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The Brunt Ice Shelf, Antarctica, accelerated rapidly from a velocity of 900 to 1500 m a -1 during 6 months, following calving of a 1500 km 2 iceberg on 22 January 2023. The immediate response to calving was observed as a change to the rate of acceleration and not to velocity directly. Acceleration increased by a factor of 10, with a second, smaller calving at the end of June 2023, leading to further tripling of acceleration. The acceleration was caused by the reduction of buttressing at the McDonald Ice Rumples, leading to high localised strain rates, which reduce the strength of the remaining ice shelf. [ABSTRACT FROM AUTHOR]

The Brunt Ice Shelf, Antarctica, accelerated rapidly from 900 m a-1 to 1500 m a-1 during six months following the calving of a 1500 km2 iceberg on 22nd January 2023. Initially, the rate of acceleration increased by a factor of ten, with a second, smaller calving at the end of June 2023 leading to further tripling of acceleration. The acceleration is caused by reduction of buttressing at the McDonald Ice Rumples due to loss of contact with the sea floor and has led to high strain rates to the south, with potential consequences for the stability of the remaining ice shelf. [ABSTRACT FROM AUTHOR]

Tide-forced short-term migration of the grounding line (GL) of Antarctic ice shelves can impact ice dynamics at the ice sheet margins and obscures assessments of long-term GL advance or retreat. However, the magnitude of tidally induced GL migration is poorly known, and the spatial patterns and modes of variability are not well characterised. Here we develop and apply a technique that uses Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) repeat-track laser altimetry to locate the inland limit of tidal ice shelf flexure for each sampled tide, enabling the magnitude and temporal variability of tidal GL migration to be resolved. We demonstrate its application at an ice plain north of Bungenstockrücken, in a region of the southern Ronne Ice Shelf subject to large ocean tides. We observe a 1300 km 2 area of ephemeral grounding over which the GL migrates by up to 15 km between low and high tide and identify four distinct modes of migration: linear, asymmetric, threshold and hysteresis. The short-term movement of the GL dominates any long-term migration signal in this location, and the distribution of GL positions and modes contains information about spatial variability in the ice–bed interface. We discuss the impact of extreme tidal GL migration on ice shelf–ocean–subglacial systems in Antarctica and make recommendations for how GLs should be more precisely defined and documented in future by the community. [ABSTRACT FROM AUTHOR]

Ice shelves influence the mass balance of the Antarctic Ice Sheet by restricting the flow of ice across the grounding zone. Their ability to restrict ice flow is sensitive to changes in their extent or thickness. Full thickness fractures, known as rifts, create tabular icebergs which reduce ice shelf extent. We present a method for measuring rift width using ICESat-2 laser altimetry, as part of a larger effort to detect, catalog and measure various characteristics of Antarctic rifts. We validate the results using optical satellite imagery and data from Global Navigation Satellite System (GNSS) receivers around "Halloween Crack" on Brunt Ice Shelf, East Antarctica. During the study period a further rift, "North Rift" formed and rapidly calved a ~1270 km2 iceberg. In response to this second rift, the opening of Halloween Crack approached stagnation before returning to opening at a reduced rate. We suggest the opening rate is controlled by the ice shelf geometry and degree of contact with a pinning point at McDonald Ice Rumples, and its influence on the large-scale ice flow field. We replicate the general pattern of opening using an inverse finite element model, and discuss the response of the ice shelf to the calving. We use historical satellite imagery and previously published ice-front positions to demonstrate the importance of McDonald Ice Rumples to the long-term calving and advance cycle of Brunt Ice Shelf. [ABSTRACT FROM AUTHOR]

