HR: 0800h
AN: C51A-0091    [Abstracts]
TI: Spontaneous generation of pure ice-streams via flow instability: Role of longitudinal shear stresses and subglacial till
AU: * Sayag, R
EM: sayag@fas.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences and school of Engineering, Cambridge, MA 02138, United States
AU: Tziperman, E
EM: eli@eps.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences and school of Engineering, Cambridge, MA 02138, United States
AB: A significant portions of the ice discharge in ice sheets is drained through ice streams, with subglacial sediment (till) acting as a lubricant. The known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses. The effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice-till model in two horizontal dimensions. A power law-viscous constitutive relation is used, explicitly including longitudinal shear stresses. The till, which has compressible-viscous rheology, affects the ice flow through bottom friction. We examine the possibility that pure ice streams develop via a spontaneous instability of ice flow. We demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters, and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams. Instabilities occur due to basal friction and melt water production at the ice-till interface. The most unstable wavelength arise due to selective dissipation of both short and long perturbation scales. Longitudinal stresses stabilize modes with short transverse wavelengths while a physical process that can be described as a gravitationally driven flow divergence feedback stabilizes long transverse wavelengths. These results do not change qualitatively for a Newtonian ice flow law, indicating no significant role to shear thinning, although this may very well be due to the restrictive assumptions of the model and analysis.
DE: 0726 Ice sheets
DE: 0730 Ice streams
DE: 0774 Dynamics
DE: 3215 Instability analysis
DE: 4445 Nonlinear differential equations
SC: Cryosphere [C]
MN: 2007 Fall Meeting