Description: We introduce the coupled model of the Green- land glacial system IGLOO 1.0, including the polythermal ice sheet model SICOPOLIS (version 3.3) with hybrid dy- namics, the model of basal hydrology HYDRO and a param- eterization of submarine melt for marine-terminated outlet glaciers. The aim of this glacial system model is to gain a better understanding of the processes important for the future contribution of the Greenland ice sheet to sea level rise under future climate change scenarios. The ice sheet is initialized via a relaxation towards observed surface elevation, impos- ing the palaeo-surface temperature over the last glacial cycle. As a present-day reference, we use the 1961–1990 standard climatology derived from simulations of the regional atmo- sphere model MAR with ERA reanalysis boundary condi- tions. For the palaeo-part of the spin-up, we add the temper- ature anomaly derived from the GRIP ice core to the years 1961–1990 average surface temperature field. For our pro- jections, we apply surface temperature and surface mass bal- ance anomalies derived from RCP 4.5 and RCP 8.5 scenar- ios created by MAR with boundary conditions from simula- tions with three CMIP5 models. The hybrid ice sheet model is fully coupled with the model of basal hydrology. With this model and the MAR scenarios, we perform simulations to estimate the contribution of the Greenland ice sheet to future sea level rise until the end of the 21st and 23rd centuries. Fur- ther on, the impact of elevation–surface mass balance feed- back, introduced via the MAR data, on future sea level rise is inspected. In our projections, we found the Greenland ice sheet to contribute between 1.9 and 13.0 cm to global sea level rise until the year 2100 and between 3.5 and 76.4 cm until the year 2300, including our simulated additional sea level rise due to elevation–surface mass balance feedback. Translated into additional sea level rise, the strength of this feedback in the year 2100 varies from 0.4 to 1.7 cm, and in the year 2300 it ranges from 1.7 to 21.8 cm. Additionally, taking the Helheim and Store glaciers as examples, we inves- tigate the role of ocean warming and surface runoff change for the melting of outlet glaciers. It shows that ocean temper- ature and subglacial discharge are about equally important for the melting of the examined outlet glaciers.

Description: Pine Island Glacier is one of the fastest changing glaciers of the Antarctic Ice Sheet and therefore of scientific interest. The glacier holds enough ice to raise the global sea level significantly (~ 0.5 m) when fully melted. The question addressed by numerous modelling studies of the glacier focuses on whether the observed changes are a start of an uncontrolled and accelerating retreat. The movement of the glacier is, in the fast-flowing areas, dominated by basal motion. In modelling studies the parametrisation of the basal motion is therefore crucial. Inversion methods are commonly applied to reproduce the complex surface flow structure of Pine Island Glacier by using information of the observed surface velocity field to constrain, among other things, basal sliding. We introduce two different approaches of combining a physical parameter, the basal roughness, with basal sliding parametrisations. This way basal sliding is again connected closer to its original formulation. We show that the basal roughness is an important and helpful parameter to consider and that many features of the flow structure can be reproduced with these approaches.

Description: Subglacial hydrology plays an important role in ice sheet dynamics as it determines the sliding velocity. It also drives freshwater into the ocean, leading to undercutting of calving fronts by plumes. Modeling subglacial water has been a challenge for decades. Only recently have new approaches been developed such as representing subglacial channels and thin water sheets by separate layers of variable hydraulic conductivity. We extend this concept by modeling a confined–unconfined aquifer system (CUAS) in a single layer of an equivalent porous medium (EPM). The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms of governing the opening and closure of the system. Furthermore, we applied the model to the Northeast Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500km downstream from the ice divide. Using the effective pressure from the hydrology model, the Ice Sheet System Model (ISSM) showed considerable improvements in modeled velocities in the coastal region.

Description: Calving is a major means of ice discharge of the Antarctic and Greenland Ice Sheets. The breaking off of icebergs changes the ice front configuration of marine terminating glaciers, which affects the stress regime of their upstream areas. Recent observations show the close correlation between the ice front position and the behaviour of many outlet glaciers. However, modelling of a glacier subject to calving poses various challenges. No universal calving rate parametrisation is known, and tracking of a moving ice front and the related boundary conditions in two or three spatial dimensions is non-trivial. Here, we present the theoretical and technical framework for a Level-Set Method, an implicit boundary tracking scheme, which we implemented into the Ice Sheet System Model (ISSM). The scheme allows us to study the dynamic response of a drainage basin to user-defined front ablation rates. We apply the method in a suite of experiments to Jakobshavn Isbræ, a major marine terminating outlet glacier of the western Greenland Ice Sheet. The model robustly reproduces the high sensitivity of the glacier to frontal ablation in form of calving. We find that enhanced calving is able to trigger significant acceleration of the ice stream. Upstream acceleration is sustained through a combination of various feedback mechanisms. However, lateral stress and ice influx into the trough are able to stabilise the ice stream. This study contributes to the present discussion on causes and effects of the continued changes occurring at Jakobshavn Isbræ, and emphasises that the incorporation of seasonal calving and dynamic lateral effects is key for realistic model projections of future global sea level rise on centennial time scales.

Description: As ice flows over a subglacial lake, the drop in bed resistance leads to an increase in ice velocities and a draw down of isochrones and cold ice. The ice surface flattens as it adjusts to the lack of resisting forces at the base. The rapid transition in velocity induces changes in ice viscosity and releases deformation energy that can raise the temperature locally. Recent studies of Antarctic subglacial lakes indicate that many lakes experience very fast and possibly episodic drainage, during which the lake size is rapidly reduced as water flows out. Questions that arise are what effect this would have on internal layers within the ice and whether such past drainage events could be inferred from isochrone structures downstream. Here, we study the effect of a subglacial lake on ice dynamics as well as the influence that such short timescale drainage would have on the internal layers of the ice. To this end, we use a full Stokes, polythermal ice flow model. An enthalpy-gradient method is used to account for the evolution of temperature and water content within the ice. We find that a rapid transition between slow-moving ice outside the lake, and full sliding over the lake, can release considerable amounts of deformational energy, with the potential to form a temperate layer at depth in the transition zone. In addition, we provide an explanation for a characteristic surface feature commonly seen at the edges of subglacial lakes, a hummocky surface depression in the transition zone between little to full sliding. We also conclude that rapid changes in the horizontal extent of subglacial lakes and slippery patches, compared to the average ice column velocity, can create a traveling wave at depth within the isochrone structure that transfers downstream with the advection of ice, thus indicating the possibility of detecting past drainage events with ice penetrating radar.

Description: Full-Stokes (FS) ice sheet models provide the most sophisticated formulation of ice sheet flow. However, their applicability is often limited due to the high computational demand and numerical challenges. To balance computational demand and accuracy, the so-called Blatter–Pattyn (BP) stress regime is frequently used. Here, we explore the dynamic consequences of using simplified approaches by solving FS and the BP stress regime applied to the Northeast Greenland Ice Stream. To ensure a consistent comparison, we use one single ice sheet model to run the simulations under identical numerical conditions. A sensitivity study to the horizontal grid resolution (from 12.8 to a resolution of 0.1 km) reveals that velocity differences between the FS and BP solution emerge below ∼ 1 km horizontal resolution and continuously increase with resolution. Over the majority of the modelling domain both models reveal similar surface velocity patterns. At the grounding line of the 79∘ North Glacier the simulations show considerable differences whereby the BP model overestimates ice discharge of up to 50 % compared to FS. A sensitivity study to the friction type reveals that differences are stronger for a power-law friction than a linear friction law. Model differences are attributed to topographic variability and the basal drag, in which neglected stress terms in BP become important.