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Projections of sea‐level rise from ice‐sheet shrinkage in a warming world have large uncertainties, linked to limited knowledge of changes at the ocean‐ice sheet interface. This interface most typically is modeled as a grounding line, across which still‐connected ice flows into the ocean to float as an ice shelf, or where icebergs calve from a cliff before the ice begins to float. But, extensive and rapidly increasing evidence shows that this is really a grounding zone, and that processes in this grounding zone omitted from many models could exert major controls on sea‐level rise. Plain Language Summary: Marine‐terminating glaciers flow into the ocean across extensive grounding zones. These kilometers‐long and glacier‐wide zones represent the last broad region of glacier contact with the rock and sediments below before the ice enters the ocean as a floating ice shelf or calved icebergs. Loss of this basal drag along with enhanced basal melting caused by tidally driven seawater intrusion leads to faster outflow and rapid thinning of the overlying ice. As a result of the local thinning, grounding zones retreat inland and sea level rises with more loss of previously grounded ice. Most ice‐sheet models used in sea‐level projections do not include grounding‐zone processes, but rather stop their ice‐ocean interactions at a grounding line. They are thereby omitting important dynamic feedbacks and underestimating future sea‐level contributions from the marine‐based sectors of the Greenland and Antarctic ice sheets. Key Points: Tidally modulated seawater intrusion leads to loss of ice‐bed contact as well as significant (maximal) basal melting within grounding zonesThe future dynamics of marine outlet glaciers are ultimately controlled by coupled processes operating within and through grounding zonesDespite the importance of grounding zones to ice‐sheet dynamics, most ice‐sheet models used in sea‐level projections do not include them [ABSTRACT FROM AUTHOR]

Recent studies have revealed the presence of a complex freshwater system underlying the Aurora Subglacial Basin (ASB), a region of East Antarctica that contains ∼7 m of global sea level potential in ice mainly grounded below sea level. However, the impact that subglacial freshwater has on driving the evolution of the dynamic outlet glaciers that drain this basin has yet to be tested in a coupled ice sheet‐subglacial hydrology numerical modeling framework. Here, we project the evolution of the primary outlet glaciers draining the ASB (Moscow University Ice Shelf, Totten, Vanderford, and Adams Glaciers) in response to an evolving subglacial hydrology system and to ocean forcing through 2100, following low and high CMIP6 emission scenarios. By 2100, ice‐hydrology feedbacks enhance the ASB's 2100 sea level contribution by ∼30% (7.50–9.80 mm) in high emission scenarios and accelerate the retreat of Totten Glacier's main ice stream by 25 years. Ice‐hydrology feedbacks are particularly influential in the retreat of the Vanderford and Adams Glaciers, driving an additional 10 km of retreat in fully coupled simulations relative to uncoupled simulations. Hydrology‐driven ice shelf melt enhancements are the primary cause of domain‐wide mass loss in low emission scenarios, but are secondary to ice sheet frictional feedbacks under high emission scenarios. The results presented here demonstrate that ice‐subglacial hydrology interactions can significantly accelerate retreat of dynamic Antarctic glaciers and that future Antarctic sea level assessments that do not take these interactions into account might be severely underestimating Antarctic Ice Sheet mass loss. Plain Language Summary: Recent studies have revealed that ice sheet basal meltwater flows beneath a large sector of the East Antarctic Ice Sheet that contains enough ice to raise global sea levels by ∼7 m if it were to all melt. Here, we develop a coupled ice sheet and subglacial hydrology model and use it to simulate the evolution of a large East Antarctic basin through 2100 under low and high climate forcing scenarios. Interactions between the ice sheet and the subglacial freshwater system cause this sector of East Antarctica to contribute 30% more to global sea level rise by 2100 in high emission scenarios. In addition, these interactions also drive accelerated retreat of up to 10 km of the coastal outlet glaciers that drain this ice sheet sector. Overall, we find that freshwater flowing beneath the Antarctic Ice Sheet can significantly accelerate retreat and the subsequent global sea level rise contribution of Antarctica's vulnerable outlet glaciers. Modeling studies that have attempted to quantify Antarctica's future contribution to global sea level have yet to take into account these ice sheet‐freshwater interactions, which may lead to an underestimate in future sea levels. Key Points: The 21st century evolution of the Aurora Subglacial Basin is modeled in a coupled ice sheet‐subglacial hydrology modelIce‐hydrology feedbacks accelerate projected grounding line retreat and mass loss of coastal outlet glaciersEnhanced ocean melt due to subglacial discharge is particularly influential under cooler ocean temperatures [ABSTRACT FROM AUTHOR]

Understanding grounding line dynamics is critical for projecting glacier evolution and sea level rise. Observations from satellite radar interferometry reveal rapid grounding line migration forced by oceanic tides that are several kilometers larger than predicted by hydrostatic equilibrium, indicating the transition from grounded to floating ice is more complex than previously thought. Recent studies suggest seawater intrusion beneath grounded ice may play a role in driving rapid ice loss. Here, we investigate its impact on the evolution of Petermann Glacier, Greenland, using an ice sheet model. We compare model results with observed changes in grounding line position, velocity, and ice elevation between 2010 and 2022. We match the observed retreat, speed up, and thinning using 3‐km‐long seawater intrusion that drive peak ice melt rates of 50 m/yr; but we cannot obtain the same agreement without seawater intrusion. Including seawater intrusion in glacier modeling will increase the sensitivity to ocean warming. Plain Language Summary: Relatively warm seawater melts marine‐terminating glaciers from below. Recent observations suggest that seawater flows below grounded ice at high tide. The presence of seawater at this boundary, referred to as seawater intrusion, has the potential to increase glacier mass loss. We test this hypothesis on Petermann Glacier, Greenland, using an ice sheet flow model. We run the model to reconstruct the glacier's behavior from 2010 to 2022 with and without seawater intrusion. We compare the results with satellite observations of velocity, grounding line position, and ice thinning. When we use enhanced ice melt rates from kilometer‐scale seawater intrusion, we match the observed retreat, speed up, and thinning. When we do not, the model fails to replicate the observations. Seawater intrusion may play a critical role in glacier evolution. Adding this process to ice flow models will increase their sensitivity to ocean warming and projections of ice mass loss and sea level rise. Key Points: Ice melt caused by seawater intrusion in the grounding zone explains the observed grounding line retreat of Petermann GlacierWithout seawater intrusion in the grounding zone, we do not replicate the full extent of observed retreatIncluding seawater intrusion in the grounding zone increases glacier mass loss [ABSTRACT FROM AUTHOR]

