Description: Ice shelves play a crucial role in helping controlling the current rate of mass loss from the Antarctic ice sheet through their buttressing potential. This control is modulated by variations in ice shelf mass balance, which cause changes in the thickness of the ice shelf, and therefore in its ability to restrain flow from the grounded ice sheet. We present results showing temporal variability in sub-shelf melting using autonomous phase-sensitive radio-echo sounders (ApRES) near the grounding line of the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica. When combined with additional oceanographic evidence of seasonal variation in stratification and the amplification of the diurnal tides around the Gunnerus Bank, the results suggest an intricate mechanism in which topographic waves control the seasonal melt rate variability near the grounding line of the Roi Baudouin Ice Shelf.

Description: Ice shelves play a crucial role in helping controlling the current rate of mass loss from the Antarctic ice sheet through their buttressing potential. This control is modulated by variations in ice shelf mass balance, which cause changes in the thickness of the ice shelf, and therefore in its ability to restrain flow from the grounded ice sheet. We present results showing temporal variability in sub-shelf melting using autonomous phase-sensitive radio-echo sounders (ApRES) near the grounding line of the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica. When combined with additional oceanographic evidence of seasonal variation in stratification and the amplification of the diurnal tides around the Gunnerus Bank, the results suggest an intricate mechanism in which topographic waves control the seasonal melt rate variability near the grounding line of the Roi Baudouin Ice Shelf.

Description: Ice shelves play a crucial role in helping controlling the current rate of mass loss from the Antarctic ice sheet through their buttressing potential. This control is modulated by variations in ice shelf mass balance, which cause changes in the thickness of the ice shelf, and therefore in its ability to restrain flow from the grounded ice sheet. We present results showing temporal variability in sub-shelf melting using autonomous phase-sensitive radio-echo sounders (ApRES) near the grounding line of the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica. When combined with additional oceanographic evidence of seasonal variation in stratification and the amplification of the diurnal tides around the Gunnerus Bank, the results suggest an intricate mechanism in which topographic waves control the seasonal melt rate variability near the grounding line of the Roi Baudouin Ice Shelf.

Description: Ice shelves control the dynamic mass loss of ice sheets through buttressing and their integrity depends on the spatial variability of their basal mass balance (BMB) i.e., the difference between refreezing and melting. Here, we present an improved technique - based on satellite observations - to capture the small-scale variability in the BMB of ice shelves. As a case study, we apply the methodology to the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica, and derive its yearly averaged BMB at 10 m horizontal gridding. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes in flow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. Our BMB field exhibits much spatial detail and ranges from -14.7 to 8.6 m/a ice equivalent. Highest melt rates are found close to the grounding line where the pressure melting point is high, and the ice-shelf slope is steep. The BMB field agrees well with on-site measurements from phase-sensitive radar, although independent radar profiling indicates unresolved spatial variations in firn density. We show that an elliptical surface depression (10 m deep and with an extent of 0.7 km ×1.3 km) lowers by 0.5 to 1.4 m/a , which we tentatively attribute to a transient adaptation to hydrostatic equilibrium. We find evidence for elevated melting beneath ice shelf channels (with melting being concentrated on the channel's flanks). However, farther downstream from the grounding line, the majority of ice shelf channels advect passively (i.e. no melting nor refreezing) toward the ice shelf front. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This study highlights expected challenges for a full coupling between ice and ocean models.

Description: We would like to assess how the ELRA models behaves in comparison to both Self-Gravitating Viscoelastic solid Earth Models (SGVEMs) outputs and to observations. First, we compare the present-day uplift rates predicted when a spatially-uniform ELRA model is forced with the ICE-6G (Argus et al., 2014) and W12 ice loading histories (Whitehouse et al., 2012a,2012b) with the uplift rates reproduced in Fig 6a from Argus et al. (2014) and Fig 10a from Whitehouse et al. (2012b), respectively, where SGVEMs with uniformly stratified solid Earth are used. Next, we compare the present-day uplift rates reproduced by a spatially-uniform ELRA model forced with two different ice loading histories (ICE-6G; Argus et al. (2014) and W12; Whitehouse et al. (2012a,2012b)) for each of the 2000 Monte Carlo ELRA configurations of our ensemble with GPS observations of present-day uplift rates from Whitehouse et al. (2012b).

