Feedbacks between ice and ocean dynamics at the West Antarctic Filchner-Ronne Ice Shelf in future global warming scenarios

The ice flow at the margins of the West Antarctic Ice Sheet 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, ocean simulations for several of the IPCC's future climate scenarios demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf within the next decades. In this study, we couple the finite elements ocean circulation model FESOM and the three-dimensional thermomechanical ice flow model RIMBAY to investigate the complex interactions between ocean and ice dynamics at the Filchner-Ronne Ice Shelf. We focus on the impact of a changing ice shelf cavity on ocean dynamics as well as the feedback of the resulting sub-shelf melting rates on the ice shelf geometry and implications for the dynamics of the adjacent marine-based Westantarctic Ice Sheet. Our simulations reveal the high sensitivity of grounding line migration to ice-ocean interactions within the Filchner-Ronne Ice Shelf and emphasize the importance of coupled model studies for realistic assessments of the Antarctic mass balance in future global warming scenarios

The role of ice streams in a coupled ice flow-ocean modeling approach at the Filchner-Ronne Ice Shelf, Antarctica

The ice flow at the margins of the Antarctic Ice Sheet (AIS) is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the AIS and thus its contribution to sea level rise. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the continental slope front 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. The increase of water temperature in the sub-shelf cavity is estimated to dramatically raise the basal shelf melting. Coupled simulations with a finite elements ocean model and a three-dimensional thermomechanical ice flow model reveal that the consequent thinning of the FRIS would lead to an extensive grounding line retreat associated with a vast mass loss of the AIS. In this subsequent study, we aim for an enhanced understanding of the complex feedbacks between ocean circulation and ice dynamics of the grounded AIS. Therefor, 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 streams 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. Based on satellite-observed ice surface velocity patterns we identify such areas with low basal drag and parametrize the ice flow model accordingly. As a result, the modeled ice flow patterns will 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 on increased sub-shelf melting rates in future coupled simulations

Response to Filchner–Ronne Ice Shelf cavity warming in a coupled ocean–ice sheet model – Part 1: The ocean perspective

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

The interplay of ocean circulation and ice dynamics at the Filchner-Ronne Ice Shelf, Antarctica

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

Response of the cryosphere to ocean warming �below Filchner Ronne Ice Shelf (Antarctica)

Simulations of ice shelf - ocean interaction for several IPCC future climate change scenarios have revealed the potential of a rapidly increasing basal mass loss for the Filchner-Ronner Ice Shelf (FRIS) in the Weddell Sea. This result is consistent between two independent sea ice - ice shelf - ocean models forced with identical atmospheric data sets. However, both models assume a steady-state ice shelf geometry. To study ice-ocean interaction in a more consistent way, the ice flow model RIMBAY has been configured in a model domain that comprises the FRIS and the grounded ice in the relevant catchment area up to the ice divides. At the base of the model ice shelf, melt rates from the finite-element sea ice – ice shelf – ocean model FESOM are prescribed. For present-day conditions with ice shelf basal melting obtained from a 20th-century simulation with FESOM, the ice model yields a quasi-steady state with an ice shelf geometry and grounding line location very close to the presently observed configuration. With FESOM’s increasing melt rates modelled for future climate warming scenarios, the ice model predicts an accelerated grounding line retreat between the Möller and Institute Ice Streams. Simulated discharge of (formerly) grounded ice is converted to an estimated contribution to global sea level rise. The sub-ice shelf cavity geometry in FESOM is adjusted according to the ice thickness evolution in RIMBAY to investigate the effect of a dynamically varying ice shelf topography on simulated basal melt rates. A two-way coupling between the two models will be conducted as a natural next step

Antarctic subglacial hydrology - interactions of subglacial lakes, basal water flow and ice dynamics

