A general ocean circulation model is coupled with a 3D-thermodynamical ice-sheet/shelf model to simulate the response of the Filchner-Ronne Ice Shelf (FRIS, Antarctica) and coastal parts of its catchment basin to a postulated inflow of Warm Deep Water into the ice-shelf cavity on a 1000-yr timescale. Prescribed ocean warming (based on climate projections) enters the ice-shelf cavity in the up to 1500 m deep Filchner Trough and penetrates deep into the sub-ice cavity. Increasing basal melt rates induce geometry changes of the cavity, which in turn have an impact on the ocean circulation and therefore the modelled melt rates. Highest melt rates of about 20 m yr(-1) follow the (up to 180 km) retreating grounding line. Basal mass loss reaches about 250 km(3) yr(-1), doubling the present-day value. The most vulnerable areas below the FRIS are the Bailey Ice Stream and the area between the Institute and Moeller Ice Streams, where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock. The potential additional contribution to the eustatic sea level rise due to the grounded-ice loss, simulated in an ensemble approach against a transient control experiment, is about 0.05 mm yr(-1) during the first 500 yr and about 0.17 mm yr(-1) thereafter. (C) 2015 Elsevier B.V. All rights reserved.

Interactions between Antarctic ice shelf regions and the ocean and their contribution to oceanic water mass formation: Ice shelves represent the main areas of ice discharge from the Antarctic Ice Sheet into the Southern Ocean. At the transition between ice and ocean, ice shelves affect both, the mass balance and dynamics of the ice sheet and the water mass formation in the Southern Ocean by ice shelf melting and calving at the shelf front, respectively. Under constant boundary conditions (mass flux from the ice sheet, ice accumulation and melting) a balance between mass gain and mass loss exists, and the ice shelves, except for recurring calving events, establish an almost invariant shape. In addition, ice shelves exert a buttressing force on the ice sheet and, therefore, are a regulating factor of ice discharge. Under changing climatic conditions, manifested for example in a changing oceanic circulation and temperature, this equilibrium state might be disturbed. In a model study with a coupled ocean - ice shelf - ice sheet model, the influence of changing climate boundary conditions on the mass loss in the Atlantic sector of the Antarctic Ice Sheet, the water mass formation in the Weddell Sea, and the sea level is investigated. Starting from scenario runs of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report IV (AR4), which describe the climate development to the year 2100, the dynamic coupling of ocean circulation and ice sheet dynamics predict a significant mass loss for the Filchner-Ronne Ice Shelf and in particular a retreat of its southern grounding line. This mass loss is equivalent to a doubling of today’s observed losses of glaciers and ice caps and sets the previously assumed stability of the East Antarctic Ice Sheet to climate change into question.

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.

Glaciers and ice caps exhibit currently the largest cryospheric contributions to sea level rise. Modelling the dynamics and mass balance of the major ice sheets is therefore an important issue to investigate the current state and the future response of the cryosphere in response to changing environmental conditions, namely global warming. This requires a powerful, easy-to-use, versatile multi-approximation ice dynamics model. Based on the well-known and established ice sheet model of Pattyn (2003) we develop the modular multi-approximation thermomechanic ice model RIMBAY, in which we improve the original version in several aspects like a shallow ice–shallow shelf coupler and a full 3D-grounding-line migration scheme based on Schoof's (2007) heuristic analytical approach. We summarise the full Stokes equations and several approximations implemented within this model and we describe the different numerical discretisations. The results are cross-validated against previous publications dealing with ice modelling, and some additional artificial set-ups demonstrate the robustness of the different solvers and their internal coupling. RIMBAY is designed for an easy adaption to new scientific issues. Hence, we demonstrate in very different set-ups the applicability and functionality of RIMBAY in Earth system science in general and ice modelling in particular.

We apply a global finite element sea ice/ice shelf/ocean model (FESOM) to the Antarctic marginal seas to analyze projections of ice shelf basal melting in a warmer climate. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM. Results from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. While trends in sea ice coverage, ocean heat content, and ice shelf basal melting are small in simulations forced with ECHAM5 data, a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity of basal melting to increased ocean temperatures is found for the ice shelves in the Amundsen Sea. For the cold-water ice shelves in the Ross and Weddell Seas,decreasing convection on the continental shelf in the HadCM3 scenarios leads to an erosion of the continental slope front and to warm water of open ocean origin entering the continental shelf. As this water reaches deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. Highest melt rates at the deep FRIS grounding line causes a retreat of > 200km, equivalent to an land ice loss of 110 Gt/yr.

The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line through submarine glacial troughs and causing high rates of melting at deep ice-shelf bases. The interaction between ocean currents, continental bathymetry, and shelf hydrography is thus likely to influence future rates of ice loss. The evolution of basal loss in a warming climate is presented for ten Antarctic ice shelves, based on the output of two coupled ice–ocean models (BRIOS and FESOM) both forced by the IPCC-SRES E1 and A1B scenario-related atmospheric outputs of the HadCM3 and ECHAM5/MPIOM climate models. Projections of future ice shelf basal melting are similar with regard to the scenarios applied but differ substantially between the climate models used, with the HadCM3 output causing the most significant changes in continental shelf temperatures. All ice shelves face a possible increase in basal melting with the biggest changes occurring at the base of the Filchner-Ronne Ice Shelf. A redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century may lead to increased flow of warm open ocean waters into the deep southern ice-shelf cavity. Here, water temperatures can increase by more than 2 oC boosting average basal melting from 0.2 m/yr, or 82 Gt/yr, to almost 4 m/yr, or 1,600 Gt/yr. The analysis of the results suggests that the changes are caused primarily by the freshening of the shelf water masses and an increase in ocean surface stress in the southeastern Weddell Sea, both due to reduced sea ice formation and a thinning of the formerly consolidated sea-ice cover. A projected further increase of ice loss at the base of the Filchner–Ronne Ice Shelf to 2,500 Gt/yr for the year 2199 is caused by a gradual warming of the deep Weddell Sea in FESOM and does not occur in the regional BRIOS simulation.