The West Antarctic Ice Sheet is experiencing rapid thinning of its floating ice shelves, largely attributed to oceanic basal melt. Numerical models suggest that the Bellingshausen Sea has a key role in setting water properties in the Amundsen Sea and further downstream. Yet, observations confirming these pathways of volume and tracer exchange between coast and shelf break and their impact on inter‐sea exchange remain sparse. Here we analyze the circulation and distribution of glacial meltwater at the boundary between the Bellingshausen Sea and the Amundsen Sea using a combination of glider observations from January 2020 and hydrographic data from instrumented seals. Meltwater distributions over previously unmapped western regions of the continental shelf and slope reveal two distinct meltwater cores with different optical backscatter properties. At Belgica Trough, a subsurface meltwater peak is linked with hydrographic properties from Venable Ice Shelf. West of Belgica Trough, the vertical structure of meltwater concentration changes, with peak values occurring at greater depths and denser isopycnals. Hydrographic analysis suggests that the western (deep) meltwater core is supplied from the eastern part of Abbot Ice Shelf, and is exported to the shelf break via a previously‐overlooked bathymetric trough (here named Seal Trough). Hydrographic sections constructed from seal data reveal that the Antarctic Coastal Current extends west past Belgica Trough, delivering meltwater to the Amundsen Sea. Each of these circulation elements has distinct dynamical implications for the evolution of ice shelves and water masses both locally and downstream, in the Amundsen Sea and beyond. Plain Language Summary: Floating ice shelves in West Antarctica are thinning, which is largely due to melting of the ice shelf base by the ocean. Here, measurements of ocean temperature, salinity, and dissolved oxygen, collected by a remotely‐controlled underwater vehicle (a glider), are used to estimate the amount of ice shelf meltwater released in the Bellingshausen Sea. Distinct cores of meltwater can be distinguished by the amount of suspended material that is present in the water, which we attribute to meltwater following different circulation pathways after entering the ocean. Historical data from seals equipped with temperature and salinity sensors provide additional information about how the meltwater circulates in the region. The seal data show the presence of a narrow coastal current that brings meltwater from the Bellingshausen Sea into the Amundsen Sea. The pathways of meltwater revealed in this study suggest an important influence of the Bellingshausen Sea on ice shelves throughout West Antarctica. Key Points: Hydrographic observations identify both shelf‐break and coastal meltwater pathways from the western Bellingshausen Sea into the Amundsen SeaDifferences in optical backscatter properties associated with meltwater are related to distinct coast‐to‐shelf break pathwaysThe main pathway to the shelf break is via Seal Trough, identified as the de facto western boundary of the Bellingshausen Sea [ABSTRACT FROM AUTHOR]

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

In recent decades, the Greenland ice sheet has been losing mass through glacier retreat and ice flow acceleration. This mass loss is linked with variations in submarine melt, yet existing ocean models are either coarse global simulations focused on decadal‐scale variability or fine‐scale simulations for process‐based investigations. Here, we unite these scales with a framework to downscale from a global state estimate (15 km) into a regional model (3 km) that resolves circulation on the continental shelf. We further downscale into a fjord‐scale model (500 m) that resolves circulation inside fjords and quantifies melt. We demonstrate this approach in Scoresby Sund, East Greenland, and find that interannual variations in submarine melt at Daugaard‐Jensen glacier induced by ocean temperature variability are consistent with the decadal changes in glacier ice dynamics. This study provides a framework by which coarse‐resolution models can be refined to quantify glacier submarine melt for future ice sheet projections. Plain Language Summary: Over the past several decades, the Greenland ice sheet has been losing ice and contributing to sea‐level rise. About half of this ice loss is induced by melt that occurs where glaciers meet the ocean. Using coarse‐scale ocean models that simulate circulation around the globe, previous studies have noted a strong link between ocean temperature and enhanced glacier ice loss. However, due to the small scale of Greenland's fjords, coarse models are unable to directly quantify circulation in these fjords and melt on submerged glaciers. In this study, we develop a new framework to "zoom in" on a fjord, using high‐resolution models driven by larger coarse‐resolution models. In this approach, we simulate melt on one of Greenland's biggest glaciers and find that periods of higher melt coincide with more ice loss as observed from satellites. Since this framework is adaptable to other regions, it could also be used to simulate melt on other glaciers and support estimates of future sea‐level rise. Key Points: Subsurface temperature variability is simulated in a narrow fjord network using regional models downscaled from a global state estimateModeled increases in ocean melt at Daugaard‐Jensen glacier coincide with the onset of acceleration in 2005 and retreat and thinning in 2011Model variations in shelf‐to‐fjord ocean properties match with observations, providing a basis to estimate ocean forcing in ice projections [ABSTRACT FROM AUTHOR]

Abstract Projections of sea‐level rise from ice‐sheet shrinkage in a warming world have large uncertainties, linked to limited knowledge of changes at the ocean‐ice sheet interface. This interface most typically is modeled as a grounding line, across which still‐connected ice flows into the ocean to float as an ice shelf, or where icebergs calve from a cliff before the ice begins to float. But, extensive and rapidly increasing evidence shows that this is really a grounding zone, and that processes in this grounding zone omitted from many models could exert major controls on sea‐level rise.

Abstract Understanding grounding line dynamics is critical for projecting glacier evolution and sea level rise. Observations from satellite radar interferometry reveal rapid grounding line migration forced by oceanic tides that are several kilometers larger than predicted by hydrostatic equilibrium, indicating the transition from grounded to floating ice is more complex than previously thought. Recent studies suggest seawater intrusion beneath grounded ice may play a role in driving rapid ice loss. Here, we investigate its impact on the evolution of Petermann Glacier, Greenland, using an ice sheet model. We compare model results with observed changes in grounding line position, velocity, and ice elevation between 2010 and 2022. We match the observed retreat, speed up, and thinning using 3‐km‐long seawater intrusion that drive peak ice melt rates of 50 m/yr; but we cannot obtain the same agreement without seawater intrusion. Including seawater intrusion in glacier modeling will increase the sensitivity to ocean warming.

Buoyancy fluxes and submarine melt rates at vertical ice‐ocean interfaces are commonly parameterized using theories derived for unbounded free plumes. A Large Eddy Simulation is used to analyze the disparate dynamics of free plumes and wall‐bounded plumes; the distinctions between the two are supported by recent theoretical and experimental results. Modifications to parameterizations consistent with these simulations are tested and compared to results from numerical and laboratory experiments of meltwater plumes. These modifications include 50% weaker entrainment and a distinct plume‐driven friction velocity in the shear boundary layer up to 8 times greater than the externally‐driven friction velocity. Using these updated plume parameter modifications leads to 40 times the ambient melt rate predicted by commonly used parameterizations at vertical glacier faces, which is consistent with observed melt rates at LeConte Glacier, Alaska. Plain Language Summary: Over the past two decades, the outward flow of tidewater glaciers has accelerated, which has contributed to sea level rise. There is growing evidence that this acceleration has been triggered by melting at ice‐ocean interfaces, where the ocean comes into contact with and drives the melting of glaciers. In particular, commonly used models and theories describing the ocean turbulence and melt dynamics at vertical ice‐ocean interfaces underestimate observed melt rates by an order of magnitude. This study tests proposed changes to existing theories and uses a turbulence‐resolving ocean model to validate this alternative (plume with a wall) theory instead of commonly used (plume without a wall) theories; the first type is more appropriate and better takes into account how ocean turbulence drives the melting of a vertical ice wall. We show that these proposed changes are consistent with existing melt observations and are an important step toward understanding a critical process that may help us improve sea level rise predictions. Key Points: A modified wall‐bounded plume parameterization motivated by recent numerical/lab work is proposed as an alternative to free plume theorySubglacial discharge plume simulations at a vertical ice face are consistent with entrainment/plume dynamics from wall‐bounded plume theoryMelt rate estimates using updated parameters are consistent with observations at LeConte Glacier (40 times greater than standard estimates) [ABSTRACT FROM AUTHOR]

