Quantifying melt rates in Antarctic grounding zones using flexural inversion

The Antarctic grounding zone, which is the transition between the fully grounded ice sheet to freely floating ice shelf, plays a critical role in ice sheet instability, mass budget calculations and ice sheet model projections. It is therefore important to continuously monitor its location and migration over time. Here we present the first ICESat-2-derived high-resolution grounding zone product of the Antarctica Ice Sheet, including three important boundaries: the inland limit of tidal flexure (Point F), inshore limit of hydrostatic equilibrium (Point H) and the break-in-slope (Point Ib). This dataset was derived from automated techniques developed in this study, using ICESat-2 laser altimetry repeat tracks between 30 March 2019 and 30 September 2020. The new grounding zone product has a near complete coverage of the Antarctica Ice Sheet with a total of 21346 Point F, 18149 Point H and 36765 Point Ib identified, including the difficult to survey grounding zones, such as the fast-flowing glaciers draining into the Amundsen Sea Embayment. The locations of newly derived ICESat-2 landward limit of tidal flexure agree well with the most recent differential synthetic aperture radar interferometry (DInSAR) observations in 2018, with the mean absolute separation and standard deviation of 0.02 and 0.02 km, respectively. By comparing the ICESat-2-derived grounding zone with the previous grounding zone products, we find up-to 15 km grounding line retreat on the Crary Ice Rise of Ross Ice Shelf and the pervasive landward grounding line migration along the Amundsen Sea Embayment during the past two decades. We also identify the presence of ice plain on the Filchner-Ronne Ice Shelf and the influence of oscillating ocean tides on the grounding zone migration. The product derived from this study is available at https://doi.org/10.5523/bris.bnqqyngt89eo26qk8keckglww (Li et al., 2021) and is archived and maintained at the National Snow and Ice Data Center.

Warming of the ocean waters surrounding Greenland plays a major role in driving glacier retreat and the contribution of glaciers to sea level rise. The melt rate at the junction of the ocean with grounded ice-or grounding line-is, however, not well known. Here, we employ a time series of satellite radar interferometry data from the German TanDEM-X mission, the Italian COSMO-SkyMed constellation, and the Finnish ICEYE constellation to document the grounding line migration and basal melt rates of Petermann Glacier, a major marine-based glacier of Northwest Greenland. We find that the grounding line migrates at tidal frequencies over a kilometer-wide (2 to 6 km) grounding zone, which is one order of magnitude larger than expected for grounding lines on a rigid bed. The highest ice shelf melt rates are recorded within the grounding zone with values from 60 ± 13 to 80 ± 15 m/y along laterally confined channels. As the grounding line retreated by 3.8 km in 2016 to 2022, it carved a cavity about 204 m in height where melt rates increased from 40 ± 11 m/y in 2016 to 2019 to 60 ± 15 m/y in 2020 to 2021. In 2022, the cavity remained open during the entire tidal cycle. Such high melt rates concentrated in kilometer-wide grounding zones contrast with the traditional plume model of grounding line melt which predicts zero melt. High rates of simulated basal melting in grounded glacier ice in numerical models will increase the glacier sensitivity to ocean warming and potentially double projections of sea level rise.

Numerical studies of ice flow have consistently identified the grounding zone of outlet glaciers and ice streams (the region where ice starts to float) as crucial for predicting the rate of grounded ice loss to the ocean. Owing to the extreme environments and difficulty of access to ocean cavities beneath ice shelves, field observations are rare. Estimates of melt rates derived from satellites are also difficult to make near grounding zones with confidence. Therefore, numerical ocean models are important tools to investigate these critical and remote regions. The relative inflexibility of structured grid models means, however, that they can struggle to resolve these processes in irregular cavity geometries near grounding zones. To help solve this issue, we present a new nonhydrostatic unstructured mesh model for flow under ice shelves built using the Firedrake finite element framework. We demonstrate our ability to simulate full ice shelf cavity domains using the community standard ISOMIP+ Ocean0 test case and compare our results against those obtained with the popular MITgcm model. Good agreement is found between the two models, despite their use of different discretisation schemes and the sensitivity of the melt rate parameterisation to grid resolution. Verification tests based on the Method of Manufactured Solutions (MMS) show that the new model discretisation is sound and second-order accurate. A main driver behind using Firedrake is the availability of an automatically generated adjoint model. Our first adjoint calculations, of sensitivities of melt rate with respect to different inputs in an idealised grounding zone domain, are promising and point to the ability to address a number of important questions on ocean influence on ice shelf vulnerability in the future.

