An Investigation of Controls on Ice Stream Flow Using Models Constrained by Satellite and Ground Observations
This investigation's major goal is to develop and use models constrained by satellite and ground observations to study the controls on fast ice stream flow. In keeping with this goal, much of the early effort was focused on applying control-method inversions to solve for the basal shear stress beneath the eight major ice streams feeding the Filchner-Ronne Ice Shelf [Joughin et al, 2006]. These inversions indicate that a weak bed (~4 to 20 kPa) underlies large parts of these ice streams. Compared to the Ross Ice Streams, however, the distribution of this weak subglacial till is more heterogeneous, with "sticky spots" providing much of the resistance to flow. A weak bed beneath Recovery Ice Stream extends several hundred kilometres inland with flow. Along this ice stream, discrepancies between thickness measurements and flux estimates suggest the existence of a deep (-1400 m) trough not resolved by existing maps of subglacial topography.
Later efforts concentrated on determining basal shear stress and melt for rapidly thinning Pine Island and Thwaites glaciers, located along Amundsen Coast [Joughin et al., 2009]. The results indicate strong basal melting in areas above the grounding lines of both glaciers where there are fast speeds and high basal shear stresses. Farther inland, both glaciers have mixed bed conditions with apparent extensive areas of both crystalline bed rock and weak till. In particular, there are weak areas of till along much of its main trunk that could prove unstable if Pine Island Glacier retreats past the band of strong bed just above its current grounding line. In agreement with earlier studies, our model shows a strong sensitivity to small perturbations in the grounding line position. These results also reveal a strong sensitivity to the assumed sliding model, with non-linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear-viscous till. Finally, our results using a plastic-bed model are at least compatible with the limited observational constraints, which is consistent with recent theoretical work that shows basal shear stress cannot grow without bound. We are now in the process of using these results to constrain a time dependent model for Pine Island Glacier.