Abstract At marine‐terminating glaciers, both buoyant plumes and local currents energize turbulent exchanges that control ice melt. Because of challenges in making centimeter‐scale measurements at glaciers, these dynamics at near‐vertical ice‐ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically bolted to an underwater ice face, and use these to elucidate the interplay between buoyancy and externally forced currents in meltwater plumes. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near‐boundary stress. When external currents strengthen, the plume diffuses far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in‐situ observations to demonstrate how buoyant melt plumes energize near‐boundary turbulence, and why their dynamics are critical in predicting ice melt.

Abstract In recent decades, the Greenland ice sheet has been losing mass through glacier retreat and ice flow acceleration. This mass loss is linked with variations in submarine melt, yet existing ocean models are either coarse global simulations focused on decadal‐scale variability or fine‐scale simulations for process‐based investigations. Here, we unite these scales with a framework to downscale from a global state estimate (15 km) into a regional model (3 km) that resolves circulation on the continental shelf. We further downscale into a fjord‐scale model (500 m) that resolves circulation inside fjords and quantifies melt. We demonstrate this approach in Scoresby Sund, East Greenland, and find that interannual variations in submarine melt at Daugaard‐Jensen glacier induced by ocean temperature variability are consistent with the decadal changes in glacier ice dynamics. This study provides a framework by which coarse‐resolution models can be refined to quantify glacier submarine melt for future ice sheet projections.

Abstract Buoyancy fluxes and submarine melt rates at vertical ice‐ocean interfaces are commonly parameterized using theories derived for unbounded free plumes. A Large Eddy Simulation is used to analyze the disparate dynamics of free plumes and wall‐bounded plumes; the distinctions between the two are supported by recent theoretical and experimental results. Modifications to parameterizations consistent with these simulations are tested and compared to results from numerical and laboratory experiments of meltwater plumes. These modifications include 50% weaker entrainment and a distinct plume‐driven friction velocity in the shear boundary layer up to 8 times greater than the externally‐driven friction velocity. Using these updated plume parameter modifications leads to 40 times the ambient melt rate predicted by commonly used parameterizations at vertical glacier faces, which is consistent with observed melt rates at LeConte Glacier, Alaska.

Little is known about Antarctic subglacial hydrology, but based on modeling, theory and indirect observations it is thought that subglacial runoff enhances submarine melt locally through buoyancy effects. However, no studies to date have examined effects of runoff on sea ice and oceanography on the continental shelf. Here we use modeled and observational estimates of runoff to force a regional model of the Amundsen Sea Embayment. We find that runoff enhances melt locally (i.e., within the ice‐shelf cavity), increasing melt at Thwaites ice shelf by up to 15 Gt/a given estimates of steady runoff, and up to 25 Gt/a if runoff is episodic as remote sensing measurements suggest. However runoff also has smaller non‐local effects on melt through freshwater influence on flow and stratification. We further find that runoff reduces summer sea‐ice volume over the continental shelf (by up to 10% with steady runoff but over 30% with episodic runoff). Furthermore runoff is much more effective at reducing sea ice than an equivalent volume of ice‐shelf meltwater—due in part to the latent heat loss associated with submarine melting. Results suggest that runoff may play an important role in continental shelf dynamics, despite runoff flux being small relative to ice‐shelf melting—and that runoff‐driven melt and circulation may be an important process missing from regional Antarctic ocean models. Plain Language Summary: A number of floating ice shelves in Antarctica are exposed to warm waters which lead to strong melting, limiting shelves' ability to buttress against ice flow from the continent and also bringing warmed and freshened waters to the ocean surface—waters which in turn influence seasonal sea ice near the continent and insulate the deep ocean from the cold atmosphere. Meanwhile, large rivers under the Antarctic ice sheet carry melt from the ice sheet base to the ocean cavities beneath ice shelves. While a few studies have looked at the role these rivers play in ice‐shelf melt, they do not examine the effects on ocean properties and sea ice in the open ocean. We find that these "rivers" not only amplify melting under ice shelves, but have far‐reaching impacts on ocean circulation. We find that while the direct contribution of water from these subglacial rivers is small compared to that of ice‐shelf melt, there is a disproportionate impact on seasonal sea ice. Key Points: Modeled and observed subglacial runoff estimates are used to force a regional model of the Amundsen Sea EmbaymentRunoff influences melt rates regionally as well as locally, and leads to reduction of summer sea ice volume on the continental shelfRunoff impacts on sea ice differ qualitatively from those of an equivalent volume of ice‐shelf melting [ABSTRACT FROM AUTHOR]

Antarctic ice shelves are losing mass at increasing rates, yet the ice‐ocean interactions that cause significant ice loss are not well understood. A new approach of high‐resolution phase‐change simulations is used to model vertical ice melting into a stratified ocean. The ocean dynamics show complicated interplay between a turbulent buoyant meltwater plume and double‐diffusive layers, while the ice actively melts and changes topography. At room temperatures, the double‐diffusive layer thickness is closely linked to ice scalloping. At lower, more realistic ocean temperatures, the meltwater plume becomes prominent with a laminar‐to‐turbulent transition imprinting an indent on the melting ice. The double‐diffusive layer thickness is consistent with scaling prediction, while the real‐world application demonstrates reasonably good matching of the scaling prediction for some Antarctic regions. Our study is a key first step toward the future use of high‐resolution phase‐change fluid dynamics simulations to better understand Antarctic ice shelves in a changing climate. Plain Language Summary: Future climate scenarios and sea level rise are closely tied to the accelerating loss of Antarctic ice shelves, which lose significant mass by melting into the surrounding ocean. However, the extent of ice shelf mass loss in a changing climate is currently not well understood, with lack of knowledge on the fine‐scale ice‐ocean interactions presenting a key restriction on the accuracy of climate predictions. Here, we use a new suite of numerical computer simulations to model an ice face melting into the ocean. In particular, our simulations allow the ice face to melt back and change shape, which previous numerical simulations could not attempt. We see interesting ocean dynamics known as "double‐diffusive layers" that occur because temperature and salinity both affect the water density. In addition, we also see a buoyant meltwater plume evolve next to the ice face. Both the double‐diffusive layers and meltwater plume can influence the ice melting and shape. Our results are a first step in using these new phase‐change simulations to model ice‐ocean interactions, and will help to better understand the Antarctic response to a changing climate. Key Points: High‐resolution phase‐change simulations are used to examine a vertical ice face melting into a stratified ocean at low temperaturesA distinct laminar‐to‐turbulent transition occurs in the meltwater plume as it rises, accompanied by an indent in the ice at the transitionDouble‐diffusive layers adjacent to the turbulent plume are consistent with scaling predictions and are relevant to the ocean application [ABSTRACT FROM AUTHOR]