Ice shelves can control the flux of ice across the grounding line of the Antarctic Ice Sheet (AIS), and hence the rate of mass loss, through the process of ice-shelf buttressing. Recent, increased mass loss from the AIS, particularly in the Amundsen Sea Embayment and the Antarctic Peninsula, has been attributed to a reduction in buttressing due to ice-shelf thinning, calving or ice-shelf collapse events. To determine how further changes in ice-shelf geometry might affect the contribution of the AIS to global sea levels, it is therefore important to quantify the total amount of buttressing that the ice shelves currently provide and to determine where within the ice shelves that buttressing is generated.Previous work has sought to characterise the buttressing of Antarctic ice shelves by, for example, calculating the sensitivity of grounding line flux (GLF) to small perturbations in ice-shelf thickness, or defining regions of passive shelf ice. In this work, we calculate the total buttressing capacity of all Antarctic ice shelves for the first time and then explore the spatial distribution of that total buttressing capacity within each ice shelf.We use the ice-flow model Úa to conduct a series of diagnostic, idealised calving experiments on a present-day, Antarctic-wide model domain, with high spatial resolution over ice shelves and grounding lines. We calculate the total buttressing capacity of each ice shelf as the relative change in GLF in response to the complete removal of the shelf and find that the total buttressing capacity varies by over two orders of magnitude around the ice sheet.We then conduct a series of idealised calving perturbations, using a range of procedures for generating new calving front locations, and explore the spatial distribution of the total buttressing capacity within each ice shelf. We find that the vast majority of the buttressing is typically generated in ice shelf regions within a few kilometres of the grounding line. Thus, we suggest that a greater area of Antarctica’s ice shelves could be considered passive than previously proposed.      

We present a technique for investigating the grounding zone (GZ) of Antarctic ice shelves using laser altimetry from the Ice, Cloud and land Elevation Satellite (ICESat). Most surface height variability in the GZ is easily resolved by the ICESat laser's ∼65 m footprint and ∼172 m along‐track spacing. Comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide‐forced ice flexure, providing GZ location and width information for each track. Using ICESat data in the Institute Ice Stream region of southern Ronne Ice Shelf, we demonstrate that the location of the GZ based on feature identification in satellite imagery or digital elevation models may be in error by several km. Our results show that ICESat will contribute significantly to improving knowledge of GZ structure and to studies requiring accurate GZ locations, e.g., ice mass balance calculations and ice‐sheet/ocean modeling.

<p>Mapping the ice bed interface with radar is challenging in many alpine glaciers where the ice is temperate, and in-ice absorption is high. It is also difficult in selected regions of polar ice sheets such as near grounding zones and in ice streams where clutter and rough beds increase incoherent volume scattering. The lack of information for the ice geometry impedes our process understanding, e.g., basal sliding (requires knowledge about the basal roughness) and the routing of subglacial water flow (requires knowledge on basal smoothness). The lack of observations to constrain variations in ice thickness on the sub-kilometre scale is thus still a bottleneck to confidently predict ice dynamics and expected rates of sea-level rise.</p><p>A recent development in radioglaciology, namely the application of phase-coherent polarimetric radar, provides an excellent opportunity to overcome these limitations. Radar polarimetry has made significant strides in the last few years to constrain internal ice structure and their impact on the deformation of ice sheets, including the reconstruction of ice micro-structure parameters previously obtained from ice cores. Here, we suggest that the ice-bed interface can be identified in characteristic patterns of the polarimetric coherence phase. This new metric provides information in areas where the backscattered power amplitude does not show any signatures of the ice-bed interface. We provide examples for this across a wide range of glaciological settings, including cold (Colle Gnifetti, Switzerland) and temperate (Hintereisferner, Austria) alpine glaciers, thin grounding zones (Ekström Ice Shelf, East Antarctica) and thick ice domes (Dome C, East Antarctica). If this holds, then the ice thickness mapping in challenging glaciological settings should preferably be done using a quad-polarimetric acquisition geometry. For ground-based surveys, this can be done using an autonomous ice rover, for which we provide a proof-of-concept study on the Ekström Ice Shelf in Antarctica.</p>

