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Glaciologist Rob DeConto's research on polar climate change, specifically focusing on glaciers and ice sheets, is discussed in this article. DeConto explains the differences between Antarctica and Greenland and the challenges of understanding how the Antarctic ice sheet will behave in a warmer world. He also discusses the potential dangers of marine ice sheet instability and the impact of melting ice shelves on sea-level rise. The article emphasizes the uncertainties surrounding the rate of sea-level rise and the need for further research in this area. It highlights the term "deep uncertainty" used by the Intergovernmental Panel on Climate Change (IPCC) to describe the situation and the conservative nature of current projections. The author emphasizes the need for action to address the melting of ice sheets, as it poses a threat to infrastructure worldwide. [Extracted from the article]

The Antarctic marginal ice zone, the regularly wave‐affected outer band of the sea ice covered Southern Ocean, typically contains an unconsolidated ice cover comprised of smaller, thinner floes than the inner ice pack. Thus, it is a highly dynamic region and susceptible to rapid expansion and contraction, making it a focal area for understanding and predicting the response of Antarctic sea ice to a changing climate. This novel study uses unsupervised statistical clustering of sea ice data simulated by a global sea ice model (standalone CICE6 combined with a wave propagation module and prescribed ocean) to address the outstanding challenge of separating the marginal ice zone from the inner ice pack in sea ice data sets. The method identifies a marginal ice zone with the desired characteristics and floe size is shown to be the key variable in the classification. Simulated marginal ice zone widths are similar to those derived from satellite observations of wave penetration distances, but contrast with those using the standard 15%–80% areal sea ice concentration proxy, particularly during austral winter. The simulated marginal ice zone is found to undergo a seasonal transition due to new ice formation in winter, increased drift in spring, and increased rates of wave‐induced breakup and melting in summer. The understanding gained from the study motivates incorporation of wave and floe‐scale processes in sea ice models, and the methods are available for application to outputs from high‐resolution and coupled sea ice–ocean–wave models for more detailed studies of the marginal ice zone (in both hemispheres). Plain Language Summary: The record‐setting responses of Antarctic sea ice extent to climate change over recent decades have generated intense research interest in the marginal ice zone at the outskirts of the ice cover. The presence of ocean waves in this region creates a granular ice cover composed of small floes, affecting heat fluxes between the atmosphere and ocean, whilst shielding inner‐pack ice, fast ice, and ice shelves from waves. Studies of the marginal ice zone are hindered by the lack of a pragmatic approach to extract the marginal ice zone from sea ice data, which aligns with its characterization as the regularly wave‐affected sea ice region. Most studies use a concentration‐based proxy, although it overlooks large wave‐affected areas. We develop new insights into the evolution of the marginal ice zone by applying a modern machine‐learning approach to model outputs, leveraging recent modeling advances. Our findings reveal a marginal ice zone composed of young, small floes during winter and older, fractured floes during summer. Simulated widths are consistent with wave‐penetration distance observations and provide evidence of the marginal ice zone's unique properties, including high melt rates. The proposed approach may provide the basis for future studies on its contribution to Antarctic sea ice variability. Key Points: A new method to identify the Antarctic marginal ice zone is proposed based on statistical clustering of sea ice propertiesSimulated marginal ice zone widths are similar to satellite observations of wave penetration distancesNew understanding gained of differences in ice melt rates and drift speeds between the marginal ice zone and the interior ice pack [ABSTRACT FROM AUTHOR]

