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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.

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.

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 presentation, the role of atmospheric forcing in the calving of the Brunt Ice Shelf (BIS) in the Weddell Sea sector of Antarctica in February 2021 is discussed 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 warmer and more 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 newly-formed iceberg A-74 in the Weddell Sea at a speed of up to 700 m/day. The findings of this work highlight 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., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

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, Str...

Dynamics of polar outlet glaciers vary with ocean tides, providing a natural laboratory to understand basal processes beneath ice streams, ice rheology and ice-shelf buttressing. We apply Terrestrial Radar Interferometry to close the spatiotemporal gap between localized, temporally well-resolved GNSS and area-wide but sparse satellite observations. Three-hour flowfields collected over an eight day period at Priestley Glacier, Antarctica, validate and provide the spatial context for concurrent GNSS measurements. Ice flow is fastest during falling tides and slowest during rising tides. Principal components of the timeseries prove upstream propagation of tidal signatures $>$ 10 km away from the grounding line. Hourly, cm-scale horizontal and vertical flexure patterns occur $>$6 km upstream of the grounding line. Vertical uplift upstream of the grounding line is consistent with ephemeral re-grounding during low-tide impacting grounding-zone stability. On the freely floating ice shelves, we find velocity peaks both during high- and low-tide suggesting that ice-shelf buttressing varies temporally as a function of flexural bending from tidal displacement. Taken together, these observations identify tidal imprints on ice-stream dynamics on new temporal and spatial scales providing constraints for models designed to isolate dominating processes in ice-stream and ice-shelf mechanics.

The full length of Parker Ice Tongue on the Victoria Land Coast, Antarctica, calved in March 2020. Calving of this magnitude (18 km) is not previously seen for this location. The mean growth rate (...

Surface crevasses covered by snow bridges can be mapped remotely on ice sheets and glaciers using active microwave synthetic aperture radar. They are highlighted against the surrounding snow due to increased scattering from the side-walls and base of snow bridges and usually appear as linear features. The contrast between crevasses and crevasse-free regions depends on the design of the sensor, the image acquisition parameters and the properties of the snow. Here we quantify how crevassed regions are represented at X-band for different polarizations, look directions and incidence angles, and discuss whether additional information about their physical properties can be gained from their radar signature. Snow bridge thicknesses and crevasse widths are measured on the ground in the McMurdo Shear Zone and Brunt Ice Shelf by ground-penetrating radar and excavation. TerraSAR-X is shown to reliably distinguish crevasse location, balancing penetration into the snow and horizontal resolution. We provide recommendations for radar imaging parameters that optimize the identification of individual crevasses and crevassed regions.

In Antarctica, basal melting in the ice-sheet’s interior generates subglacial water that is routed via the subglacial hydrological system towards the margins. At the grounding zone, the subglacial meltwater comes into contact with ocean water subject to tides. The mixing of the two water masses may be one reason for velocities variations on tidal timescales, providing a window into processes of basal sliding. With this goal in mind, we instrumented a flowline across the grounding zone of Priestley glacier, Antarctica, with 4 differential GNSS stations co-located with advanced phase sensitive radars (ApRES) and tiltmeters all measuring continuously over several months. Moreover, we installed a Terrestrial Radar Interferometer (TRI) overlooking the glacier from an adjacent rock outcrop. The to our knowledge first-time deployment of the TRI in Antarctica reveals a stunning picture of grounding-zone dynamics providing spatially coherent 1D flowfields every 3 hours over a time period of 10 days. This enables interpretations of velocity changes measured by GNNS in an unprecedented spatial context. We complement our on-site geophysical dataset with airborne ice-penetrating radar as well as spaceborne InSAR data using timeseries from TanDEM-X, Sentinel-1A, and the ERS satellites.TRI and GNSS stations jointly detect tidal velocity fluctuations (> 50 % around the mean) which decay landwards with increasing distance from the grounding line. Triple differences in satellite interferometry reveal transient bull’s eye patterns far upstream of the grounding line quantifying localized surface lowering together with adjacent surface uplift. We interpret this as a result from abruptly migrating subglacial water pockets cascading over obstacles in the basal topography. The TRI also shows such bull’s eye patterns pulsating in our highly resolved time series. Moreover, all GNSS stations and the TRI detect a short-lived acceleration event (~100 % horizontal speedup over 2 hours) paired with spatially coherent surface uplift (~15 cm). Magnitude and duration of this event suggests operation of hydraulic jacking, a mechanism explaining short-lived speed-ups with pressure variations in a linked-cavity system. However, usually this is pre-conditioned to the existence of significant surface meltwater entering the subglacial hydrological system, which is not the case at our study site. Our joint observations with multiple sensors and instruments therefore provide unique observations to further develop our understanding of basal sliding, particularly it's dependency on upstream water supply and ocean tides.

