Mechanisms of fast flow in Jakobshavns Isbræ, West Greenland: Part II. Modeling of englacial temperatures

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Abstract
A model for the calculation of two-dimensional temperature fields is described and applied along the central flowline of Jakobshavns Isbræ, West Greenland, and along a flowline through the adjacent ice sheet. The model calculates the velocity-depth distribution based on Glen’s flow law and subject to the condition that the calculated velocities agree with the measured surface velocity and the estimated sliding velocity. The model allows for two-dimensional conduction and advection, for deformational energy dissipation and for the development of a basal layer of temperate ice. The results of modeling are compared to the englacial temperatures measured in boreholes reaching a depth of 1550 m which corresponds to 60% of the total depth at the center line. While there is a good agreement of the measured and modeled minimum temperatures, the shape of the temperature—depth profiles is quite different. We attribute this difference in shape to a characteristic three-dimensional ice deformation taking place in the convergent sub-surface channel of the actual ice stream. The model does not account for this three-dimensional effect. Adjustment of the modeled central temperature profile, so that its shape matches that of the measured profile, leads to an increase of thickness of the temperate basal layer by about 30%. Hence, the predicted temperate basal layer in the ice stream is likely to be about 300 m thick while the two-dimensional model suggests about 230 m. Such a thickening of the temperate basal layer by three-dimensional ice deformation may be an important mechanism of fast ice-stream flow.