Short-term tidal grounding line (GL) migration in Antarctica can impact ice dynamics at the ice sheet margins and obscures assessments of long-term GL advance or retreat. However, the magnitude of tidally-induced GL migration is poorly known, and the spatial pattern and modes of variability are not well characterised. Here we develop and apply a technique that uses ICESat-2 repeat-track laser altimetry to locate the inland limit of tidal ice shelf flexure for each sampled tide, enabling the magnitude and temporal variability of tidal GL migration to be resolved. We demonstrate its application at an ice plain north of Bungenstockrücken, in a region of the southern Ronne Ice Shelf subject to large ocean tides. We observe a 1,300 km² area of ephemeral grounding over which the GL migrates by up to 15 km between low and high tide, and identify four distinct modes of migration: “linear”, “asymmetric”, “threshold” and “hysteresis”. The short-term movement of the GL dominates any long-term migration signal in this location, and the distribution of GL positions and modes contains information about spatial variability in the ice-bed interface. We discuss the impact of extreme tidal GL migration on ice shelf-ocean-subglacial systems in Antarctica and make recommendations for how GLs should be more precisely defined and documented in future by the community. [ABSTRACT FROM AUTHOR]

The calving of Antarctic ice shelves remains unpredictable to date due to a lack of understanding of the role of the different climatic components in such events. In this study, the role of atmospheric forcing in the calving of the Brunt Ice Shelf (BIS) in February 2021 is investigated using a combination of observational and reanalysis data. The occurrence of a series of extreme cyclones around the time of the calving induced an oceanward sea‐surface slope of >0.08° leading to the calving along a pre‐existing rift. The severe storms were sustained by the development of a pressure dipole on both sides of the BIS associated with a La Niña event and the positive phase of the Southern Annular Mode. Poleward advection of warm and moist low‐latitude air over the BIS area just before the calving was also observed in association with atmospheric rivers accompanying the cyclones. Immediately after the calving, strong offshore winds continued and promoted the drift of the iceberg A‐74 in the Weddell Sea at a speed up to 700 m day−1. This study highlights the contribution of local atmospheric conditions to ice‐shelf dynamics. The link to the larger scale circulation patterns indicates that both need to be accounted for in the projections of Antarctic ice shelf evolution. Plain Language Summary: A calving event is the process by which a large block of ice gets separated from an ice shelf or glacier and forms an iceberg. Large calving events from marine‐terminating ice shelves around Antarctica remain to date highly unpredictable. This process is typically associated with the glaciological cycle of the ice shelves as well as ocean dynamics. However, atmospheric forcing in triggering such events has been largely overlooked. This is investigated for the calving of iceberg A‐74 from the Brunt Ice Shelf (BIS) in February 2021. We found that strong near‐surface winds associated with intense cyclones promoted the event via an increased sea‐surface slope toward the open ocean, which amplified the stress on the pre‐existing rift and led to the calving. After the calving, the iceberg drifted westwards in the Weddell Sea at a speed of 700 m day−1 aided by strong offshore winds. The extreme weather conditions leading to the calving were associated with an alternation of a series of high (blockings) and low (troughs) pressure systems around the BIS region. Key Points: An intense and stationary cyclone around the ice shelf, which was part of a wave train occurred at the time of the calvingStrong near‐surface winds associated with the cyclone promoted the calving of the Brunt Ice Shelf via oceanward sea‐surface slopeCalving occurred on 26 February when the ice accelerated significantly in a westward direction from a velocity of 4–6 to 700 m day−1 [ABSTRACT FROM AUTHOR]

Fox Glacier/Te Moeka o Tuawe is a fast-responding maritime glacier that has undergone multiple advance and retreat phases during recent decades. Here we use a combination of repeat photography, Structure from Motion (SfM), and ice discharge measurement, to identify key morphological differences associated with these repeated phase changes, and assess how much of the current terminus is still dynamically active. Increasing surface-debris cover at the margins and topographic shading result in the asymmetry of the retreating terminus, with central portions receding faster than the margins. In 2019, the glacier is shorter than at any time in recorded history, and ice flux is insufficient to sustain the current glacier length, with a further ∼300 m of the glacier terminus region potentially vulnerable to retreat. However, due to the high climate sensitivity of this maritime glacier, even a slight shift towards increasing mass flux could see this trend reverse. [ABSTRACT FROM AUTHOR]

