ALBERT

Description: This data set comprises the geographical coordinates of modelled outlines of 31 new subglacial lakes in Dronning Maud Land, Antarctica, using the general hydraulic potential equation (Shreve 1972), Bedmap2 data sets (Fretwell and others, 2013), and AWI airborne radio-echo sounding data collected between 1994–2013. The modelling was conducted in 2015 using the Bedmap2 bedrock topography and ice thickness data (Fretwell and others, 2013) at 5 km grid cell size. Determined sinks in the hydraulic potential were checked for coverage by AWI airborne radio-echo sounding data. Based on the structure of the subglacial reflexions, sinks were classified as subglacial lakes. The outline of like that identified subglacial lakes are presented here. Not all calculated sinks were covered by AWI airborne radio-echo sounding data.

Description: Lakes beneath the Antarctic Ice Sheet are known to decrease traction at the ice base and therefore can have a great impact on ice dynamics. However, the total extent of Antarctic subglacial lakes is still unknown. We address this issue by combining modeling and remote-sensing strategies to predict potential lake locations using the general hydraulic potential equation. We are able to reproduce the majority of known lakes, as well as predict the existence of many new and so far undetected potential lakes. To validate our predictions, we analyzed ice-penetrating radar profiles from radio-echo sounding flights acquired over 1994-2013 in Dronning Maud Land, East Antarctica, and this led to the identification of 31 new subglacial lakes. Based on these findings, we estimate the total number of Antarctic subglacial lakes to be similar to 1300, a factor of three higher than the total number of lakes discovered to date. We estimate that only similar to 30% of all Antarctic subglacial lakes and similar to 65% of the total estimated lake-covered area have been discovered, and that lakes account for 0.6% of the Antarctic ice/bed interface.

Description: The Regional Antarctic ice and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice-shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner–Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase in ice-shelf basal melt rates.

Description: A Regional Antarctic and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner-Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in-situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase of ice-shelf basal melt rates.

Description: The Regional Antarctic ice and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice-shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner–Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase in ice-shelf basal melt rates.

Description: The discovery of many subglacial lakes provides clear evidence for the presence of water beneath the Antarctic ice sheet. Recent observations also indicate interactions between lakes over several hundred kilometers. It is important to understand this widespread subglacial hydrologic network as it is a key parameter with respect to basal lubrication in ice flow modeling and hence, crucial to predict the impact of climate change on sea level rise. Earlier models already estimated the basal melting and routed subglacial water by applying simple balance flux algorithms, but none was mass conservative on typical mountainous bedrock topographies. They weren't able to model the evolution of subglacial lakes or route water through sinks in the hydraulic potential resulting from bedrock topography and ice pressure. Here we present a new algorithm balancing the subglacial meltwater, provided by the numerical thermodynamic ice flow model RIMBAY, and routing it iteratively along the hydraulic potential. This new flux algorithm is fully mass conservative. We can estimate the balance of melted water, water stored in subglacial lakes, and basal water, which is routed out of the sub-ice-sheet domain. The amount of fresh-water entering the oceans is of fundamental importance for the ocean circulation, in particular in the Weddell Sea, Antarctica and southern Greenland. Furthermore the water flux is coupled to the basal friction law in the ice model RIMBAY, lubricating the base of the ice sheet and thereby accelerating the overburden ice. In the present study we thoroughly investigate the impact of the subglacial water flux on the ice flow dynamics in an idealized setup. We are able to model the evolution of subglacial lakes, ice streams and a mass conservative hydrologic basal flux system. The comparison with earlier balance flux algorithms indicates the significance of our advanced incorporation of hydrological processes at the bedrock-ice interface in ice sheet modeling because of considerable impacts on ice volume and dynamics.