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.

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.

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.

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 time of satellite data acquisition. Here, we combine the sub-centimetre accuracy and spatial benefits of DInSAR with the temporal benefits of tide models to infer the spatiotemporal 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. 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 match DInSAR to 0.84 mm; generally improve traditional tide models by up to −39 % from 10.8 cm to 6.7 cm RMSE against GPS data from areas where ice is in local hydrostatic equilibrium with the ocean; and up to −74 % from 21.4 cm to 5.6 cm RMSE against GPS data in feature-rich coastal areas where contemporary tide-models are most inaccurate. Numerical modelling then reveals a Young’s modulus of E = 1.0 GPa and an ice viscosity of 10 TPa s when finite-element simulations of tidal flexure are matched to 16 days of tiltmeter data; supporting the theory that strain dependent anisotropy may significantly decrease effective viscosity compared to isotropic polycrystalline ice on large spatial scales. Applications of our method range from (i) refining coarsly-gridded tide models to resolve small-scale features at the spatial resolution and vertical accuracy of SAR imagery, to (ii) separating elastic and viscoelastic contributions in the satellite derived flexure measurement and (iii) gaining information about large-scale ice heterogenity in Antarctic ice-shelf grounding zones, the missing key to improve 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.

Ice-shelf grounding zones link the Antarctic ice-sheets to the ocean. Differential interferometric synthetic aperture radar (DInSAR) is commonly used to monitor grounding-line locations, but also contains information on grounding-zone ice thickness, ice properties and tidal conditions beneath the ice shelf. Here, we combine in-situ data with numerical modeling of ice-shelf flexure to investigate 2-D controls on the tidal bending pattern on the Southern McMurdo Ice Shelf. We validate our results with 9 double-differential TerraSAR-X interferograms. It is necessary to make adjustments to the tidal forcing to directly compare observations with model output and we find that when these adjustments are small (

The article of record as published may be found at http://dx.doi.org/10.1029/2018JC013987 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

Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, both linearly and non-linearly. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge, which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, the importance of which is highlighted here in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This variability decreases rapidly to

[1] Previous investigations have linked changes in the multiyear sea ice area of McMurdo Sound, Antarctica, from 1213 km2 in 2003 to 4923 km2 in 2005, to the passage of large tabular icebergs preventing the annual sea ice breakout. This maximum coverage then gradually diminished, by 2009 covering 1453 km2. This investigation employs the use of the Ice, Cloud, and land Elevation Satellite (ICESat) laser altimeter to derive freeboard of sea ice in McMurdo Sound from 2003 to 2009 and hence infer thickness changes over this time period. Two techniques for freeboard retrieval are compared. Method-1 (M-1) follows those previously presented in the literature using the lowest elevations to construct an estimate of sea surface height. However, the lack of leads in the study area motivated the development of Method-2 (M-2) which utilizes tide models. Each year is divided into two investigation periods from September to December and February to June, and these investigations were further segmented by sea ice type, first-year (FY), and multiyear (MY). Both applied methods reveal a statistically significant linear increase in multiyear sea ice freeboard. For M-1, the mean freeboard increased over the study period from 0.53 to 1.00 m and for M-2 from 0.46 to 0.95 m. Evidence is presented that the multiyear sea ice freeboard increase is strongly linked to the development and incorporation of a subice platelet layer. No statistically significant trends were observed for first-year sea ice. ICESat derived freeboards over first-year and multiyear sea ice areas compare within one standard deviation of airborne measured freeboard in November 2009.