Description: We perform simulations of the Antarctic ice sheet over a time span of 5000 years, starting from present-day geometry, under the four extended RCP scenarios (Golledge et al., 2015) with the "fast Elementary Thermomechanical Ice Sheet" model (f.ETISh; Pattyn et al., 2017) v1.4. All simulations are performed at a spatial resolution of 25 km. We perform a probabilistic assessment of the impact of uncertainties in Antarctic solid Earth rheological properties on the response of the Antarctic ice sheet for each RCP scenario. We consider four ELRA parameters (D_W, D_E, tau_W, tau_E) to be uncertain parameters and we assume the (prior) marginal probability distributions to follow log-uniform distributions. We determine probabilistic projections of the grounded-ice volume (Vg) as well as marginal probabilities of being ungrounded and Sobol sensitivity indices using Monte carlo estimation with 2000 samples. Each Monte carlo sample can be interpreted as a plausible solid Earth configuration for Antarctica, randomly sampled within the ELRA parameter space. For each of the 2000 Monte Carlo samples, we estimate the change in grounded-ice volume using the f.ETISh model and we determine the median values and quantiles of the projections as the sample medians and quantiles. Here, we provide time series (data every year) of the Antarctic grounded-ice volume for the ensemble of 2000 Monte Carlo simulations under RCP 2.6, 4.5, 6.0 and 8.5. These time series can be compared with those produced with the NOGIA (forced run without bedrock and sea-level adjustments) and UNIBED (ELRA model with fixed uniform parameters) experiments. We also provide gridded masks that define the limit of grounded ice in Antarctica at 7000 CE for the ensemble of 2000 Monte Carlo simulations under all RCP scenarios.

Description: The ApRES (Brennan et al., 2014; Nicholls et al., 2015) was deployed from January to December in 2016 on the RBIS about 90 km from the ice shelf front and 5 km seaward from the grounded ice on the fast-flowing portion of the West Ragnhild glacier, which is the third largest outlet glacier along the Dronning Maud Land Coast (Callens et al., 2014). The ice thickness at the site was ∼300 m in the trough of a channel (Drews, 2015) but increases up to 600 m in the grounding zone upstream, and ice flow velocities in this region range between 250 and 300 m/a (Rignot et al., 2013). By transmitting an electromagnetic signal and receiving the echo, the radar system can detect the ice base (ice-ocean interface) as well as relatively weak internal reflecting layers that are due to changes in ice permittivity. Between two consecutive measurements, the relative vertical motion of internal layers and the base can therefore be tracked. The displacements of the internal layers determine how the thickness of the column evolves due to vertical strain and bottom melting.

Description: Ice shelves control the dynamic mass loss of ice sheets through buttressing and their integrity depends on the spatial variability of their basal mass balance (BMB), i.e. the difference between refreezing and melting. Here, we present an improved technique – based on satellite observations – to capture the small-scale variability in the BMB of ice shelves. As a case study, we apply the methodology to the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica, and derive its yearly averaged BMB at 10 m horizontal gridding. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes in flow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. Our BMB field exhibits much spatial detail and ranges from −14.7 to 8.6 m a−1 ice equivalent. Highest melt rates are found close to the grounding line where the pressure melting point is high, and the ice shelf slope is steep. The BMB field agrees well with on-site measurements from phase-sensitive radar, although independent radar profiling indicates unresolved spatial variations in firn density. We show that an elliptical surface depression (10 m deep and with an extent of 0.7 km × 1.3 km) lowers by 0.5 to 1.4 m a−1, which we tentatively attribute to a transient adaptation to hydrostatic equilibrium. We find evidence for elevated melting beneath ice shelf channels (with melting being concentrated on the channel's flanks). However, farther downstream from the grounding line, the majority of ice shelf channels advect passively (i.e. no melting nor refreezing) toward the ice shelf front. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This study highlights expected challenges for a full coupling between ice and ocean models.

Description: Ice shelves control the dynamic mass loss of ice sheets through buttressing and their integrity depends on the spatial variability of their basal mass balance (BMB), i.e. the difference between refreezing and melting. Here, we present an improved technique – based on satellite observations – to capture the small-scale variability in the BMB of ice shelves. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes inflow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. After testing our technique on the Roi Baudouin Ice Shelf, East Antarctica, we test our methodology on other ice shelves, with different flow regimes. Whereas the detected large-scale pattern in the BMB is very similar to previous and coarser studies, we are nevertheless able detect small-scale features in the BMB with unprecedented detail (10 m gridding). Examples include elevated melting at an ice-shelf channel’s flank and surface lowering of an elliptical surface depression. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This work highlights expected challenges for a full coupling between ice and ocean models.

Description: Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.