Abstract Antarctic ice shelves are losing mass at increasing rates, yet the ice‐ocean interactions that cause significant ice loss are not well understood. A new approach of high‐resolution phase‐change simulations is used to model vertical ice melting into a stratified ocean. The ocean dynamics show complicated interplay between a turbulent buoyant meltwater plume and double‐diffusive layers, while the ice actively melts and changes topography. At room temperatures, the double‐diffusive layer thickness is closely linked to ice scalloping. At lower, more realistic ocean temperatures, the meltwater plume becomes prominent with a laminar‐to‐turbulent transition imprinting an indent on the melting ice. The double‐diffusive layer thickness is consistent with scaling prediction, while the real‐world application demonstrates reasonably good matching of the scaling prediction for some Antarctic regions. Our study is a key first step toward the future use of high‐resolution phase‐change fluid dynamics simulations to better understand Antarctic ice shelves in a changing climate.

Glacier-front dynamics is an important control on Greenland's ice mass balance. Warmer ocean waters trigger ice-front retreats of marine-terminating glaciers, and the corresponding loss in resistive stress leads to glacier acceleration and thinning. Here we present an approach to quantify the sensitivity and vulnerability of marine-terminating glaciers to ocean-induced melt. We develop a plan view model of Store Gletscher that includes a level set-based moving boundary capability, a parameterized ocean-induced melt, and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt. We find that the glacier is stabilized by a sill at its terminus. The glacier is dislodged from the sill when ocean-induced melt quadruples, at which point the glacier retreats irreversibly for 27 km into a reverse bed. The model suggests that ice-ocean interactions are the triggering mechanism of glacier retreat, but the bed controls its magnitude.

Tidal modulation of ice streams and their adjacent ice shelves is a real‐world experiment to understand ice‐dynamic processes. We observe the dynamics of Priestley Glacier, Antarctica, using Terrestrial Radar Interferometry (TRI) and GNSS. Ocean tides are predominantly diurnal but horizontal GNSS displacements also oscillate semi‐diurnally. The oscillations are strongest in the ice shelf and tidal signatures decay near‐linearly in the TRI data over >10 km upstream of the grounding line. Tidal flexing is observed >6 km upstream of the grounding line including cm‐scale uplift. Tidal grounding line migration is small and <40% of the ice thickness. The frequency doubling of horizontal displacements relative to the ocean tides is consistent with variable ice‐shelf buttressing demonstrated with a visco‐elastic Maxwell model. Taken together, this supports previously hypothesized flexural ice softening in the grounding‐zone through tides and offers new observational constraints for the role of ice rheology in ice‐shelf buttressing. Plain Language Summary: Temperatures in Antarctica's interior are below zero, so that ice continuously accumulates through snowfall. This mass gain is balanced by ice transport from the interior toward the coast. There, floating ice shelves form on the ocean and ice is eventually lost through iceberg calving and ocean induced melting. Small changes in any of these processes impact global ocean circulation and mean sea level. The speed of the ice varies with ocean tides. We use a specialized instrument that detects this variability and tidal flexing area‐wide and every few hours. We find that although low‐ and high‐tide only occur once a day in this area, ice flow is largest twice a day. This can be explained because ice softens when it is flexed by the ocean tides so that it can flow episodically faster. This is important because the deformation of ice shelves determines to an extent the stability of the entire ice sheet. Key Points: Terrestrial radar interferometry and GNSS identify dynamic changes at Priestley Glacier, Antarctica, forced by diurnal ocean tidesHorizontal displacements vary semi‐diurnally, are strongest in the shelf and decay near‐linearly >10 km upstream of the grounding lineGrounding line migration is small and flexural softening in ice‐shelf shear margins can explain the semi‐diurnal ice flow component [ABSTRACT FROM AUTHOR]

The stability of ice shelves and drainage of ice sheets they buttress is largely determined by melting at their atmospheric and oceanic interfaces. Subglacial bathymetry can impact ice shelf stability because it influences the onset and the pattern of warm ocean water incursions into the cavities between them and the seafloor. Bathymetry is further important at pinning points, which significantly retard the flow of ice shelves. This effect can be lost instantaneously if basal and surface melting cause an ice sheet to thin and lift off its pinning points. With all this in mind, we have developed a model of bathymetry beneath the western Roi Baudouin and central and eastern Borchgrevink ice shelves in Dronning Maud Land based on inversion from gravity data and tied to available depth references offshore and subglacial topography inland of the grounding line. The model shows deep glacial troughs beneath the ice shelves and bathymetric sills close to the continental shelf. The central Borchgrevink Ice Shelf overhangs the continental slope by around 50 km, exposing its northern parts to the open ocean and higher ocean temperatures. Continuous troughs traverse the central Borchgrevink and western Roi Baudouin ice shelves at depths greater than the offshore thermocline and thus present a risk of Warm Deep Water intrusions into their cavities under the current and future oceanographic regimes. Differing bathymetric characteristics might explain the ice shelves' contrasting dominant mass loss processes. Plain Language Summary: The rate at which Antarctica's ice sheets flow off the continent is largely stabilized by floating ice shelves that form where they meet the surrounding ocean. Assessing the stability of this interconnected system strongly depends on correctly quantifying ice gain processes, such as snowfall, and ice mass loss processes, such as melting at the bases of the ice shelves. This basal melting strongly depends on the flow of warm ocean water into the cavity between the ice shelf and the seafloor below, which is in turn influenced by the shape of the seabed. Using sparse direct measurements together with small variations in the pull of gravity measured from airplanes, we have generated a model of the formerly unknown topography beneath the Borchgrevink and Roi Baudouin ice shelves in East Antarctica. The modeled seabed shows deep troughs beneath the ice shelves and topographic sills along the continental shelf. Gateways within these sills potentially allow for the intrusion of warm water into the cavities, representing a threat to future ice shelf stability. Key Points: We have generated bathymetric models based on gravity inversion beneath the Roi Baudouin and Borchgrevink ice shelvesResults are similar to ice shelves throughout the entire Dronning Maud Land, which are all crossed by deep troughs and bathymetric sillsDeep gateways leading from the open ocean into ice shelf cavities possibly allow for the intrusion of Warm Deep Water into these cavities [ABSTRACT FROM AUTHOR]

