Abstract
Abstract. Understanding the response of fast flowing ice streams or outlet glaciers to changing climate is crucial in order to make reliable projections of sea level change over the coming decades. Motion of fast outlet glaciers occurs largely through basal motion governed by physical processes at the glacier bed, which are not yet fully understood. Various subglacial mechanisms have been suggested for fast flow but common to most of the suggested processes is the requirement of presence of liquid water, and thus temperate conditions. We use a combination of modelling, field, and remote observations in order to study links between different heat sources, the thermal regime and basal sliding in fast flowing areas on Vestfonna ice cap. A special emphasis lies on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate recently. We use the ice flow model Elmer/Ice including a Weertman type sliding law and a Robin inverse method to infer basal friction parameters from observed surface velocities. Firn heating, i.e. latent heat release through percolation of melt water, is included in our model; its parameterisation is calibrated with the temperature record of a deep borehole. We found that strain heating is negligible, whereas friction heating is identified as one possible trigger for the onset of fast flow. Firn heating is a significant heat source in the central thick and slow flowing area of the ice cap and the essential driver behind the ongoing fast flow in all outlets. Our findings suggest a possible scenario of the onset and maintenance of fast flow on the Vestfonna ice cap based on thermal processes and emphasise the role of latent heat released through refreezing of percolating melt water for fast flow. However, these processes cannot yet be captured in a temporally evolving sliding law. In order to simulate correctly fast flowing outlet glaciers, ice flow models not only need to account fully for all heat sources, but also need to incorporate a sliding law that is not solely based on the basal temperature, but also on hydrology and/or sediment physics.
Highlights
Recent studies suggest that an important contribution to sea level rise over the coming decades will be cryospheric mass loss in the form of discharge from fast flowing ice streams or outlet glaciers (Meier et al, 2007; Moon et al, 2012; Jacob et al, 2012; Tidewater Glacier Workshop Report, 2013)
For surface elevations we use the digital elevation model (DEM) from the Norwegian Polar Institute (NPI) (1 : 100 000, 1990, UTM zone 33N, WGS 1984), which is based on topographic maps derived from aerial photography (Fig. 1)
We present the basal friction parameter distribution obtained on the VSF ice cap for 1995, 2008 and 2011 from the inversion of observed velocity fields using Elmer/Ice
Summary
Recent studies suggest that an important contribution to sea level rise over the coming decades will be cryospheric mass loss in the form of discharge from fast flowing ice streams or outlet glaciers (Meier et al, 2007; Moon et al, 2012; Jacob et al, 2012; Tidewater Glacier Workshop Report, 2013). While many large-scale flow models use spatially uniform parameters for the friction law, but only allow sliding when ice at the bed reaches the pressure melting point (Greve, 1997; Ritz et al, 2001; Quiquet et al, 2013), we solve an inverse problem to constrain spatially varying friction law parameters by determining the best match between model and observed surface velocities This approach allows quantification of the basal sliding velocity, which can help to constrain the in situ processes (Morlighem et al, 2010; Pralong and Gudmundsson, 2011; Jay-Allemand et al, 2011; Habermann et al, 2012, 2013).
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