Abstract Ice flow from the ice sheets to the ocean contains the maximum potential contributing to future eustatic sea-level rise. In Antarctica most mass fluxes occur via the extended ice-shelf regions covering more than half the Antarctic coastline. The most extended ice shelves are the Filchner–Ronne and Ross Ice Shelves, which contribute ~30% to the total mass loss caused by basal melting. Basal melt rates here show small to moderate average amplitudes of <0.5ma–1. By comparison, the smaller but most vulnerable ice shelves in the Amundsen and Bellinghausen Seas show much higher melt rates (up to 30 ma–1), but overall basal mass loss is comparably small due to the small size of the ice shelves. The pivotal question for both characteristic ice-shelf regions, however, is the impact of ocean melting, and, coevally, change in ice-shelf thickness, on the flow dynamics of the hinterland ice masses. In theory, ice-shelf back-pressure acts to stabilize the ice sheet, and thus the ice volume stored above sea level. We use the three-dimensional (3-D) thermomechanical ice-flow model RIMBAY to investigate the ice flow in a regularly shaped model domain, including ice-sheet, ice-shelf and open-ocean regions. By using melting scenarios for perturbation studies, we find a hysteresis-like behaviour. The experiments show that the system regains its initial state when perturbations are switched off. Average basal melt rates of up to 2 ma–1 as well as spatially variable melting calculated by our 3-D ocean model ROMBAX act as basal boundary conditions in time-dependent model studies. Changes in ice volume and grounding-line position are monitored after 1000 years of modelling and reveal mass losses of up to 40 Gt a–1.

The redirection of warm water under the Filchner–Ronne Ice Shelf during the second half of this century could cause the ice-shelf base to melt at a rate 20 times higher than at present. Warm ocean currents are known to erode ice shelves from below, but changes in currents can be forced by many different mechanisms, leading to uncertain outcomes. This study highlights the vulnerability to climate change of a small Antarctic coastal region, which has potentially severe consequences for the mass balance of a large Antarctic ice shelf. Hellmer et al. use climate modelling to show that the projected loss of sea ice in the Weddell Sea (east of the Antarctic Peninsula) leads to an increase in wind stress, which in turn accelerates a warm ocean current far underneath the vast Filchner–Ronne Ice Shelf. The authors predict that the increased warmth could increase melt by a factor of 20, with possible consequences for ice-stream dynamics in the East Antarctic Ice Sheet. The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line1,2 through submarine glacial troughs and causing high rates of melting at the deep ice-shelf bases3,4. The interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice-mass loss. Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice–ocean model forced by a range of atmospheric outputs from the HadCM35 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner–Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance6. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise.

Simulations of the ocean dynamics in the cavity under the Amery Ice Shelf, Antarctica, were carried out using a three‐dimensional numerical ocean model. Two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front. The simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography. The circulation is driven by the density gradient in the cavity, which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice‐ocean interface, and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front. The interaction at the ice‐ocean interface allows the basal component of the mass loss of the Amery Ice Shelf to be estimated. In the two simulations the computed losses were 5.8 Gt yr−1 and 18.0 Gt yr−1, values consistent with observations. The bulk of the melting occurred near the southern grounding line of the ice shelf, although substantial melting also occurred in areas where heat transport by horizontal circulation was large. Accretion was restricted to areas where water, from upstream melting, became supercooled as it ascended the ice shelf base.

A high-resolution three-dimensional ocean circulation model is applied to the cavity beneath Filchner-Ronne Ice Shelf (FRIS). The model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands. The surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses, in the former case leading to freezing, while melting at the ice shelf base results in the latter case. The pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath FRIS. An anticyclonic circulation around the Korff and Henry Ice Rises with melting west of Korff Ice Rise and freezing on the eastern side and north of Henry Ice Rise is the main source for an ice-pumping mechanism that produces the observed large marine ice body in the central Filchner-Ronne Ice Shelf. The estimates for net melting for realistic conditions at the open ocean boundary are 40-50 km(3) yr(-1) indicating that ice shelf-ocean interaction is an important contribution to the mass balance of the ice shelf.

The circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean. Hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity, which at least provides the potential for melting and freezing processes. Applying a three‐dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier, we found an important parameter controlling the interaction between these two systems. Idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction, because of the sudden decrease in water column thickness. Since the water column thickness and the Coriolis force determine the characteristics for geostrophic flow, separated circulation systems arise for the open ocean and the ice shelf cavity. Only in areas where constant water column thickness and, from the oceanographic point of view, constant ƒ/H contours can be observed across the barrier, an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf. This is only the case at lateral sloping sidewalls or at deep depressions, which can be found, for example, in the southern Weddell Sea. In all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier.