Currently Pine Island Glacier (PIG) is responsible for 20% of the total ice loss offdischarge from the West Antarctic Ice Sheet (WAIS) ([22]; [30]). The accelerated thinning observed since the 1980s has essentially been attributed to enhanced sub-ice shelf melting [21] induced by the recent alteration of Circumpolar Deep Water circulation [10]. This has reduced the buttressing exerted by the ice shelf, leading to the acceleration of the ice stream and the ongoing retreat of the grounding line (GL) along the glacier's trunk observed since 1992 [17]. Today the GL lies over bedrock that has a steep retrograde slope [29] (Figure 1c) raising the possibility that PIG may already be engaged in an irrevocable retreat. ProvidedAssuming that ice flow is dominated * durand@lgge.obs.ujf-grenoble.fr 2 by basal sliding and lateral variation can be ignored, grounding lines located on retrograde slopes are always unstable [24,3], but in realistic, three-dimensional geometries lateral drag and buttressing in the ice shelf can act to prevent unstable retreat [9]. Assessing the stability of PIG therefore requires numerical models that accurately represent these additional forces.Models designed to study the evolution of PIG have been reported, though limited to flowline geometries [7] or extreme forcings [11]. Overall, the short-term behaviour of PIG is not well understood, and projections vary wildly, ranging from modest retreat to almost full collapse of the main trunk within a century [11,7].Here, we evaluate the potential instability of PIG and its short-term contribution to sea-level rise (SLR) using state-of-the-art ice flow models. To decide whether PIG is subject to Marine Ice Sheet Instability (MISI) at present, we must answer two questions: (i) to what extent is the dynamic response of PIG to changes in its ice shelf dictated by the bedrock topography rather than the type and amplitude of the perturbation, and, (ii) can the GL be stabilized on the retrograde slope? Confidence in the answers we propose is of course affected by the accuracy of both the physics implemented in the models that we use and our estimates of poorly constrained parameters. We addressed these questions using three different ice-flow models: the full Stokes For all three models, the geometry is relaxed over 15 years to remove unphysical surface undulations induced by remaining uncertainties in the model initial conditions [6]. Surface accumu-3 lation is given by the regional atmospheric model RACMO (1980RACMO ( -2004) and sub-ice shelf melting is imposed as a piecewise linear function of the lower surface elevationwater depth with a maximum melting rate of 100 m a −1 below -800 m depth, linearly decreasing to no melt above -400 m. This melt-rate parametrisation, which we will refer to as m0 control, is in reason- The recent retreat of PIG is now firmly attributed to acceleration of the glacier in response to sub-ice shelf melting. To evaluate the consequences of melting on PIG dynamics, fivefour different melt-rate perturbations are tested. These ar...

ABSTRACT. This paper discusses results from the second phase of the European Ice Sheet Modelling Initiative (EISMINT). It reports the intercomparison of ten operational ice-sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour. A schematic, circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate. The major finding is that the radial symmetry implied in the experimental design can, under certain circumstances, break down with the formation of distinct, regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt.These features also manifest themselves in the thickness and velocity distributions predicted by the models. They appear to be a common feature to all of the models which took part in the intercomparison, and may stem from interactions between ice temperature, flow and surface form. The exact nature of these features varies between models, and their existence appears to be controlled by the overall thermal regime of the ice sheet. A second result is that there is considerable agreement between the models in their predictions of global-scale response to imposed climate change.

Fluid-sediment interactions control river channel forms and processes. Analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour. In this paper, the hydraulic processes active in a natural pool-riffle sequence are simulated using a three-dimensional computational fluid dynamics (CFD) model. Methods employed for the prescription of model boundary conditions are outlined. Model calculations are assessed using comparisons with field observations acquired over a range of flows. Simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near-bankfull and an intermediate flow event. Results are used to assess existing theories that seek to explain the development and maintenance of pool-riffle sequences. Simulated results suggest that near-bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases. Model simulations indicate that secondary flow acts to route near-bed flow over the downstream side of riffles and into the poolhead away from the centre of pools. Implications for sediment transport and pool maintenance are discussed.

[1] Predicting ice sheet mass balance is challenging because of the complex flow of ice streams. To address this issue, we have coupled a three-dimensional higher-order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress. The model was tested for its sensitivity to regional water availability. First, with an assumed undrained bed, the ice stream oscillates between active and stagnant phases, solely as a result of thermodynamic feedbacks occurring at the ice-till interface. However, the velocity amplitude decreases over time, as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing "ice sheet mode." Second, we assume that the till is able to assimilate water from a hypothetical regional hydrological system. This leads to significantly different long-term behavior, as a continuously oscillating "ice stream mode" is maintained. The extra water incorporated in the till leads to higher velocities, triggering stronger thermodynamic feedbacks between the ice and till layer. Results also suggest that fast-flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase. Similarly, till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase. Our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system, underscoring the need to accurately describe the coupling between ice dynamics, basal conditions and regional subglacial hydrology in ice sheet models.

Subglacial hydrology in East Antarctica is poorly understood, yet may be critical to the manner in which ice flows. Data from a new regional airborne geophysical survey (ICECAP) have transformed our understanding of the topography and glaciology associated with the 287,000 km2 Aurora Subglacial Basin in East Antarctica. Using these data, in conjunction with numerical ice sheet modeling, we present a suite of analyses that demonstrate the potential of the 1000 km‐long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin. We present results from our analysis of basal topography, bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures. Although no clear‐cut subglacial lakes are found within the Aurora Basin itself, dozens of lake‐like reflectors are observed that, in conjunction with other results reported here, support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the Dome C ice divide to reach the coast via the Totten Glacier.