Internal wave-driven mixing is an important factor in the balance of heat and salt fluxes in the polar regions of the ocean. Transformation of internal waves at the edge of the ice cover can enhance the mixing and melting of ice in the Arctic Ocean and Antarctica. In the polar oceans, internal solitary waves (ISWs) are generated by various sources, including tidal currents over bottom topography, the interaction of ice keels with tides, time-varying winds, vortices, and lee waves. In this study, a numerical investigation of the transformation of ISWs propagating from open water in the stratified sea under the edge of the ice cover is carried out to compare the depression ISW transformation and loss of energy on smooth ice surfaces, including those on the ice shelf and glacier outlets, with the processes beneath the ridged underside of the ice. They were carried out using a non-hydrostatic model that is based on the Reynolds-averaged Navier–Stokes equations in the Boussinesq approximation for a continuously stratified fluid. The Smagorinsky turbulence model extended for stratified fluid was used to describe the small-scale turbulent mixing explicitly. Two series of numerical experiments were carried out in an idealized 2D setup. The first series aimed to study the processes of the ISWs of depression transformation under an ice cover of constant submerged ice thickness. Energy loss was estimated based on a budget of depth-integrated pseudoenergy before and after the wave transformation. The transformation of ISWs of depressions is controlled by the blocking parameter β , which is the ratio of the minimum thickness of the upper layer under the ice cover to the incident wave amplitude. The energy loss was relatively small for large positive and large negative values of β. The maximal value of energy loss was about 38 %, and it was reached at β≈0 for ISWs. In the second series of experiments, a number of keels were located on the underside of the constant-thickness ice layer. The ISW transformation under ridged ice also depends on the blocking parameter β. For large keels (β<0), more than 40 % of energy is lost on the first keel, while for relatively small keels (β>0.3), the losses on the first keel are less than 6 %. Energy losses due to all keels depend on the distance between them, which is characterized by the parameter μ , i.e. the ratio of keel depth to the distance between keels. In turn, for a finite length of the ice layer, the distance between keels depends on the keel quantity. [ABSTRACT FROM AUTHOR]

Glacial fjords often contain ice mélange, a frozen conglomeration of icebergs, brash ice, and sea ice, that have been postulated to influence both glacier dynamics and fjord circulation through coupled mechanical and thermodynamic processes. Ice mélange meltwater can alter stratification of the water column by releasing cool, fresh water across a range of depths in the upper layer of the fjord. This meltwater input can subsequently modify the depth at which the subglacial discharge plume reaches neutral buoyancy and therefore the underlying buoyancy-driven fjord circulation and heat exchange with warm ocean shelf waters. Despite a spate of recent modelling studies exploring these proposed feedbacks, we lack in situ observations quantifying changes to the water column induced by ice mélange meltwater. Here we use a novel dataset collected before and after the melt, breakup, and down-fjord transport of an ephemeral ice mélange in front of Kangilliup Sermia (Rink Isbræ) to directly investigate the extent to which ice mélange meltwater can modify glacier-adjacent water properties. We find that even a short-lived ice mélange (4 days) can cause substantial cooling (0.18 °C) and freshening (0.25 g kg-1) of the water column that leads to stratification change down to the depth of the outflowing discharge plume. We compare our observations to an adjacent fjord, Kangerlussuup Sermia, where ice mélange seldom forms in the summertime, and show that the presence or absence of ice mélange melt creates fundamental differences in their upper layer hydrography. These observations provide critical constraints for recent modelling studies that have suggested ice mélange meltwater needs to be included in ocean circulation models for glaciers with deep grounding lines and high ice fluxes, which are precisely the glaciers exhibiting the largest magnitude terminus retreats at present. [ABSTRACT FROM AUTHOR]

The freezing point of water is negatively dependent on pressure; therefore in any ocean without external forcing it is warmest at the surface and grows colder with depth. Below floating ice on Earth (e.g., ice shelves or sea ice), this pressure dependence combines with gradients in the ice draft to drive an ice redistribution process termed the "ice pump": submerged ice melts, upwells, and then refreezes at shallower depths. Ice pumping is an exchange process between the ocean and overhead ice that results in unique ice compositions and textures and influences the distribution of sub‐ice habitats on Earth. Here, we scale recent observations from Earth's ice shelves to planetary conditions and find that ice pumping is expected for a wide range of possible sub‐ice shell pressures and salinity at other ocean worlds such as Europa and Enceladus. We show how ice pumping would affect hypothetical basal ice shell topography and ice thickness under varying ocean conditions and demonstrate how remote sensing of the ice shell draft can be used to estimate temperature gradients in the upper ocean ahead of in situ exploration. For example, the approximately 22 km gradient observed in Enceladus' ice shell draft between the south pole and the equator suggests a temperature differential of 0.18 K at the base of the ice shell. These concepts can extend the interpretation of observations from upcoming ocean world missions, and link ice shell topography to ice‐ocean material exchange processes that may prove important to overall ocean world habitability. Plain Language Summary: The freezing point of water depends on pressure. As pressure increases, the freezing point decreases, which can influence the melting or freezing of ice in an ocean. A helpful way to conceptualize this dependency is to recall that water expands as it freezes. As pressure increases, this expansion requires more work to displace the higher pressure surroundings, so the water must be even colder to freeze—a decrease in the freezing point. If ice is submerged, the deeper ice where the freezing point is colder can melt faster. This forms freshened meltwater that may rise to shallower depths where it is now colder than the shallower, lower pressure freezing point and can refreeze underwater. This process is referred to as an "ice pump", because it acts to equilibrate topography in submerged ice. In ice‐covered oceans on Earth, the ice pump is an important process that influences the composition and texture of the ice, and therefore the sub‐ice ecosystems. Here, we find that ice pumping is also likely at other ocean worlds in our solar system where it may similarly influence potential sub‐ice ecosystems and show how observations of planetary ice shell thicknesses can be used to bound ocean conditions. Key Points: When ice is submerged, a melting and freezing exchange process termed the "ice pump" can affect ice composition, texture, and thicknessWe find that ice pumping is likely beneath the ice shells of several ocean worlds in our solar systemThe ice pump concept enables inversion between ocean world ice shell thickness and ice‐ocean interface temperature ranges [ABSTRACT FROM AUTHOR]

Satellite radar interferometry data reveals that the grounding line of Petermann Glacier, Greenland migrates by several kilometers during the tidal cycle, bringing pressurized, subsurface, warm ocean waters in regular contact with a large sector of grounded ice. We use the Massachusetts Institute of Technology general circulation model in two dimensions to calculate the ice melt rates as a function of grounding zone (GZ) length and ocean Thermal Forcing (TF). Ice melt rates are found to be higher in the GZ cavity than anywhere else in the ice shelf cavity. The melt rates increase sub‐linearly with the length of the GZ and ocean TF. The model results agree well with remote sensing estimates of ice melt. High basal ice melt rates in tidally flushed grounding zones imply that marine‐terminating glaciers are more sensitive to ocean TF than anticipated, which will increase their projected contribution to sea level rise. Plain Language Summary: The traditional view of ice melting in contact with ocean waters is that melt rates drop to zero at the grounding line, which is a semi‐fixed boundary at the junction between grounded ice and the ocean. In reality, the grounding line migrates by kilometers during the tidal cycle, more than 10 times beyond the range expected from hydrostatic equilibrium, which brings warm, saline water in rapid contact with broad sectors of grounded ice. We use an ocean model to calculate the melt rates caused by seawater intrusions. We find that the melt rates in the GZ are higher than anywhere else in the ice shelf cavity and increase as the GZ becomes wider and the ocean gets warmer. Ice melt in kilometer‐size grounding zones will reduce the basal resistance to flow and will increase the sensitivity of the glacier flow to ocean warming, hence projections of sea level rise from the glacier will go up. Key Points: First modeling of ice melt rates from seawater intrusions in the kilometer‐size grounding zone (GZ) of Petermann Glacier using an ocean modelModeled melt rates are highest in the GZ and increase linearly with GZ width and ocean thermal forcingHigh melt rates in kilometer‐size grounding zones imply a higher sensitivity of glaciers to ocean warming than anticipated [ABSTRACT FROM AUTHOR]

Internal wave-driven mixing is an important factor in the balance of heat and salt fluxes in the polar regions of the ocean. The breaking internal waves at the edge of the ice cover can essentially enhance the mixing and melting of ice in the Arctic Ocean and Antarctica. The internal solitary waves (ISWs) are generated by various sources, including tidal currents over the bottom topography, the interaction of ice keels with tides, varying in time wind, vortices, and lee waves. In the study, a numerical investigation of the transformation of ISW propagating from open water in the stratified sea under the edge of the ice cover is carried out to compare the depression ISW transformation and loss of energy on smooth ice surfaces, including those on the ice shelf and glacier outlets, with the processes beneath the ridged underside of the ice. They were carried out using a nonhydrostatic model which is based on the Reynolds averaged Navier-Stokes equations in the Boussinesq approximation for a continuously stratified fluid. The Smagorinsky turbulence model extended for stratified fluid was used to explicitly describe the small-scale turbulent mixing. Two series of numerical experiments were carried out in an idealized 2D setup. The first series aimed to study processes of the ISW-depression transformation under ice cover of constant submerged ice thickness. A loss of energy was estimated based on the budget of depth-integrated pseudoenergy before and after the wave transformation. The transformation of depression ISW is controlled by the blocking parameter β. For large positive and large negative values of parameter β which is the ratio of the height of the minimum depth of the upper layer under the ice cover to the incident wave amplitude. The energy loss was relatively small for large positive and large negative values of β. The maximal value of energy loss was about 38 % and it is reached at β ≈ 0 for ISW. In the second series of experiments, a number of keels were located underside of the ice layer of constant thickness. The ISW transformation under ridged ice also depends on the blocking parameter β. For large keels (β<0), more than 40 % is lost on the first keel, while for relatively small keels (β>0.3), the losses on the first keel are less than 6 %. Energy losses due to all keels depend on the distance between them which is characterized by the parameter μ which is the ratio keel depth to the distance between keels. If the tidal flow around the large keels is the source of internal waves, then under conditions of strongly ridged ice the waves excited by the tidal flow are dispersed in the vicinity of their formation. [ABSTRACT FROM AUTHOR]