Description: The discovery of many subglacial lakes provides clear evidence for the presence of water beneath the Antarctic ice sheet. Recent observations also indicate interactions between lakes over several hundred kilometers. It is important to understand this widespread subglacial hydrologic network as it is a key parameter with respect to basal lubrication in ice flow modeling and hence, crucial to predict the impact of climate change in sea level rise. Earlier models already estimated the basal melting and routed subglacial water by applying simple balance flux algorithms, but none was mass conservative on typical mountainous bedrock topographies. They weren’t able to model the evolution of subglacial lakes or route water through sinks in the hydraulic potential resulting from bedrock topography and ice pressure. Here we present a new algorithm balancing the subglacial meltwater, provided by the numerical thermodynamic ice flow model RIMBAY, and routing it iteratively along the hydraulic potential. This new flux algorithm is fully mass conservative. We can estimate the balance of melted water, water stored in subglacial lakes, and basal water, which is routed out of the sub-ice-sheet domain. The amount of fresh-water entering the oceans is of fundamental importance for the ocean circulation, in particular in the Weddell Sea, Antarctica and southern Greenland. Furthermore the water flux is coupled to the basal friction law in the ice model RIMBAY, lubricating the base of the ice sheet and thereby accelerating the overburden ice. In the present study we thoroughly investigate the impact of the subglacial water flux on the ice flow dynamics in an idealized setup. We are able to model the evolution of subglacial lakes, ice streams and a mass conservative hydrologic basal flux system. The comparison with earlier balance flux algorithms indicates the significance of our advanced incorporation of hydrological processes at the bedrock-ice interface in ice sheet modeling because of considerable impacts on ice volume and dynamics.

Description: Simulations of ice-shelf basal melting in future climate scenarios from the IPCC’s Fourth Assessment Report (AR4) have revealed a large uncertainty and the potential of a rapidly increasing basal mass loss particularly for the large cold-water ice shelves in the Ross and Weddell Seas. The large spread in model results was traced back to uncertainties in the freshwater budget on the continental shelf, which is governed by sea-ice formation. Differences in sea-ice formation, in turn, follow the regional differences between the atmospheric heat fluxes imprinted by the climate models. A more recent suite of BRIOS and FESOM model experiments was performed with output from two members of the newer generation of climate models engaged in the IPCC’s Fifth Assessment Report (AR5). Comparing simulations forced with output from the AR5/CMIP5 models HadGem2 and MPI-ESM, we find that uncertainties arising from inter-model differences in high latitudes have reduced considerably. Projected heat fluxes and thus sea-ice formation over the Southern Ocean continental shelves have converged to an ensemble with a much smaller spread than between the AR4 experiments. For most of the ten larger ice shelves in Antarctica, a gradual (but accelerating) increase of basal melt rates during the 21st century is a robust feature throughout the various realizations. Both with HadGem2 and with MPI-ESM forcing, basal melt rates for the Filchner–Ronne Ice Shelf in FESOM increase by a factor of two by the end of the 21st century in the RCP85 scenario. For the smaller, warm-water ice shelves, inter-model differences in ice-shelf basal mass loss projections are still slightly larger than differences between the scenarios RCP45 and RCP85; compared with AR4 projections, however, the model-dependent spread has been strongly reduced.

Description: The ice flow at the margins of the West Antarctic Ice Sheet (WAIS) is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the WAIS and thus its contribution to sea level rise. The stability of these ice shelves results from the balance of mass gain by accumulation and ice flow from the adjacent ice sheet and mass loss by calving and basal melting due to the ocean heat flux. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the submarine slope front of the Antarctic Continent and boost basal ice shelf melting. In particular, simulations demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf (FRIS) within the next decades. In coupled simulations with a finite elements ocean model and a three-dimensional thermomechanical ice flow model we reveal that the consequent thinning of the FRIS would lead to an extensive grounding line retreat associated with a vast mass loss of the WAIS. In a subsequent study, we focus on the ice streams which are draining into the FRIS and dominating the mass transport from grounded to floating ice. For a better representation of these fast-flowing ice features we expand the above ice flow model by the incorporation of local processes at the ice base. There, sediment deformation and lubrication by subglacial hydrology locally allow high basal sliding rates and thus create the precondition for the development of ice streams. A parametrization of basal sliding properties by the simulated basal melt water fluxes allows us to depict velocity and locations of observed ice streams in the catchment of the FRIS more realistically. We present first results of this advanced ice-flow modeling approach, anticipating an even larger response of the AIS to increased sub-shelf melting rates in future coupled simulations.