The Antarctic Ice Sheet influences the global temperature and sea level by complex interactions with the atmosphere and the ocean and is thus an important factor in the Earth's climate system. Recent climate assessments reveal a steady increase of global temperatures and an on-going shrinking of glaciers and ice sheets. Because the total Antarctic ice volume has the potential to raise the global sea level by about 58 meters, it is of particular interest to understand the ice dynamics regarding the mass export and thus the contribution to sea level rise. Observations of the last decades reveal a widespread hydrological system of subglacial lakes and drainage networks beneath the Antarctic Ice Sheet which is recognized to have a large impact on the ice dynamics. The aim of this thesis is to investigate this subglacial hydrological environment and its interactions with the ice flow dynamics of the overlying ice sheet. For reaching this aim, the ice flow model RIMBAY is enhanced by a subglacial hydrology module which provides the simulation of basal water flow and the identification of positions and extents of subglacial lakes. This model is then applied to the Antarctic Ice Sheet. A subsequent validation by the analysis of ice-penetrating radar profiles in Dronning Maud Land leads to the identification of 31 new potential subglacial lake locations. Based on these findings, the total number of Antarctic subglacial lakes is estimated to be 1300±300, a factor of three more than what has been discovered so far. Their overall extent is assessed to cover about 0.6% of the Antarctic ice-bed interface. Furthermore, strong correlations are found between modeled pathways of basal water flow and observed locations of ice streams. In a detailed investigation of the Ross Ice Streams at the Antarctic Siple Coast the local basal driver of fast ice flow is identified as water saturated and unconsolidated sediment. The assessment of the basal flow regime enables the simulation of basal drainage patterns which are clearly associated with current patterns of fast ice flow. The application of satellite-observed ice surface elevation changes to the present-day ice sheet geometry additionally allows prognostic water flow simulations. They reveal a high dynamic of basal water pathways. In particular, a major hydraulic tributary of the Kamb and Whillans Ice Stream is redirected towards the Bindschadler Ice Stream within the next 200 years, possibly resulting in future increase of ice velocities within the Bindschadler Ice Stream. In order to gain further insights into the complex feedback mechanisms between an ice sheet and its subglacial environment, ice dynamics and subglacial hydrology are modeled in a coupled approach for a synthetic domain. A new hydrological concept is developed and implemented in RIMBAY, providing the dynamic generation of subglacial lakes and covering the spatial and temporal variability of basal drainage systems. The impact of basal hydrology on the ice dynamic is estimated in various experiments, considering distinct feedback mechanisms. It is demonstrated, that a coupling at full complexity leads to a considerably negative mass balance of the investigated synthetic ice sheet. The results reveal the capabilities of the new hydrological concept and emphasize the necessity to incorporate subglacial hydrology in ice sheet models

The evolution of subglacial water pathways and catchment areas derived from observed ICESat and CryoSat-2 ice surface elevation changes at the Siple Coast, Antarctica

The mass export of the West Antarctic Ice Sheet (WAIS) is dominated by fast flowing ice streams which transport ice from the interior of the ice sheet towards its coast lines with velocities of several hundred meters per year. Understanding their dynamics is considered as a key to estimate the contributions of the WAIS to global sea level rise. This study focuses on the Ross Ice Streams (RIS) at the Siple Coast where observations reveal a high variability of ice stream pathways and velocities in the past. A widely spread and meters thick basal layer of unconsolidated sediments beneath the ice sheet creates the precondition for high basal sliding rates by sediment deformation. However, the exact locations of the RIS are determined by the pathways of basal melt water flow. We compute the subglacial water flow paths for the present-day ice sheet geometry with a balance flux approach and find high correlations between areas of enhanced subglacial water flow and the locations of the RIS. Moreover, the ice flow velocities of the particular ice streams are found to be correlated with the sizes of the water catchment areas draining underneath. For projections we apply surface elevation change rates observed by ICESat and CryoSat-2 to the present-day ice sheet geometry for 200 years and thus estimate the evolution of basal water pathways and catchment areas at the Siple Coast. The results of the simulations using the elevation change rates derived by the particular satellite campaigns show a high consistency. According to them, a major hydraulic tributary of the Kamb and Whillans Ice Stream (KIS and WIS) will be redirected underneath the Bindschadler Ice Stream (BIS) within the next 200 years. The water catchment area feeding underneath the BIS is estimated to grow by about 50% while the lower part of the stagnated KIS becomes increasingly separated from the upper hydraulic tributaries of the Siple Coast. This might be a continuation of the subglacial hydraulic processes which caused the past stagnation of the KIS and could also explain the observed deceleration of the WIS. Furthermore, this might also lead to a future increase of the ice velocities within the BIS and an increased ice drainage of the corresponding hinterland

The response of the West Antarctic Ice Sheet to ocean warming beneath the Filchner-Ronne Ice Shelf

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, ocean simulations for several of the IPCC's future climate scenarios 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 this study, we couple the finite elements ocean circulation model FESOM and the three-dimensional thermomechanical ice flow model RIMBAY to investigate the sensitivity of the ice dynamics within the entire FRIS catchment to simulated future basal shelf melt rates. Our simulations indicate a high sensitivity of the ice dynamics for the Möller and the Institute Ice Stream but only very little response of other ice streams like Rutford, Foundation and Recovery Ice Stream to enhanced basal shelf melting. The grounding line between the Möller and Institute Ice Streams is located on a submarine ridge in front of a deep trough further inland. In this area, basal shelf melting causes a local thinning of the FRIS. The consequent initial retreat of the grounding line continues once it reaches the adjacent reverse-sloped bedrock. We state, that a possible 'point of no return' for a vast grounding line retreat along this steep reverse bedrock slope might have been crossed already even for simulated present-day melt rates, indicating that the WAIS is currently not in equlibrium. Furthermore, our simulations show an accelerated grounding line retreat in this sector of the FRIS as an answer to modeled future cavity warming scenarios leading to an additional mass loss of the WAIS