Differential interferometric synthetic aperture radar (DInSAR) is an essential tool for detecting ice-sheet motion near Antarctica's oceanic margin. These space-borne measurements have been used extensively in the past to map the location and retreat of ice-shelf grounding lines as an indicator for the onset of marine ice-sheet instability and to calculate the mass balance of ice sheets and individual catchments. The main difficulty in interpreting DInSAR is that images originate from a combination of several SAR images and do not indicate instantaneous ice deflection at the times of satellite data acquisitions. Here, we combine the sub-centimetre accuracy and spatial benefits of DInSAR with the temporal benefits of tide models to infer the spatio-temporal dynamics of ice–ocean interaction during the times of satellite overpasses. We demonstrate the potential of this synergy with TerraSAR-X data from the almost-stagnant southern McMurdo Ice Shelf (SMIS). We then validate our algorithm with GPS data from the fast-flowing Darwin Glacier, draining the Antarctic Plateau through the Transantarctic Mountains into the Ross Sea. We are able to reconstruct DInSAR-derived vertical displacements to 7 mm mean absolute residual error and generally improve traditional tide-model output by up to 39 % from 10.8 to 6.7 cm RMSE against GPS data from areas where ice is in local hydrostatic equilibrium with the ocean and by up to 74 % from 21.4 to 5.6 cm RMSE against GPS data in feature-rich coastal areas where tide models have not been applicable before. Numerical modelling then reveals Young's modulus of E=1.0±0.56 GPa and an ice viscosity of ν=10±3.65 TPa s when finite-element simulations of tidal flexure are matched to 16 d of tiltmeter data, supporting the hypothesis that strain-dependent anisotropy may significantly decrease effective viscosity compared to isotropic polycrystalline ice on large spatial scales. Applications of our method include the following: refining coarsely gridded tide models to resolve small-scale features at the spatial resolution and vertical accuracy of SAR imagery, separating elastic and viscoelastic contributions in the satellite-derived flexure measurement, and gaining information about large-scale ice heterogeneity in Antarctic ice-shelf grounding zones, the missing key to improving current ice-sheet flow models. The reconstruction of the individual components forming DInSAR images has the potential to become a standard remote-sensing method in polar tide modelling. Unlocking the algorithm's full potential to answer multi-disciplinary research questions is desired and demands collaboration within the scientific community. [ABSTRACT FROM AUTHOR]

Ocean‐driven melting of ice shelves is a primary mechanism for ice loss from Antarctica. However, due to the difficulty in accessing the sub‐ice shelf ocean cavity, the relationship between ice shelf melting and ocean conditions is poorly understood, particularly near the grounding zone, where the ice transitions from grounded to floating. We present the first borehole oceanographic observations from the grounding zone of the Ross Ice Shelf, Antarctica's largest ice shelf by area. Contrary to predictions that tidal currents near grounding zones mix the water column, we found that Ross Ice Shelf waters were vertically stratified. Current velocities at middepth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle. The observed stratification resulted in low melt rates near this portion of the grounding zone, inferred from phase‐sensitive radar observations. These melt rates were generally <10 cm/year, which is lower than average for the Ross Ice Shelf (~20 cm/year). Melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing. Stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting. Since diffusive convection influences vertical heat and salt fluxes differently than shear‐driven turbulence, this process may affect ice shelf melting and merits further consideration in ocean models of sub‐ice shelf circulation. Plain Language Summary: Ice shelf melting is an important player in ice loss from the Antarctic Ice Sheet, affecting sea level rise. Ice shelf melting is controlled by ocean properties and processes, but sparse observations of the sub‐ice shelf ocean cavity limit our understanding of these controls and thus limit our ability to predict sea level rise. This study presents rare ocean observations deep below the largest ice shelf by area, the Ross Ice Shelf, far from the open ocean. The observed ocean setting is surprisingly quiescent, and waters are cold, around −2 °C. This study also presents new, highly localized ice shelf melting measurements at the site that show that these ocean conditions lead to slow ice shelf melting of only centimeters per year. These observations reveal the ways in which the Ross Ice Shelf contrasts with rapidly melting ice shelves affected by warmer seawater elsewhere in West Antarctica. Thus, they adds nuance to our scientific understanding of ice‐ocean interactions around the Antarctic continent. Key Points: The ocean cavity near the grounding zone of the Ross Ice Shelf is vertically stratified with a boundary layer freshened by ice meltingA tidally mixed zone and tidal currents are absent from this 10‐m‐thick ocean cavity in the flexure zoneThe observed stratification and low current velocities result in low melt rates (7 cm/year) [ABSTRACT FROM AUTHOR]