This investigation employs the use of ICESat to derive freeboard measurements in McMurdo Sound in the western Ross Sea, Antarctica, for the time period 2003-2009. Methods closely follow those previously presented in the literature but are complemented by a good understanding of general sea ice characteristics in the study region from extensive temporal ground investigations but with limited spatial coverage. The aim of remote sensing applications in this area is to expand the good knowledge of sea ice characteristics within these limited areas to the wider McMurdo Sound and western Ross Sea region. The seven year Austral Spring (September, October, and November) investigation is presented for sea ice freeboard alone. An interannual comparison of mean freeboard indicates an increase in multiyear sea ice freeboard from 1.08 m in 2003 to 1.15 m in 2009 with positive and negative variation in between. No significant trend was detected for first year sea ice freeboard. Further, an Envisat imagery investigation complements the freeboard assessment. The multiyear sea ice was observed to increase by 254 % of its original 2003 area, as firstyear sea ice persisted through the 2004 melt season into 2005. This maximum coverage then gradually diminished by 2009 to 20 % above the original 2003 value. The mid study period increase is likely attributed to the passage of iceberg B-15A minimising oceanic pressures and preventing sea ice breakout in the region.

Very high precision satellite altimeter measurements from the Geoscience Laser Altimeter System onboard NASA's Ice, Cloud and Land Elevation Satellite (ICESat) have allowed a method of feature tracking to be developed for floating ice which relies on recording the movement of large surface undulations. This method is applied to a section of the Ross Ice Shelf downstream of the grounding line of the Beardmore Glacier, Antarctica. The altimetry method has benefits over established optical and interferometric remote sensing techniques due to high pointing accuracy for geo-location, ability to deal with tidal fluctuations and to measure velocity where visible surface features are absent. Initial processing of a single sequence of ICESat tracks gives encouraging results for unidirectional ice flow with correlations between surface profiles in consecutive years exceeding 90% and producing high internal consistency in velocity between independent tracks. Velocities of 331 ± 28 m a−1 near to the grounding line are also consistent with available ground measurements for the area.

Twelve years of horizontal wind data from the Scott Base MF radar and the Halley SuperDARN radar recorded between January 1996 and December 2007 are analysed to study the interannual variability of the migrating (S=2) and non-migrating (S=1) components of the semidiurnal tide around 78°S in the Antarctic upper mesosphere. Significant quasi-biennial modulation of the summer time S=1 component is observed. During early summer the amplitude of the component is up to 4 ms−1 stronger during the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) measured at 30 hPa. No statistically significant effect is seen in amplitude of the migrating component of the tide, or in the phase (time of maximum) of either component. These results are discussed in the light of previous observations of the interannual variability of the semidiurnal tide.

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.

The longitudinal variability of the semidiurnal tide in the Antarctic upper mesosphere is investigated by comparison of observations from two radars at approximately opposite sides of Antarctica. Under the assumption that the tide is composed of an S = 2 (migrating) and S = 1 (westward-propagating, non-migrating) component only, the relative phases of the components are shown to vary with season such that the waves are typically in constructive interference during the winter (summer) months at longitudes around 0 degrees E (180 degrees E). We show that this has profound effects on the seasonal behaviour of the semidiurnal tide around 78 degrees S dependent on the longitude, and that no single-station observations at this latitude can be considered representative of a "zonal mean". The superposition of these two waves is used to interpret differences in previously-published ground-based climatologies of the tide. Citation: Hibbins, R. E., O. J. Marsh, A. J. McDonald, and M. J. Jarvis (2010), A new perspective on the longitudinal variability of the semidiurnal tide, Geophys. Res. Lett., 37, L14804, doi:10.1029/2010GL044015.