Future projections of global mean sea level change are uncertain, partly because of our limited understanding of the dynamics of Greenland’s outlet glaciers. Here we study Nioghalvfjerdsbræ, an outlet glacier of the Northeast Greenland Ice Stream that holds 1.1 m sea-level equivalent of ice. We use GPS observations and numerical modelling to investigate the role of tides as well as the elastic contribution to glacier flow. We find that ocean tides alter the basal lubrication of the glacier up to 10 km inland of the grounding line, and that their influence is best described by a viscoelastic rather than a viscous model. Further inland, sliding is the dominant mechanism of fast glacier motion, and the ice flow induces persistent elastic strain. We conclude that elastic deformation plays a role in glacier flow, particularly in areas of steep topographic changes and fast ice velocities.
Abstract. Ice crystals are mechanically and dielectrically anisotropic. They progressively align under cumulative deformation, forming an ice crystal orientation fabric that, in turn, impacts ice deformation. However, almost all the observations of fabric are from ice core analysis and its interplay with the flow is unclear. Here, we present a non-linear inverse approach that combines radar polarimetry with vertical changes in anisotropic reflection to extract, for the first time, the full orientation tensor. The orientation tensor is routinely used to synthesize fabric information and it is used in anisotropic ice flow models. We validate our approach at two Antarctic ice-core sites (EDC and EDML) in contrasting flow regimes. Spatial variability of ice-fabric characteristics in the dome-to-flank transition near Dome C is quantified with 20 more sites located along a 36 km long cross-section. Local horizontal anisotropy increases under the dome summit and decreases away from the dome summit. We suggest that this is a consequence of the non-linear rheology of ice also known as Raymond effect. On larger spatial scales, horizontal anisotropy increases with increasing distance from the dome. At most of the sites, the main driver of ice-fabric evolution is vertical compression, yet our data show that ice fabric horizontal distribution is consistent with the present horizontal flow. Our method, which uses co- and cross polarimetric radar data suitable for profiling radar applications, can constrain ice-fabric distribution on a spatial scale comparable to ice flow observations and models.
This study investigates seasonal ice dynamics of Nioghalvfjerdsfjorden or 79 • N Glacier, one of the major outlet glaciers of the North East Greenland Ice Stream. Based on remote sensing data and in-situ GPS measurements we show that surface melt water is quickly routed to the ice-bed interface with a direct response on ice velocities measured at the surface. From the temporally highly resolved GPS time series we found summer peak velocities of up to 22% faster than their winter baseline. These average out to 9% above winter velocities when relying on temporally lower resolved velocity estimates from TerraSAR-X intensity offset tracking. From our GPS time series we also found short term ice acceleration after the melt season. By utilizing optical satellite imagery and interferometrically derived digital elevation models we were able to link the post melt season speed-up to a rapid lake drainage event (<24 h) with an estimated drainage volume of 28×10 6 m 3 . We further highlight that GPS measurements are needed to resolve short term velocity fluctuations with low amplitudes, whereas remote sensing estimates are rather useful for the calculation of general trends in velocity behavior.
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