The effect of the North-East Greenland Ice Stream (NEGIS) on the Greenland ice sheet in changing climates

The North-East Greenland Ice Stream (NEGIS) was discovered as a large fast-flow feature in north-east Greenland by synthetic aperture radar (SAR) imaginary of the ERS-1 satellite. In this study, the NEGIS is implemented in the ice-sheet model SICOPOLIS (Simulation Code for POLythermal Ice Sheets), which simulates the large-scale dynamics and thermodynamics of an ice sheet three-dimensionally and over time. In the first part of my study, paleoclimatic simulations are carried out. We simulate the evolution of the ice sheet on a 20-km grid for the period from 250 ka BP until today. Spin-up simulations are from 422 ka BP until 250 ka BP, and the time step for all model components is 5 a. It is driven by a climatology reconstructed from a combination of present-day observations and GCM results for the past. In the 20-km resolution runs, we employ two different types of simulations, one is variation of the basal sliding coefficient in the Weertman sliding law for the NEGIS area, and the other is consideration of the effect of surface melt-water to basal sliding at NEGIS. We assume that the NEGIS area is characterized by enhanced basal sliding compared to the "normal", slowly-flowing areas of the ice sheet, and optimize the basal sliding law in the NEGIS area by minimizing the misfit between simulated and observed surface elevations and velocities. Tuning the basal sliding coefficient shows a very clear effect on the NEGIS, and by comparison with velocity data, provides a reasonable value of the coefficient. On the other hand, the surface melt-water affects only the ice margin, and therefore cannot explain the onset of the NEGIS as far upstream as observed. In the second step, 10-km resolution simulations are carried out with a modified domain map based on Fahnestock et al. (1993) in order to have a more detailed representation of the NEGIS. The model time is from 127 ka BP until today without spin-up simulations, and the time step for all model components is 1 a. Like for the 20-km simulations, fast sliding in the NEGIS area is implemented by an enhanced basal sliding coefficient. By comparison with date for the velocity and the ice thickness, it is concluded that an enhancement by a factor three gives the best results. Then, the role of the NEGIS for the possible decay of the Greenland ice sheet in future warming climates is investigated. Recent observations of accelerated ice flow in Greenland indicate that surface melt-water percolating to the base may play a crucial role in provoking a fast reaction of ice-sheet flow on increased surface temperatures. By employing a simple, plausible parameterization of this process for the NEGIS area, we simulate the evolution of the ice sheet over the next centuries in response to standard greenhouse-gas-emission scenarios, and look into the question whether an accelerating NEGIS can affect the stability of the ice sheet as a whole. For future warming scenarios, the effect of the NEGIS (the best basal sliding coefficient) and the variation of the surface meltwater coefficient are investigated. Moreover, we model the surface meltwater effect on the entire ice sheet with the same settings of the basal sliding coefficient of the NEGIS and the climate conditions. From the result of the runs, we find that the surface meltwater effect (when limited to the NEGIS) on the total volume of ice sheet is quite small, while the variation of global warming scenarios affect the ice volume strongly. However, the surface meltwater effect is pronounced when applied to the entire ice sheet and very high values for the surface meltwater coefficient are assumed. It is obvious that ice melts strongly in large areas especially in southern Greenland where temperatures are generally higher.