Totten Glacier (TG), the primary ice discharger of East Antarctica, contains 3.85 m sea level rise equivalent (SLRe) ice mass and has displayed ocean‐driven dynamic change since at least the early 2000s. We project TG's evolution through 2100 in an asynchronously coupled ice‐ocean model, forced at the ocean boundaries with anomalies in CMIP6 projected temperature, salinity, and velocity. Consistent with previous studies, the Antarctic Slope Current continues to modulate warm water inflow toward TG in future simulations. Warm water (−0.5 – 1°C) accesses TG's sub‐ice shelf cavity through depressions along the eastern ice front, driving sustained retreat of TG's eastern grounding zone that cannot be captured in uncoupled models. In high emission scenarios, warm water overcomes topographic barriers and dislodges TG's southern grounding zone around 2070, increasing the rate of grounded ice loss 3.5‐fold (10–35 Gt/yr) and resulting in a total 4.20 mm SLRe loss by 2100. Plain Language Summary: Totten Glacier (TG), East Antarctica, has been losing ice mass and retreating as a result of warm ocean water melting the underside of this glacier. To simulate this glacier's future evolution in response to realistic ocean forcing, we combine an ice sheet model of TG with a newly developed East Antarctic ocean model configuration and run simulations through 2100. Consistent with previous modeling studies, we find that the strength of the Antarctic Slope Current controls the presence of warm water near TG in future simulations. Significant retreat is projected on TG's eastern and southern sectors. On the eastern side, warm ocean water flows directly along the transition of floating to grounded ice, causing sustained melt throughout the entire simulation in all experiments. In contrast, warm ocean water can only access and promote retreat of TG's southern sector in the strongest warming scenarios by 2070, significantly accelerating TG's rate of mass loss and resulting in a total sea level rise contribution of 4.20 mm by 2100. Key Points: Simulations of Totten Glacier (TG) over the 21st century are performed in a coupled ice‐ocean modelA 3.5‐fold increase in grounded ice loss follows retreat of TG's southern grounding zone, resulting in a 4.2 mm sea level rise contributionCoupled model results capture the spatial distribution of ocean‐driven basal melting and lead to retreat on the eastern flank [ABSTRACT FROM AUTHOR]

Marine-terminating glaciers control most of Greenland's ice discharge into the ocean, but little is known about the geometry of their frontal regions. Here we use side-looking, multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough. Deep glacier grounding enables contact with subsurface, warm, salty Atlantic waters (AW) which melts ice at rates of meters per day. We detect cavities undercutting the base of the calving faces at the sites of subglacial water (SGW) discharge predicted by a hydrological model. The observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of SGW transport warm AW to the ice faces. Glacier undercutting likely enhances iceberg calving, impacting ice front stability and, in turn, the glacier mass balance.

Mass loss from Antarctica's three largest ice shelves is dominated by calving, primarily of large tabular icebergs every few decades. Smaller, more frequent calving events also occur, but it is more difficult to detect them and quantify their contribution to total ice‐shelf mass loss. We used surface elevation data from NASA's ICESat‐2 laser altimeter to examine the structure of the Ross Ice Shelf front between October 2018 and July 2020. Profiles frequently show a depression a few meters deep about 200–800 m upstream of the front, with higher values on the eastern portion of the ice shelf. This structure results from bending due to buoyancy of a submerged ice bench generated by ice‐front melting near the waterline when warm water is present in summer. These bending stresses may cause small‐scale calving events whose frequency would change as summer sea ice and atmosphere–ocean heat exchanges vary over time. Plain Language Summary: Mass loss from Antarctica's floating ice shelves, which form as the ice sheet extends into the Southern Ocean, influences how quickly grounded ice flows into the ocean. Estimating future sea‐level change from grounded‐ice loss therefore requires understanding, and developing models for, the processes that affect ice shelves. We used measurements of surface height from NASA's recently launched ICESat‐2 mission to explore one such process, the calving of small icebergs due to upper‐ocean melting of the ice front. We focus on the large Ross Ice Shelf. This local melting leads to bending of the ice shelf that can be seen in ICESat‐2 profiles that cross the ice front. The bending may also fracture the ice shelf to create small icebergs. We found that these surface structures are generally larger on the eastern portion of Ross Ice Shelf than on the western portion. We suggest that this pattern is due to differences in ice, ocean, and sea ice conditions that promote or impede the melting responsible for the ice‐shelf bending. ICESat‐2 will allow us to monitor changes in these small‐scale structures and any associated calving events, which will provide clues about how ice shelves will change in the future. Key Points: We used ICESat‐2 laser altimetry to map prevalent rampart‐moat (R‐M) structures along the Ross Ice Shelf frontR‐M structures indicate a submerged buoyant bench seaward of the aerial ice front and ice stresses that may cause small‐scale calvingAlong‐front variation of R‐M height differences provides insight into sensitivity to oceanic and glaciological conditions [ABSTRACT FROM AUTHOR]

Changes in Greenland's marine-terminating outlet glaciers have led to changes in the flux of icebergs into Greenland's coastal waters, yet icebergs remain a relatively understudied component of the ice-ocean system. We developed a simple iceberg delineation algorithm for Landsat imagery. A machine learning-based cloud mask incorporated into the algorithm enables us to extract iceberg size distributions from open water even in partially cloudy scenes. We applied the algorithm to the Landsat archive covering Disko Bay, West Greenland, to derive a time series of iceberg size distributions from 2000–02 and 2013–15. The time series captures a change in iceberg size distributions, which we interpret as a result of changes in the calving regime of the parent glacier, Sermeq Kujalleq (Jakobshavn Isbræ). The change in calving style associated with the disintegration and disappearance of Sermeq Kujalleq's floating ice tongue resulted in the production of more small icebergs. The increased number of small icebergs resulted in increasingly negative power law slopes fit to iceberg size distributions in Disko Bay, suggesting that iceberg size distribution time series provide useful insights into changes in calving dynamics.

We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1. Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (~ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay).

