which is in close agreement with the observed trend. However, our simulations do not capture sea ice loss in other seasons, pointing to important model biases in winter and spring. Plain Language Summary: Arctic sea ice has declined strikingly over the past 40 years. This reduction has occurred due to both external forcing (including anthropogenic greenhouse gas emissions and aerosols) and natural climate variability, but the precise contribution of each remains uncertain. Here, we nudge a global coupled climate model toward observed winds in the high latitudes. We find that the model reproduces many features of Arctic climate change and summer Arctic sea ice loss, although it does not capture observed sea ice trends in other seasons. These experiments reveal the impact of observed atmospheric circulation variability on Arctic sea ice and climate relative to the model's response to external forcing. Our results may help to better understand and predict changes in the warming Arctic. Key Points: We nudge winds in a fully coupled climate model to observed anomalies over the satellite periodCirculation trends that are not part of the modeled forced response result in an enhanced September sea ice decline close to observationsWinter and spring sea ice trend biases remain after nudging winds, suggesting ice‐ocean biases in the model [ABSTRACT FROM AUTHOR]

Using the climate model MIROC4m, we simulate self‐sustained oscillations of millennial‐scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep‐water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere‐sea ice‐ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection. Plain Language Summary: Ice core data indicate the existence of millennial‐scale climate fluctuations, known as Dansgaard‐Oeschger (DO) events, with global‐scale impacts during the last glacial period. Evidence suggests that DO events are closely related to abrupt changes in the Atlantic meridional overturning circulation (AMOC). Here, using the climate model, MIROC4m, we simulate DO‐like oscillations of millennial‐scale periodicity with a mid‐glacial atmospheric CO2 level and suitable obliquity and glacial ice sheet topography but without external freshwater forcing. When the AMOC is in a weak (strong) state, subsurface temperatures in the North Atlantic increase (reduce) gradually, which weakens (strengthens) stratification of the North Atlantic Ocean and leads to AMOC recovery (collapse). At the same time, salt advection with the North Atlantic subpolar gyre changes greatly with the abrupt changes in the AMOC and works as a positive feedback. Furthermore, we examine the direct effects of the precessional component of orbital forcing on the AMOC by comparing the two cases of extreme climatic precession, which can change the seasonality of the atmosphere‐ocean system. Our simulations show that when the precession shifts to stronger boreal seasonality, the period of the oscillation shortens through changes in sea ice thickness and subsurface temperature in the North Atlantic. Key Points: Dansgaard‐Oeschger‐like oscillations of millennial‐scale are simulated with a coupled climate model, MIROC4m, under glacial conditionsThe period of the simulated oscillations shortens as the climatic precession shifts to stronger boreal seasonalityThe period of the oscillations can be influenced by the seasonal sea ice change and the subsurface temperature change in the North Atlantic [ABSTRACT FROM AUTHOR]

The Pre‐Industrial climate model simulation is intended as an equilibrium control experiment that uses constant external forcing, yet in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model HadGEM3‐GC31‐LL a distinct change in climate state occurs around year 500 of the 2000 year simulation. The global mean near surface air temperature increases by almost 0.5 K associated with a reduction in southern hemisphere sea ice area of almost 20%. Here we show this step change in the state of the climate to be a consequence of the onset of deep convection in the Weddell and Ross Sea gyres. The delayed onset of convection in the gyres is a consequence of a positive, downward, top‐of‐atmosphere radiative balance, and continual ocean heat up‐take during the model spin‐up and the initial pre‐industrial control simulation. Consequently, model spin‐up strategy should be revised to initialize pre‐industrial simulations that are in energy balance. Plain Language Summary: The Pre‐industrial control simulation of climate models is a reference for climate change simulations. Consequently it should simulate a stable climate. However, if the model is not in radiative equilibrium, the ocean continually accumulates heat, becoming unstable at high latitudes and generating large climate fluctuations. This results in a control simulation that is not a steady state. Consequently the methodologies used to initialize and formulate climate models should be revised to ensure a steady state is achieved. Key Points: The long timescales associated with deep ocean heat uptake suggests simulations may not be equilibriated before forcing is perturbedSouthern Ocean venting of heat reduces sea ice cover and increases global mean temperature, leading to a warmer climate stateNet top‐of‐atmosphere energy imbalance can have implications for the methodologies used for climate model spin‐up [ABSTRACT FROM AUTHOR]