The retreating Pine Island Glacier (PIG), West Antarctica, presently contributes ~5-10% of global sea-level rise. PIG's retreat rate has increased in recent decades with associated thinning migrating upstream into tributaries feeding the main glacier trunk. To project future change requires modelling that includes robust parameterisation of basal traction, the resistance to ice flow at the bed. However, most ice-sheet models estimate basal traction from satellite-derived surface velocity, without a priori knowledge of the key processes from which it is derived, namely friction at the ice-bed interface and form drag, and the resistance to ice flow that arises as ice deforms to negotiate bed topography. Here, we present highresolution maps, acquired using ice-penetrating radar, of the bed topography across parts of PIG. Contrary to lower-resolution data currently used for ice-sheet models, these data show a contrasting topography across the ice-bed interface. We show that these diverse subglacial landscapes have an impact on ice flow, and present a challenge for modelling ice-sheet evolution and projecting global sea-level rise from ice-sheet loss. [ABSTRACT FROM AUTHOR]

We examine tidal flexure in the grounding zone of the McMurdo Ice Shelf, Antarctica, using a combination of TerraSAR-X repeat-pass radar interferometry, a precise digital elevation model, and GPS ground validation data. Satellite and field data were acquired in tandem between October and December 2014. Our GPS data show a horizontal modulation of up to 60% of the vertical displacement amplitude at tidal periods within a few kilometres of the grounding line. We ascribe the observed oscillatory horizontal motion to varying bending stresses and account for it using a simple elastic beam model. The horizontal surface strain is removed from nine differential interferograms to obtain precise bending curves. They reveal a fixed (as opposed to tidally migrating) grounding-line position and eliminate the possibility of significant upstream bending at this location. The consequence of apparent vertical motion due to uncorrected horizontal strain in interferometric data is a systematic mislocation of the interferometric grounding line by up to the order of one ice thickness, or several hundred metres. While our field site was selected due to its simple boundary conditions and low background velocity, our findings are relevant to other grounding zones studied by satellite interferometry, particularly studies looking at tidally induced velocity changes or interpreting satellite-based flexure profiles. [ABSTRACT FROM AUTHOR]

Tidal flexure in ice shelf grounding zones has been used extensively in the past to determine grounding line position and ice properties. Although the rheology of ice is viscoelastic at tidal loading frequencies, most modelling studies have assumed some form of linear elastic beam approximation to match observed flexure profiles. Here we use density, radar and DInSAR measurements in combination with full-Stokes viscoelastic modelling to investigate a range of additional controls on the flexure of the Southern McMurdo Ice Shelf. We find that inclusion of observed basal crevasses and density dependent ice stiffness can greatly alter the flexure profile and yet fitting a simple elastic beam model to that profile will still produce an excellent fit. Estimates of the effective Young's modulus derived by fitting flexure profiles are shown to vary by over 200% depending on whether these factors are included, even when the local thickness is well constrained. Conversely, estimates of the grounding line position are relatively insensitive to these considerations for the case of a steep bed slope in our study region. By fitting tidal amplitudes only, and ignoring phase information, elastic beam theory can provide a good fit to observations in a wide variety of situations. This should, however, not be taken as an indication that the underlying rheological assumptions are correct. [ABSTRACT FROM PUBLISHER]