<p>Ice shelf shape directly controls ocean heat intrusions, melting near the grounding line, and buttressing. Little is known about what determines ice-shelf shape because ice-ocean coupled simulations typically aim at projecting Antarctica's contribution to sea-level rise and they do not resolve small-scale ice-ocean interactive processes. We conduct ice-ocean coupled simulations for an idealized high-resolution, Pine-Island-like model configuration. We show that ocean melting and ice stretching caused by ice acceleration both thin the ice shelf from the grounding line toward the ice shelf front, while ice divergence from the center advects ice toward the ice shelf edges, compensating melt-driven thinning along the across-shelf direction. We separate the ice dynamical component of ice shelf thinning from melt-induced thinning, as a way to understand processes that occur around the grounding zone, where satellite measurements cannot provide a direct measure of basal melt. In the case of the simulated idealized Pine-Island-like ice shelf, ∼75% and ∼25% of ice-shelf thinning is driven by ice shelf melting and ice stretching, respectively. For observations along Pine Island Ice Shelf flowlines, we also confirm that ocean melting and ice acceleration are the two main terms shaping the ice shelf. We also highlight the importance of shallow-depth melting for determining the ice shelf front thickness and ice shelf shape. Recent studies show that the ice shelf shape close to the ice shelf front can control ice shelf buttressing, ice shelf/glacier evolutions, and sea level rise prediction. This study suggests that ice-ocean interactive processes between the entire ice shelf and the ocean alter ice shelf shape including ice shelf front thickness, despite that ice-ocean interactive processes only close to grounding zones have attracted much attention in the past decade. </p>

Quantifying vulnerability of Antarctic ice shelves to hydrofracture using microwave scattering properties

Surface meltwater on Antarctic ice shelves is comprised of slush (saturated firn), and ponded water (lakes and streams). Often, slush forms as a precursor to ponded water, and its formation leads subsequently to water collecting in basins or flowing across ice shelf surfaces. Where slush and/or ponded water refreeze at the end of a melt season, the firn air content of ice shelves may be lowered. This can increase ice shelves’ susceptibility to future meltwater ponding, making them more vulnerable to potential hydrofracture and break-up. Slush and ponded water also have a lower albedo than snow or dry firn, further increasing ice-shelf surface melt under warmer climates. To date, most satellite-derived estimates of surface water on ice shelves have identified only ponded water, potentially underestimating the extent of surface meltwater. Here, we use a previously developed random forest classifier to produce a novel, continent-wide dataset of slush and ponded water extent across all Antarctic ice shelves between 2013 and 2021. Our dataset is comprised of monthly meltwater products for the austral summers (November-March where data availability allows), from which continent-wide, regional, and individual ice-shelf trends are investigated.The continent-wide total meltwater coverage (assessed between November and February) was greatest during January 2017, reaching 6078 km2. Notably, we find that including the slush extent in total meltwater calculations increases surface water extent by a mean of 56% during the melt-season peak (January). However, we identify marked inter-regional variation, with slush accounting for 71% of January’s total surface meltwater extent in Dronning Maud Land, but only 46% in the Antarctic Peninsula. This indicates that until now, the extent of surface meltwater across Antarctica’s ice shelves has been largely under-estimated on ice shelf, regional, and continent-wide scales, which has significant repercussions for calculations of the surface and sub-surface energy and mass balance of ice shelves, the long-term storage of meltwater on ice shelves, and predictions of future ice shelf stability.