The seafloor depths under the Cook Ice Shelf and Ninnis Glacier Tongue have not been directly measured, despite their importance for understanding ocean circulation and ice shelf change. We model the bathymetry underneath the floating ice and surrounding ocean using airborne gravity data. Our model is constrained by few ship‐based seafloor measurements near the ice front and by ice‐base measurements over areas of grounded ice from radar data. Localized basins (∼1,400 m deep) are found beneath both ice shelves. The shallowest modeled bathymetry (∼200 m) represents the offshore extension of Cape Freshfield. Near the grounding line, seafloor depths are found to be deeper than the observed depth of the modified Circumpolar Deep water in the region (<350 m), key factor for basal melt analyses. From transit flight gravity anomalies, we suggest the relocation of the mapped edge of the continental shelf and a narrowing of the Cook Shelf Depression. Plain Language Summary: The knowledge of how deep the ocean floor is under the floating ice shelves that connect to grounded ice sheets, is crucial for understanding how ocean water circulates and interacts with the overlying ice. We present a new bathymetric model of the seafloor beneath two ice shelves located in East Antarctica: Cook Ice Shelf and Ninnis Glacier Tongue. Both ice shelves are inaccessible to ships due to heavy sea ice conditions, so the data used in our model were collected from airborne surveys. Our bathymetry model shows new information on the depth and shape of the seafloor that will help understanding the ocean circulation in the area and how this might impact ice thickness changes. Key Points: High resolution bathymetry model of Cook Ice Shelf, Ninnis Glacier Tongue, and surrounding open ocean from airborne gravity inversionNew bathymetry model improves the understanding of water pathways between the ice shelves and the continental shelf edgeTransit flight gravity anomalies suggest relocation of the edge of the continental shelf northwards of the currently attributed position [ABSTRACT FROM AUTHOR]

We explored the potential of fibre optics coupled with distributed acoustic sensing (DAS) to measure the thickness and Young's modulus of an ice layer, using the properties of guided seismic waves. During two winter seasons (2020 and 2021), an optical fibre was deployed over one of the frozen Roberts Mountain lakes (at 2400 m a.s.l) near Grenoble (France) and we measured both the continuous ambient seismic noise as well as signals generated by active sources (hammer), with a DAS interrogator. Following a Bayesian scheme, we inverted the dispersion curves of longitudinal and flexural guided waves retrieved from the analysis of active shot gathers and obtained Young's modulus E = 3.4 ± 0.1 GPa and ice thickness h = 47 ± 1 cm from the second-year data. The ice thickness was consistent with field measurements. Field observations of porous and/or fracture ice may explain the relatively low effective Young's modulus (relative to pure ice), which may also be affected by a snow layer not included in the model. The drastic improvements in the inversion results between the two years are related to better coupling conditions (drone deployment before early freezing), more appropriate acquisition parameters (2 m gauge length), and the upper snow layers (less thick and less heterogeneous in the second year). Moreover, we were able to use the non-dispersive low-frequency noise associated with gravity waves to estimate the lake depth H = 5 m which is compatible with independent observations. The use of DAS to record guided seismic waves could then appear as a relevant tool for monitoring environments like floating ice shelves and sea ice. [ABSTRACT FROM AUTHOR]

Understanding the dynamic behaviour of ice shelves, specifically the controls on their ability to buttress the flow of ice into the ocean, is critical for predicting future ice-sheet contributions to sea level rise. Many large ice shelves, which are predominantly composed of meteoric ice, have a basal layer of marine ice (formed from accumulated platelets at the ice–ocean interface), comprising up to 40% of their thickness locally. Differences in temperature, chemistry and microstructure between marine and meteoric ice mean the rheological properties of the ice vary throughout the ice shelf. These differences are not explicitly accounted for in ice-sheet modelling applications, and may have an important influence on ice shelf dynamics. We tested the sensitivity of a model of an idealised ice shelf to variations in temperature distribution and flow enhancement, and found that incorporating a realistic thermal profile (where the marine ice layer is isothermal) had an order of magnitude greater effect on ice mass flux and thinning than incorporating the mechanical properties of the marine ice. The presence of marine ice at the ice shelf base has the potential to significantly increase deviatoric stresses at the surface and ice mass flux across the front of an ice shelf. [ABSTRACT FROM AUTHOR]