Recent evidence has shown increasing mass loss from the Greenland ice sheet, with a general trend of accelerated mass losses extending northwards. However, different glaciers have been shown to respond differently to similar external forcings, constituting a problem for extrapolating and upscaling data. Specifically, whilst some outlet glaciers have accelerated, thinned, and retreated in response to atmospheric and oceanic warming, the behavior of other marine terminating glaciers appears to be less sensitive to climate forcing. Ryder glacier, for which only a few studies have been conducted, is located in North Greenland and terminates with a floating ice tongue in Sherard Osborn Fjord. The persistence or disintegration of floating ice tongues has impacts on glacier dynamics and stability, with ramifications beyond, including sea level rise. This study focuses on understanding the controls on calving and frontal ablation of the Ryder glacier through the use of time‐lapse imagery and satellite data. The results suggest that Ryder glacier has behaved independently of climate forcing during recent decades, with fjord geometry exerting a first order control on its calving. Plain Language Summary: Many glaciers around the world are losing ice at an accelerating rate as a result of global climatic changes. The Greenland ice sheet, an important contributor to sea‐level rise, is losing increasing amounts of mass, with regions in the North has become impacted. However, each glacier is responding differently to the climatic changes due to different local settings. All glaciers can lose mass due to surface melt. On top of this, marine terminating glaciers lose mass as a result of submarine melt and calving (the breaking off of icebergs at the front). With increasing ocean temperatures, marine terminating glaciers can be particularly sensitive to climatic changes and be important contributors to sea‐level rise. This article focuses on the Ryder glacier, a marine terminating glacier in North Greenland, to investigate controls on glacier behavior in this region. The results show that the Ryder glacier has been stable over recent decades, with periodic large calving events controlled by the shape of the fjord. Key Points: Understanding controls on individual glaciers has relevance for the whole Greenland ice sheet and associated sea level rise estimatesThe behavior of Ryder glacier seems to be controlled by fjord geometry, rather than climatic forcingLarge calving events at Ryder glacier correspond with frontal acceleration, but velocity increases do not propagate to the grounding line [ABSTRACT FROM AUTHOR]

The Antarctic Ice Sheet loses about half its mass through oceandriven melting of its fringing ice shelves. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica's contribution to global sea level. We use high-resolution large-eddy simulation to examine ocean-driven melt, in a geophysical-scale model of the turbulent ice shelf-ocean boundary layer, focusing on the ocean conditions observed beneath the Ross Ice Shelf. We quantify the role of double-diffusive convection in determining ice shelf melt rates and oceanic mixed layer properties in relatively warm and low-velocity cavity environments. We demonstrate that double-diffusive convection is the first-order process controlling the melt rate and mixed layer evolution at these flow conditions, even more important than vertical shear due to a mean flow, and is responsible for the step-like temperature and salinity structure, or thermohaline staircase, observed beneath the ice. A robust feature of the multiday simulations is a growing saline diffusive sublayer that drives a time-dependent melt rate. This melt rate is lower than current ice-ocean parameterizations, which consider only shear-controlled turbulent melting, would predict. Our main finding is that double-diffusive convection is an important process beneath ice shelves, yet is currently neglected in ocean-climate models. [ABSTRACT FROM AUTHOR]

Satellite observations of gravity anomalies, ice-surface elevation and glacier velocity show significant increases in net grounded-ice-mass loss over the past decade along the Bellingshausen Sea sector (BSS), West Antarctica, in areas where warm (>1°C) sea water floods the continental shelf. These observations provide compelling but indirect evidence that mass losses are driven primarily by reduced buttressing from the floating ice shelves caused by ocean-driven ice-shelf thinning. Here, we combine recent observations of ice velocity, thickness and thickness changes with an ice flow model to study the instantaneous dynamic response of BSS outlet glaciers to observed ice-shelf thinning, alone. Our model results show that multiple BSS outlet glaciers respond instantaneously to observed ice-shelf thinning, particularly in areas where ice shelves ground at discrete points. Increases in modeled and observed dynamic mass losses, however, account for ~5% of the mass loss rates estimated from gravity anomalies and changes in ice-surface elevation, suggesting that variations in surface mass balance may be key to understanding recent BSS mass loss. Our approach isolates the impact of ice-shelf thinning on glacier flow and shows that if ice-shelf thinning continues at or above current rates, total BSS mass loss will increase in the next decade.

A set of collocated, in situ oceanographic and glaciological measurements from Petermann Gletscher Ice Shelf, Greenland, provides insights into the dynamics of under‐ice flow driving basal melting. At a site 16 km seaward of the grounding line within a longitudinal basal channel, two conductivity‐temperature (CT) sensors beneath the ice base and a phase‐sensitive radar on the ice surface were used to monitor the coupled ice shelf‐ocean system. A 6 month time series spanning 23 August 2015 to 12 February 2016 exhibited two distinct periods of ice‐ocean interactions. Between August and December, radar‐derived basal melt rates featured fortnightly peaks of ∼15 m yr−1 which preceded the arrival of cold and fresh pulses in the ocean that had high concentrations of subglacial runoff and glacial meltwater. Estimated current speeds reached 0.20 – 0.40 m s−1 during these pulses, consistent with a strengthened meltwater plume from freshwater enrichment. Such signals did not occur between December and February, when ice‐ocean interactions instead varied at principal diurnal and semidiurnal tidal frequencies, and lower melt rates and current speeds prevailed. A combination of estimated current speeds and meltwater concentrations from the two CT sensors yields estimates of subglacial runoff and glacial meltwater volume fluxes that vary between 10 and 80 m3 s−1 during the ocean pulses. Area‐average upstream ice shelf melt rates from these fluxes are up to 170 m yr−1, revealing that these strengthened plumes had already driven their most intense melting before arriving at the study site.Plain Language Summary: Petermann Gletscher is a large glacier in northern Greenland that transports 4% of the ice sheet by area into the ocean. At its marine terminus it extends into a 16 km wide and 50 km long floating ice shelf. The ice shelf has retreated by 30% over the last decade, likely due to increasing melting of its base. Regions of faster melting result in channels carved into the ice base, where ice thicknesses are half that of the surrounding ice. Here, we investigate the cause of this melting using 6 months of oceanographic and glaciological measurements from August 2015 to February 2016 in the ice shelf's central basal channel. We find that under‐ice ocean current speeds played the primary role in mixing ocean heat upward and controlling melting in the channel. During the first 3 months of data, freshwater periodically flowed from land into the ocean beneath the ice shelf, causing currents to accelerate and melting to increase. This behavior changed in the second 3 months, when freshwater discharge stopped and tides instead drove weaker currents and low melting. Hence, the timing of subglacial freshwater outflow into the ocean controlled when the ice shelf experienced the strongest melting in its central basal channel.Key Points: Concurrent measurements of melt rate and hydrography under Petermann Gletscher Ice Shelf reveal dynamics of ice shelf meltingMaximum melt rates occur during periods of elevated buoyant under‐ice flow rather than when ocean temperatures are greatestInitiation of buoyant flow is modulated at the grounding line by the spring‐neap tidal cycle and seasonality in subglacial runoff [ABSTRACT FROM AUTHOR]

In the outer solar system, a growing number of giant planet satellites are now known to be abodes for global oceans hidden below an outer layer of ice. These planetary oceans are a natural laboratory for studying physical oceanographic processes in settings that challenge traditional assumptions made for Earth's oceans. While some driving mechanisms are common to both systems, such as buoyancy-driven flows and tides, others, such as libration, precession, and electromagnetic pumping, are likely more significant for moons in orbit around a host planet. Here, we review these mechanisms and how they may operate across the solar system, including their implications for ice-ocean interactions. Future studies should continue to advance our understanding of each of these processes as well as how they may act together in concert. This interplay also has strong implications for habitability as well as testing oceanic hypotheses with future missions.