<p>The Ross Ice Shelf (RIS) is one of the biggest Antarctic ice shelves and buttresses ice streams draining both the West and East Antarctic ice sheets. Recent  observations indicate that the melting of Antarctic ice-shelves is accelerating with great spatial heterogeneity. However, estimates of basal melting, which rely on indirect methods, are affected by large uncertainties: as for the RIS, the literature includes basal melt rates from 48 to 123 Gt/yr. To improve basal melting predictions we must understand what causes its spatio-temporal variability. Here, we use a regional configuration of the MIT general circulation model (MITgcm) to analyze the interactions between various water masses and the ice shelf, and their connection to local and global climate. The model simulates the ocean circulation in the Ross Sea and inside the RIS cavity from 1993 to 2018. In the actual configuration it does not account for tidal forcing. Basal melting of the RIS is parameterized by the three-equation formulation. The simulated RIS basal averaged melt rate is 78.6 ± 13.3 Gt/yr averaged over 1993-2018.</p><p>To better understand which local water mass causes basal melting, we developed a new methodology based on mixing ratios of endpoint-water masses. The endpoints are defined by: the High and Low Salinity Shelf Water (HSSW/LSSW), characterized by high and low salinity respectively and a near-freezing temperature; warm and salty modified Shelf Waters (mSW); warm and fresh Antarctic Surface Water (AASW); and cold and fresh Ice Shelf Water (ISW).</p><p>Our analyses show that in the long-term, HSSW causes ~45% of the total basal melting and is found mostly in the Western half of the RIS cavity. It shows a long-term trend due to the increase in the volume of cavity occupied by HSSW at the expense of LSSW. LSSW yields ~20% of the total basal melting and is mostly found in the Eastern half of the RIS cavity. As expected, melting due to mSW (~15% of the basal melting) and AASW (~7% of the basal melting) shows a strong seasonal cycle. Simulated mSW mostly reaches the Central-Eastern RIS during summer. Similarly, AASW intrudes below the RIS near Ross Island exclusively in summer. Melting attributed to ISW is only ~2%. About 11% of the simulated basal melting cannot be clearly attributed to any of the main water masses due to local mixing.</p><p>Finally, RIS basal melting and Ross Sea water masses variability inside the cavity are likely driven by a combination of local forcing (katabatic wind), large-scale wind/pressure systems (Amundsen Sea Low, Southern Annular Mode) and teleconnections (El-Niño Southern Oscillation, Pacific Decadal Oscillation), mediated by ocean-sea ice interactions, in particular by sea ice production in Western Ross Sea polynyas, and sea ice import in the Eastern Ross Sea. Identifying such climatic connections can inform which melting mode will be more important in the future climate and which region of the RIS will be more affected.</p>

<p>Ice shelves play a crucial role in controlling rates of ice discharge across Antarctica’s grounding lines. Mass loss from ice shelves, predominately due to basal melting and calving, can reduce the buttressing force provided by ice shelves, leading to increased grounded ice discharge. Despite the importance of ice shelves, existing estimates of calving and freshwater fluxes from ice shelves have utilised disparate datasets valid for inconsistent time periods or have relied on simplifying assumptions, resulting in a limited account of the health of many ice shelves and little indication of processes driving ice shelf mass imbalance.</p><p>Here, we quantify calving and basal melt fluxes at annual temporal resolution during 2010 to 2019. Our annual measurements account for annual variations in ice velocity and basal melt rate for 183 ice shelves, and annual variations calving front position for 34 major ice shelves (accounting for ~90% of the ice shelf area). On average during the study period, a calving flux of 1283±109 Gt yr<sup>-1</sup> is roughly equal to a melt flux of 1247±149 Gt yr<sup>-1</sup>. Inter-annual variations in the fluxes of both basal meltwater and calving mean that the melt contribution to ice shelf mass loss varies between 35% and 62%, with the lowest contributions in years with large calving events. These large (>100 Gt) calving events are rare (8 events during 2010-2019), yet account for 35% of the total ice shelf calving flux, highlighting the importance of large calving events for ice shelf mass balance over short time scales. Eighty percent of ice shelves, including many in East Antarctica, are melting at or faster than their balance rates, indicating that ocean-driven erosion of ice shelf grounding lines is widespread around Antarctica. Furthermore, we find a significant and strong positive correlation (R=0.68) between basal melt flux and grounding line discharge, implying that ocean-driven melt may pace grounded ice loss from Antarctica.</p>