Grounding zones are vital to ice-sheet mass balance and its coupling to the global ocean circulation. Processes here determine the mass discharge from the grounded ice sheet, to the floating ice shelves. The response of this transition zone to tidal forcing has been described by both elastic and viscoelastic models. Here we examine the validity of these models for grounding zone flexure over tidal timescales using field data from the Southern McMurdo Ice Shelf (78° 15′S, 167° 7′E). Observations of tidal movement were carried out by simultaneous tiltmeter and GPS measurements along a profile across the grounding zone. Finite-element simulations covering a 64 d period reveal that the viscoelastic model fits best the observations using a Young's modulus of 1.6 GPa and a viscosity of 1013.7 Pa s (≈ 50.1 TPa s). We conclude that the elastic model is only well-constrained for tidal displacements >35% of the spring-tidal amplitude using a Young's modulus of 1.62 ± 0.69 GPa, but that a viscoelastic model is necessary to adequately capture tidal bending at amplitudes below this threshold. In grounding zones where bending stresses are greater than at the Southern McMurdo Ice Shelf or ice viscosity is lower, the threshold would be even higher. [ABSTRACT FROM PUBLISHER]

We present a diagnostic glacier flowline model parameterized and constrained by new velocity data from ice-surface GPS installations and speckle tracking of TerraSAR-X satellite images, newly acquired airborne-radar data, and continental gridded datasets of topography and geothermal heat flux, in order to better understand two outlet glaciers of the East Antarctic ice sheet. Our observational data are employed as primary inputs to a modelling procedure that first calculates the basal thermal regime of each glacier, then iterates the basal sliding coefficient and deformation rate parameter until the fit of simulated to observed surface velocities is optimized. We find that the two glaciers have both frozen and thawed areas at their beds, facilitating partial sliding. Glacier flow arises from a balance between sliding and deformation that fluctuates along the length of each glacier, with the amount of sliding typically varying by up to two orders of magnitude but with deformation rates far more constant. Beardmore Glacier is warmer and faster-flowing than Skelton Glacier, but an up-glacier deepening bed at the grounding line, coupled with ice thicknesses close to flotation, lead us to infer a greater vulnerability of Skelton Glacier to grounding-line recession if affected by ocean-forced thinning and concomitant acceleration. [ABSTRACT FROM AUTHOR]

Ice-thickness measurements in Antarctic ice-shelf grounding zones are necessary for calculating the mass balance of individual catchments, but remain poorly constrained for most of the continent. We describe a new inverse modelling optimization approach to estimate ice thickness in the grounding zone of Antarctic outlet glaciers and ice shelves using spatial patterns of tide-induced flexure derived from differential interferometric synthetic aperture radar (InSAR). We demonstrate that the ill-posedness of the inverse formulation of the elastic-plate equations for bending can be controlled by regularization. In one dimension, the model recreates smooth, synthesized profiles of ice thickness from flexure information to within 1-2%. We test the method in two dimensions and validate it in the grounding zone of Beardmore Glacier, a major outlet glacier in the Transantarctic Mountains, using interferograms created from TerraSAR-X satellite imagery acquired in 2012. We compare our results with historic and modern ice-thickness data (radio-echo sounding from 1967 and ground-penetrating radar from 2010). We match both longitudinal and transverse thickness transects to within 50m root-mean- square error using an effective Young's modulus of 1.4GPa. The highest accuracy is achieved close to the grounded ice boundary, where current estimates of thickness based on surface elevation measurements contain a systematic bias towards thicker ice. [ABSTRACT FROM AUTHOR]