MECHANISMS OF FAST-FLOW IN JAKOBSHAVNS-ISBRAE, WEST GREENLAND .2. MODELING OF ENGLACIAL TEMPERATURES

A model for the calculation of two-dimensional temperature fields is d escribed and applied along the central flowline of Jakobshavns Isbrae, West Greenland, and along a flowline through the adjacent ice sheet. The model calculates the velocity-depth distribution based on Glen's f low law and subject to the condition that the calculated velocities ag ree with. the measured surface velocity and the estimated sliding velo city. The model allows for two-dimensional conduction and advection, f or deformational energy dissipation and for the development of a basal layer of temperate ice. The results of modeling are compared to the e nglacial 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 tempera tures, the shape of the temperature depth profiles is quite different. We attribute this difference in shape to a characteristic three-dimen sional ice deformation taking place in the convergent sub-surface chan nel of the actual ice stream. The model does not account for this thre e-dimensional effect. Adjustment of the modeled central temperature pr ofile, so that its shape matches that of the measured profile, leads t o 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 abou t 230 m. Such a thickening of the temperate basal layer by three-dimen sional ice deformation may be an important mechanism of fast ice-strea m flow.