We present a 700 km airborne electromagnetic survey of late‐spring fast ice and sub‐ice platelet layer (SIPL) thickness distributions from McMurdo Sound to Cape Adare, providing a first‐time inventory of fast ice thickness close to its annual maximum. The overall mode of the consolidated ice (including snow) thickness was 1.9 m, less than its mean of 2.6 ± 1.0 m. Our survey was partitioned into level and rough ice, and SIPL thickness was estimated under level ice. Although level ice, with a mode of 2.0 m and mean of 2.0 ± 0.6 m, was prevalent, rough ice occupied 41% of the transect by length, 50% by volume, and had a mode of 3.3 m and mean of 3.2 ± 1.2 m. The thickest 10% of rough ice was almost 6 m on average, inclusive of a 2 km segment thicker than 8 m in Moubray Bay. The thickest ice occurred predominantly along the northwestern Ross Sea, due to compaction against the coast. The adjacent pack ice was thinner (by ∼1 m) than the first‐year fast ice. In Silverfish Bay, offshore Hells Gate Ice Shelf, New Harbor, and Granite Harbor, the SIPL transect volume was a significant fraction (0.30) of the consolidated ice volume. The thickest 10% of SIPLs averaged nearly 3 m thick, and near Hells Gate Ice Shelf the SIPL was almost 10 m thick, implying vigorous heat loss to the ocean (∼90 W m−2). We conclude that polynya‐induced ice deformation and interaction with continental ice influence fast ice thickness in the western Ross Sea. Plain Language Summary: Sea ice held stationary by the coast is named fast ice. Around Antarctica, it is often observed near floating land ice and grounded icebergs. Fast ice is a critical habitat for marine life and known to be a sensitive indicator to changing weather and oceanic processes. Despite its importance, satellite measurements of its area are recent, and little is known about its thickness. Using a specialized instrument on a fixed wing aircraft, we measured the 2017 November fast ice thickness in the western Ross Sea. Most of the fast ice surveyed was 2 m thick on average and had formed in sheltered embayments. Where floating glaciers are in contact with fast ice, a 3–10 m thick layer of loose ice crystals was sometimes observed beneath, which contributed to its thickness. However, most of the thicker ice was rough ice, formed when ferocious offshore winds pushed ice from the coast. It then drifted with the prevailing ocean currents until eventually becoming frozen in place against coastal topography. This rough ice, 3.3 m thick on average and comprising half the surveyed volume, was significantly thicker than adjacent freely moving pack ice. Our survey provides a baseline from which future change can be measured. Key Points: Annual maximum fast ice and sub‐ice platelet layer thickness distributions are surveyed by airborne electromagnetics over 700 km of Ross SeaDeformed against the coast, 50% of transect volume was rough first‐year fast ice (mode 3.3 m thick), that was thicker than nearby pack iceSub‐ice platelet layers over 0.5 m thick underlay level ice over large parts of four embayments, revealing land ice‐fast ice interaction [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 vertical front of ice shelves represents a topographic barrier for barotropic currents that transport a considerable amount of heat toward the ice shelves. The blocking effect of the ice front on barotropic currents has recently been observed to substantially reduce the heat transport into the cavity beneath the Getz Ice Shelf in West Antarctica. We use an idealized numerical model to study the vorticity dynamics of an externally forced barotropic current at an ice front and the impact of ice shelf thickness, ice front steepness, and ocean stratification on the volume flux entering the cavity. Our simulations show that thicker ice shelves block a larger volume of the barotropic flow, in agreement with geostrophic theory. However, geostrophy breaks locally at the ice front, where relative vorticity and friction become essential for the flow to cross the discontinuity in water column thickness. The flow entering the cavity accelerates and induces high basal melt rates in the frontal region. Tilting the ice front, as undertaken in sigma-coordinate models, reduces this acceleration because the flow is more geostrophic. Viscous processes—typically exaggerated in low-resolution models—break the potential vorticity constraint and bring the flow deeper into the ice shelf cavity. The externally forced barotropic current can only enter the cavity if the stratification is weak, as strong vertical velocities are needed at the ice front to squeeze the water column beneath the ice shelf. If the stratification is strong, vertical velocities are suppressed and the barotropic flow is almost entirely blocked by the ice front. Significance Statement: Ice shelves in West Antarctica are thinning, mostly from basal melting through oceanic heat entering the underlying ice shelf cavities. Thinning of ice shelves reduces their ability to buttress the grounded ice resting upstream, leading to sea level rise. To model the ice sheet's contribution to sea level rise more accurately, the processes governing the oceanic heat flux into the ice shelf cavity must be articulated. This modeling study investigates the dynamics of a depth-independent current approaching the ice shelf; it corroborates previous findings on the blocking of such a current at the ice front. The amount of water that enters the cavity strongly depends on ice shelf thickness and ocean stratification. For a well-mixed ocean, the upper part of the flow can dive underneath the ice shelf and increase basal melting near the ice front. In a stratified ocean, the approaching depth-independent current is almost entirely blocked by the ice front. [ABSTRACT FROM AUTHOR]