• Timing and rate of glacier retreat is dictated by basal rheology and topography. • Retreat rate across mixed-rheology beds depends on spacing between basal bumps. • Ice-ocean interactions around isolated topographic highs impact stability. The spatially variable basal flow law suggested by geophysical data from Thwaites Glacier, West Antarctica produces modeled ice-flow response to warming that differs notably from the response of commonly assumed spatially uniform flow laws, with implications for future sea-level rise. Unlike many ice-sheet outlets, Thwaites flows across rather than along prominent topography, with hard stoss faces where form drag is understood to give a low-stress-exponent basal flow law and soft lee-side tills likely to give nearly-plastic behavior. Applying the PSU 2-D higher-order flowline model to a range of idealized Thwaites-like topographies, we test the impact of nearly plastic, nearly viscous, and spatially variable basal rheology on forced grounding-line evolution. In agreement with previous findings, for spatially uniform basal rheology, the timing and rate of grounding-line retreat depend strongly on the bed exponent, with plastic rheology promoting longer stability at the current grounding line but faster retreat once destabilized. Additionally, we find that retreat over mixed-rheology beds is sensitive to the wavelength of the basal topography. For outlet-glacier flow across long-wavelength bumps (≳10 km), behavior falls between that for uniform viscous and plastic beds, yet closer to plastic. However, over shorter-wavelength topography, at times, grounding-line retreat is slower than for either uniform-rheology end-member. Accurately accounting for variable basal conditions in ice-sheet models thus is critical for improving projections of both the timing and magnitude of retreat. [ABSTRACT FROM AUTHOR]

Changes in Greenland's marine-terminating outlet glaciers have led to changes in the flux of icebergs into Greenland's coastal waters, yet icebergs remain a relatively understudied component of the ice-ocean system. We developed a simple iceberg delineation algorithm for Landsat imagery. A machine learning-based cloud mask incorporated into the algorithm enables us to extract iceberg size distributions from open water even in partially cloudy scenes. We applied the algorithm to the Landsat archive covering Disko Bay, West Greenland, to derive a time series of iceberg size distributions from 2000–02 and 2013–15. The time series captures a change in iceberg size distributions, which we interpret as a result of changes in the calving regime of the parent glacier, Sermeq Kujalleq (Jakobshavn Isbræ). The change in calving style associated with the disintegration and disappearance of Sermeq Kujalleq's floating ice tongue resulted in the production of more small icebergs. The increased number of small icebergs resulted in increasingly negative power law slopes fit to iceberg size distributions in Disko Bay, suggesting that iceberg size distribution time series provide useful insights into changes in calving dynamics. [ABSTRACT FROM AUTHOR]

We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1. Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (~ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay). [ABSTRACT FROM AUTHOR]

Although the Greenland ice sheet is losing mass as a whole, patterns of change on both local and regional scales are complex. Spatial statistics reveal large spatial variability of dynamic thinning rates of Greenland's marine-terminating glaciers between 2003 and 2009; only 18% of glacier thinning rates co-vary with neighboring glaciers. Most spatially-correlated thinning rates are clusters of stable glaciers in the Thule, Scoresby Sund, and Southwest regions. Conversely, where spatial-autocorrelation is low, individual glaciers are more strongly controlled by local, glacier-scale features than by regional influences. We investigate possible sources of local control of oceanic forcing by combining grounding line depths and ocean model output to estimate mean ocean heat content adjacent to 74 glaciers. Linear regression models indicate stronger correlation of dynamic thinning rates with ocean heat content compared to those with grounding line depths alone. The correlation between ocean heat and dynamic thinning is robust for all of Greenland except glaciers in the West, and strongest in the Southeast (R2 ~ 0.81 ± 0.15, p = 0.009), implying that glaciers with deeper grounded termini here are most sensitive to changes in ocean forcing. In the Northwest, accounting for shallow sills in the regressions improves the correlation of water depth with glacial thinning, highlighting the need for comprehensive knowledge of fjord geometry.

Marine-terminating glaciers and ice streams are important controls of ice sheet mass balance. However, understanding of their long-term response to external forcing is limited by relatively short observational records of present-day glaciers and sparse geologic evidence for paleo-glaciers. Here we use a high-resolution ice sheet model with an accurate representation of grounding line dynamics to study the deglaciation of the marine-based south-western Norwegian sector of the Scandinavian Ice Sheet and its sensitivity to ocean and atmosphere forcing. We find that the regional response to a uniform climate change is highly dependent on the local bedrock topography, consistent with ice sheet reconstructions. Our simulations suggest that ocean warming is able to trigger initial retreat in several fjords, but is not sufficient to explain retreat everywhere. Widespread retreat requires additional ice thinning driven by surface melt. Once retreat is triggered, the underlying bedrock topography and fjord width control the rate and extent of retreat, while multi-millennial changes over the course of deglaciation are modulated by surface melt. We suggest that fjord geometry, ice-ocean interactions and grounding line dynamics are vital controls of decadal-to centennial scale ice sheet mass loss. However, we postulate that atmospheric changes are the most important drivers of widespread ice sheet demise, and will likely trump oceanic influence on future ice sheet mass loss and resulting sea level rise over centennial and longer time scales. [ABSTRACT FROM AUTHOR]

Abstract: In recent decades, tidewater glaciers in Northwest Greenland contributed significantly to sea level rise but exhibited a complex spatial pattern of retreat. Here we use novel observations of bathymetry and water temperature from NASA's Ocean Melting Greenland mission to quantify the role of warm, salty Atlantic Water in controlling the evolution of 37 glaciers. Modeled ocean‐induced undercutting of calving margins compared with ice advection and ice front retreat observed by satellites from 1985 to 2015 indicate that 35 glaciers retreated when cumulative anomalies in ocean‐induced undercutting rose above the range of seasonal variability of calving‐front positions, while two glaciers standing on shallow sills and colder water did not retreat. Deviations in the observed timing of retreat are explained by residual uncertainties in bathymetry, inefficient mixing of waters in shallow fjords, and the presence of small floating sections. Overall, warmer ocean temperature triggered the retreat, but calving processes dominate ablation (71%). [ABSTRACT FROM AUTHOR]