<p>A major source of uncertainty in future sea-level projections is the ocean-driven basal melt of Antarctic ice shelves. Remote sensing estimates of basal melt shows kilometer-scale features, and ice sheet models require kilometer-scale resolution to realistically resolve ice shelf stability and grounding line migration. Yet 3D numerical ocean models are computationally too heavy to produce melt rates at this resolution. To bridge this resolution gap, we here present the 2D numerical model LADDIE which allows for computationally efficient downscaling of basal melt rates, based on coarse 3D ocean model output. As a test case, we apply the model to downscale basal melt rates of the Crosson-Dotson ice shelf in the fast-melting Amundsen Sea region. Due to the model’s computational efficiency, parameters can be tuned extensively. We have tuned the model to a range of parameters, namely average basal melt, melt in the Kohler West grounding zone, melt along the Dotson Ice Shelf Channel, and the overturning circulation of the cavity waters. These tuning targets are taken from a range of remote sensing products and in situ ocean observations. We show that the model can be tuned to agree with all observations, providing confidence that the model contains the essential physical processes governing basal melt. We propose that (sub-)kilometer resolution basal melt rates can be used to improve the realism of ice sheet models and their simulations of contemporary and future mass loss. Here we show that LADDIE can provide these boundary conditions in a computationally efficient way.</p>

The melt rate of Antarctic ice shelves is of key importance for rising sea levels and future climate scenarios. Recent observations beneath Larsen C Ice Shelf revealed an ocean boundary layer that was highly turbulent and raised questions on the effect of these rich flow dynamics on the ocean heat transfer and the ice shelf melt rate. Directly motivated by the field observations, we have conducted large-eddy simulations (LES) to further examine the ocean boundary layer beneath Larsen C Ice Shelf. The LES was initialized with uniform temperature and salinity (T–S) and included a realistic tidal cycle and a small basal slope. A new parameterization based on previous work was applied at the top boundary to model near-wall turbulence and basal melting. The resulting vertical T–S profiles, melt rate, and friction velocity matched well with the Larsen C Ice Shelf observations. The instantaneous melt rate varied strongly with the tidal cycle, with faster flow increasing the turbulence and mixing of heat toward the ice base. An Ekman layer formed beneath the ice base and, due to the strong vertical shear of the current, Ekman rolls appeared in the mixed layer and stratified region (depth ≈ 20–60 m). In an additional high-resolution simulation (conducted with a smaller domain) the Ekman rolls were associated with increased turbulent kinetic energy, but a relatively small vertical heat flux. Our results will help with interpreting field observations and parameterizing the ocean-driven basal melting of ice shelves.

ABSTRACTGrounding 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.

The observed rapid thinning, speed-up and retreat of the ice in West Antarctica’s Amundsen Sea Embayment might indicate an early stage of a marine ice sheet instability (MISI), potentially leading to the disintegration of the West Antarctic Ice Sheet (WAIS). In general, the stability of a marine ice sheet is strongly linked to the dynamics of its buttressing ice shelves which act as a regulator of the ice discharge into the ocean. Existing numerical modeling studies usually simulated MISI-type WAIS retreat either under prescribed fixed present-day calving front positions (associated with very strong ice-shelf buttressing) or in the absence of ice shelves (neglecting ice-shelf buttressing). These approaches represent extreme cases of realizing ice-shelf buttressing in simulations of a WAIS retreat and comparison between the study results are difficult due to the use of different numerical models and experimental designs. Here we aim to investigate the influence of time-evolving calving fronts and associated buttressing changes in the course of an unfolding WAIS disintegration, based on simulations with the Parallel Ice Sheet Model (PISM). One focus will be on how ice-shelf calving affects MISI retreat rates and the extent of a potential WAIS collapse, i.e., the time evolution and magnitude of the associated sea-level contribution.