Prompted by field observation of an aufeis‐induced channel avulsion along the Hula Hula River in June 2021, we use measurements of channel migration zone width along 15 rivers flowing north across the Arctic coastal plain in Alaska, USA. We differentiated sites with aufeis that covered ≥1 km2 in early summer during the period 2017–2021 from sites without such aufeis formation. All but four of the 28 sites with aufeis have widths greater than the 95% confidence interval and 20 sites fall outside of the 95% prediction interval for channel width based on drainage area. Pairwise comparison indicates that the population of aufeis sites have significantly wider channel migration zones (p < 0.0001) than non‐aufeis sites after accounting for drainage area. Seasonal aufeis facilitates lateral channel migration and associated heterogeneity. Loss of aufeis under warming climate may reduce habitat diversity in these river corridors. Plain Language Summary: Aufeis forms in Arctic river corridors when groundwater springs continue to provide water to the surface after the river is covered by ice. The resulting shelves of ice can be up to 3 m thick and extend across the channel and floodplain. As this ice slowly melts in early summer, it can divert river flow from the active channel onto the floodplain, creating new channels. This paper examines the resulting width of the floodplain across which active channel migration occurs. Comparing sites with and without aufeis, we find that sites with aufeis are significantly wider. Loss of aufeis with warming climate will reduce the habitat diversity of Arctic rivers. Key Points: Aufeis that persists into summer on rivers of the Alaskan Arctic coastal plain can force channel avulsionSites with repeated aufeis in successive years have wider channel migration zonesLoss of aufeis as climate warms may result in narrower and less diverse river corridors [ABSTRACT FROM AUTHOR]

Accreted ice retains and preserves traces of the ocean from which it formed. In this work, we study two classes of accreted ice found on Earth—frazil ice, which forms through crystallization within a supercooled water column, and congelation ice, which forms through directional freezing at an existing interface—and discuss where each might be found in the ice shells of ocean worlds. We focus our study on terrestrial ice formed in low temperature gradient environments (e.g., beneath ice shelves), consistent with conditions expected at the ice-ocean interfaces of Europa and Enceladus, and we highlight the juxtaposition of compositional trends in relation to ice formed in higher temperature gradient environments (e.g., at the ocean surface). Observations from Antarctic sub-ice-shelf congelation ice and marine ice show that the purity of frazil ice can be nearly two orders of magnitude higher than congelation ice formed in the same low temperature gradient environment (∼0.1% vs. ∼10% of the ocean salinity). In addition, where congelation ice can maintain a planar ice-water interface on a microstructural scale, the efficiency of salt rejection is enhanced (∼1% of the ocean salinity) and lattice soluble impurities such as chloride are preferentially incorporated. We conclude that an ice shell that forms by gradual thickening as its interior cools would be composed of congelation ice, whereas frazil ice will accumulate where the ice shell thins on local (rifts and basal fractures) or regional (latitudinal gradients) scales through the operation of an "ice pump." [ABSTRACT FROM AUTHOR]

Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to ocean-driven melting and the associated disruption to glacial dynamics. The local ocean circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf and resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood. A cyclonic (i.e. clockwise) gyre circulation (27 km radius) in front of the Pine Island Ice Shelf has previously been identified in both numerical models and velocity observations. Mooring data further revealed a potential reversal of this gyre during an abnormally cold period. Here we present ship-based observations from 2019 to the west of Thwaites Ice Shelf, revealing another gyre (13 km radius) for the first time in this habitually ice-covered region, rotating in the opposite (anticyclonic, anticlockwise) direction to the gyre near Pine Island Ice Shelf, despite similar wind forcing. We use an idealised configuration of MITgcm, with idealised forcing based on ERA5 climatological wind fields and a range of idealised sea ice conditions typical for the region, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. The model driven solely by wind forcing in the presence of ice can reproduce the horizontal structure and direction of both gyres. We show that the modelled gyre direction depends upon the spatial difference in the ocean surface stress, which can be affected by the applied wind stress curl filed, the percentage of wind stress transferred through the ice, and the angle between the wind direction and the sea ice edge. The presence of ice, either it is fast ice/ice shelves blocking the effect of wind or mobile sea ice enhancing the effect of wind, has the potential to reverse the gyre direction relative to ice-free conditions. [ABSTRACT FROM AUTHOR]

Ice shelves in the Amundsen Sea are thinning rapidly as ocean currents bring warm water into the cavities beneath the floating ice. While the reported melt rates for the Getz ice shelf are comparatively low for the region, its size makes it one of the largest freshwater sources around Antarctica, with potential consequences for e.g. bottom water formation downstream. Here, we use two year long mooring records (2016-2018) and 16 year long regional model simulations to describe, for the first time, the hydrography and circulation in the vicinity of the ice front between Siple and Carney Island. We find that, throughout the mooring record, temperatures in the trough remain below 0.15 °C, more than 1 °C lower than in the neighboring Siple and Dotson Trough, and we observe a mean current (0.03 m s-1) directed towards the ice front. The variability in the heat transport towards the ice front appears to be governed by wind stress over the Amundsen Sea Polynya region, potentially through interactions with the coastal current, although this hypothesis could not be confirmed by the numerical model. The model simulations suggest that the heat content in the trough during the observed period was lower than normal, due to anomalously low summertime sea ice concentration and weak winds. [ABSTRACT FROM AUTHOR]

Observations of surface lowering on Larsen C Ice Shelf (LCIS), Antarctica, have prompted concern about its stability. In this study, an ocean model is used to investigate the extent to which changes in ocean forcing may have influenced ice loss and the distribution of stabilizing marine ice beneath LCIS. The model uses a new bathymetry, containing a southern seabed trough discovered using seismic observations. The modeled extent of marine ice, thought to stabilize LCIS, is in good agreement with observations. Experiments applying idealized ocean warming yield an increase in melting over the southern trough. This is inconsistent with lowering observed in northern LCIS, suggesting oceanic forcing is not responsible for that signal. The marine ice extent and thickness reduces significantly under ocean warming, implying a high sensitivity of LCIS stability to changes in ocean forcing. This result could have wide implications for other cold‐water ice shelves around Antarctica. Plain Language Summary: Satellite observations have revealed a lowering in recent decades of the surface of Larsen C Ice Shelf (LCIS), Antarctica, which has led to concern about its stability. By modeling ocean conditions under LCIS, we investigate the extent to which ocean melting may have caused the ice to thin, leading to the observed lowering, or altered the pattern of marine ice beneath LCIS. Marine ice forms when seawater freezes to the base of the ice shelf, and is thought to stabilize LCIS. The model uses a new seabed data set that contains a wide, deep seabed trough in the south, found by a seismic survey. In modeled ocean warming experiments, an increase in melting is concentrated in this southern region. However, greater lowering has been observed in the north, suggesting that changes in ocean conditions are not responsible for the lowering. The calculated pattern of marine ice at the base of LCIS looks similar to the observed pattern. With a warmer ocean, marine ice is significantly reduced in crucial regions of the ice shelf. This shows that the stability of LCIS is sensitive to changes in ocean conditions and other ice shelves around Antarctica are likely sensitive to these changes too. Key Points: An ocean model with new Larsen C bathymetry shows greatest melting and melt sensitivity where seismic data indicate a deep southern troughThe calculated marine ice distribution under Larsen C Ice Shelf, based on model melt/freeze rates, is in good agreement with observationsA reduction in marine ice with ocean warming implies a threat to Larsen C stability, with wide implications for cold‐water ice shelves [ABSTRACT FROM AUTHOR]