Satellite observations of gravity anomalies, ice-surface elevation and glacier velocity show significant increases in net grounded-ice-mass loss over the past decade along the Bellingshausen Sea sector (BSS), West Antarctica, in areas where warm (>1°C) sea water floods the continental shelf. These observations provide compelling but indirect evidence that mass losses are driven primarily by reduced buttressing from the floating ice shelves caused by ocean-driven ice-shelf thinning. Here, we combine recent observations of ice velocity, thickness and thickness changes with an ice flow model to study the instantaneous dynamic response of BSS outlet glaciers to observed ice-shelf thinning, alone. Our model results show that multiple BSS outlet glaciers respond instantaneously to observed ice-shelf thinning, particularly in areas where ice shelves ground at discrete points. Increases in modeled and observed dynamic mass losses, however, account for ~5% of the mass loss rates estimated from gravity anomalies and changes in ice-surface elevation, suggesting that variations in surface mass balance may be key to understanding recent BSS mass loss. Our approach isolates the impact of ice-shelf thinning on glacier flow and shows that if ice-shelf thinning continues at or above current rates, total BSS mass loss will increase in the next decade. [ABSTRACT FROM AUTHOR]

Fast ice plays important physical and ecological roles: as a barrier to wind, waves and radiation, as both barrier and safe resting place for air-breathing animals, and as substrate for microbial communities. While sea ice has been monitored for decades using satellite imagery, high-resolution imagery sufficient to distinguish fast ice from mobile pack ice extends only back to c. 2000. Fast ice trends may differ from previously identified changes in regional sea ice distributions. To investigate effects of climate and human activities on fast ice dynamics in McMurdo Sound, Ross Sea, the sea and fast ice seasonal events (1978–2015), ice thicknesses and temperatures (1986–2014), wind velocities (1973–2015) and dates that an icebreaker annually opens a channel to McMurdo Station (1956–2015) are reported. A significant relationship exists between sea ice concentration and fast ice extent in the Sound. While fast/sea ice retreat dates have not changed, fast/sea ice reaches a minimum later and begins to advance earlier, in partial agreement with changes in Ross Sea regional pack ice dynamics. Fast ice minimum extent within McMurdo Sound is significantly correlated with icebreaker arrival date as well as wind velocity. The potential impacts of changes in fast ice climatology on the local marine ecosystem are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The first fully synchronous, coupled ice shelf‐ocean model with a fixed grounding line and imposed upstream ice velocity has been developed using the MITgcm (Massachusetts Institute of Technology general circulation model). Unlike previous, asynchronous, approaches to coupled modeling our approach is fully conservative of heat, salt, and mass. Synchronous coupling is achieved by continuously updating the ice‐shelf thickness on the ocean time step. By simulating an idealized, warm‐water ice shelf we show how raising the pycnocline leads to a reduction in both ice‐shelf mass and back stress, and hence buttressing. Coupled runs show the formation of a western boundary channel in the ice‐shelf base due to increased melting on the western boundary due to Coriolis enhanced flow. Eastern boundary ice thickening is also observed. This is not the case when using a simple depth‐dependent parameterized melt, as the ice shelf has relatively thinner sides and a thicker central “bulge” for a given ice‐shelf mass. Ice‐shelf geometry arising from the parameterized melt rate tends to underestimate backstress (and therefore buttressing) for a given ice‐shelf mass due to a thinner ice shelf at the boundaries when compared to coupled model simulations. [ABSTRACT FROM AUTHOR]

Warm water masses circulating at depth off the coast of Greenland play an important role in controlling rates of mass loss from the Greenland Ice Sheet through feedbacks associated with the melting of marine glacier termini. The ability of these warm waters to reach glacier termini is strongly controlled by fjord bathymetry, which was unmapped for the majority of Greenland’s fjords until recently. In response to the need for bathymetric measurements in previously uncharted areas, we developed two companion methods to infer fjord bathymetry using icebergs as depth sounders. The main premise of our methods centers around the idea that deep-drafted icebergs will become stranded in shallow water such that estimates of iceberg surface elevation can be used to infer draft, and thus water depth, under the assumption of hydrostatic equilibrium. When and where available, surface elevations of icebergs stranded on bathymetric highs were extracted from digital elevation models (DEMs) and converted to estimates of iceberg draft. To expand the spatial coverage of our inferred water depths beyond the DEM footprints, we used the DEMs to construct characteristic depth–width ratios and then inferred depths from satellite imagery-derived iceberg widths. We tested and applied the methods in two fjord systems in western Greenland with partially constrained bathymetry, Ilulissat Isfjord and Naajarsuit Fjord, to demonstrate their utility for inferring bathymetry using remote sensing datasets. Our results show that while the uncertainties associated with the methods are high (up to ±93 m), they provide critical first-order constraints on fjord bathymetry.

The sub-gridscale floe size and thickness distribution (FSTD) is an emerging climate variable, playing a leading-order role in the coupling between sea ice, the ocean, and the atmosphere. The FSTD, however, is difficult to measure given the vast range of horizontal scales of individual floes, leading to the common use of power-law scaling to describe it. The evolution of a coupled mixed-layer-FSTD model of a typical marginal ice zone is explicitly simulated here, to develop a deeper understanding of how processes active at the floe scale may or may not lead to scaling laws in the floe size distribution. The time evolution of mean quantities obtained from the FSTD (sea ice concentration, mean thickness, volume) is complex even in simple scenarios, suggesting that these quantities, which affect climate feedbacks, should be carefully calculated in climate models. The emergence of FSTDs with multiple separate power-law regimes, as seen in observations, is found to be due to the combination of multiple scale-selective processes. Limitations in assuming a power-law FSTD are carefully analyzed, applying methods used in observations to FSTD model output. Two important sources of error are identified that may lead to model biases: one when observing an insufficiently small range of floe sizes, and one from the fact that floe-scale processes often do not produce power-law behavior. These two sources of error may easily lead to biases in mean quantities derived from the FSTD of greater than 100%, and therefore biases in modeled sea ice evolution. [ABSTRACT FROM AUTHOR]

Numerical models of ice-ocean interactions typically rely upon a parameterization for the transport of heat and salt to the ice face that has not been satisfactorily validated by observational or experimental data. We compare laboratory experiments of ice-saltwater interactions to a common numerical parameterization and find a significant disagreement in the dependence of the melt rate on the fluid velocity. We suggest a resolution to this disagreement based on a theoretical analysis of the boundary layer next to a vertical heated plate, which results in a threshold fluid velocity of approximately 4 cm/s at driving temperatures between 0.5 and [ABSTRACT FROM AUTHOR]

I explore the tidewater glacier cycle with a 1-D, depth- and width-integrated flow model that includes a mass-flux calving parameterization. The parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity. The model demonstrates variable sensitivity to climate. From an advanced, stable configuration, a small warming of the climate triggers a rapid retreat that causes large-scale drawdown and is enhanced by positive glacier-dynamic feedbacks. Eventually, the terminus retreats out of deep water and the terminus velocity decreases, resulting in reduced drawdown and the potential for restabilization. Terminus readvance can be initiated by cooling the climate. Terminus advance into deep water is difficult to sustain, however, due to negative feedbacks between glacier dynamics and surface mass balance. Despite uncertainty in the precise form of the parameterization, the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability. It also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models. [ABSTRACT FROM PUBLISHER]