1Arctic Centre, University of Lapland, Rovaniemi, Finland
2Laboratoire de Glaciologie et Géophysique de l'Environment (LGGE), UMR5183, UJF-Grenoble 1, CNRS, Grenoble, France
3Department of Earth Sciences, Air, Water and Landscape science, Uppsala University, Uppsala, Sweden
4Gamma Remote Sensing and Consulting AG, Gümligen, Switzerland
5CSC – IT Center for Science Ltd., Espoo, Finland
Abstract. The dynamic regime of Svalbard's Nordaustlandet ice caps is dominated by fast flowing outlet glaciers, making assessment of their response to climate change challenging. A key element of the challenge lies in the fact that the motion of fast outlet glaciers occurs largely through basal sliding, and is governed by physical processes at the glacier bed, processes that are difficult both to observe and to simulate. Up to now, most of the sliding laws used in ice flow models were based on uniform parameters with a condition on temperature to identify regions of basal sliding. However these models are usually not able to reproduce observed velocities with sufficient accuracy. With the development of inverse methods, it is now common to infer a spatially varying field of sliding parameters from surface ice-velocity observations. These parameter distributions usually reflect a high spatial variability and represent valuable information to understand and test various hypotheses on physical processes involved in sliding. However, in these models, basal sliding is uncoupled from the thermal regime of basal ice and the evolution of the sliding parameters in prognostic simulations remains problematic.
Here we explore the role of different heat sources (friction heating, strain heating and latent heat through percolation of melt water) on the development of sliding and fast flow through thermomechanical coupling on Nordaustlandet outlet glaciers. We focus on Vestfonna with a special emphasis on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate between 1995 and 2008 and possibly already prior to 1995. We try to reconcile the impacts of temperature and heat sources on basal friction coefficients inferred from observed surface velocities during these two periods.
Our simulations reproduce a temperature profile from borehole measurements, allowing an interpretation of the vertical temperature structure in terms of temporal evolution of climate. We identify firn heating as a crucial heat source to explain Vestfonna's temperature distribution, especially in the thick areas in the center. However, friction heating is the dominant heat source at the bed of fast flowing outlet glaciers. Our findings do not support a purely temperature dependent sliding law for Vestfonna, implying that hydrology and/or sediment physics need to be represented in order to simulate fast flowing outlet glaciers.