HR: 17:45h
AN: C44A-08    [Abstracts]
TI: Mechanical feedbacks in calving glacier retreat, thinning and acceleration.
AU: * Howat, I M
EM: ihowat@pmc.ucsc.edu
AF: Dept. of Earth Science University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Tulaczyk, S
EM: tulaczyk@pmc.ucsc.edu
AF: Dept. of Earth Science University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Joughin, I
EM: ian@apl.washington.edu
AF: APL,University of Washington, 1013 NE 40th Street, Seattle, WA 98105-6698 United States
AU: O'Neel, S
EM: shad@colorado.edu
AF: INSTAAR,University of Colorado, Campus Box 450, Boulder, CO 80309-0450 United States
AU: Scambos, T
EM: teds@icehouse.colorado.edu
AF: NSIDC,University of Colorado, Campus Box 449, Boulder, CO 80309-0449 United States
AB: Many of the world's major calving glaciers have undergone rapid retreat of the ice front over the past decade. In many cases, retreat has been accompanied by increasing surface velocities and ice thinning due to accelerating ice drainage (i.e. dynamic thinning). Since the rate of dynamic thinning can be an order of magnitude greater than that caused by increasing surface melt, rapid acceleration following ice front retreat may greatly increase the sensitivity of ice sheets and glaciers to climate change. Observed changes in the surface strain fields of several large, retreating calving glaciers suggest that retreat and acceleration may be linked through a reorganization of the glacier force budget. For a glacier flowing into a constricting fjord, resistive stress generated from shear along the valley walls may result in a driving stress significantly in excess of the basal drag. An initial ice thinning and/or retreat of the calving front due to environmental forcing may decrease the flow resistance provided by lateral shearing. To compensate for this loss, the glacier may accelerate to increase marginal shear and decrease surface slope, leading to further thinning. This dynamic thinning may in turn cause further retreat, due to lift-off at the grounding zone, leading to a cycle of acceleration and thinning. Weakening of marginal ice during acceleration, through increased shear heating and fabric development, may further reduce resistance provided by lateral drag. Thus, for a glacier with a lateral drag that is a significant portion of the driving stress, the impact of increased surface melt and calving rates on mass-balance may be greatly amplified by a mechanical feedback between thinning, retreat and acceleration. Furthermore, our data suggest that this feedback may be greatly strengthened by the geometry of outlet fjords, which tend to widen up-glacier. Due to the non-linear relationship between glacier width and lateral drag, retreat of the calving front into a wider section of a fjord may lead to a much larger decrease in resistance provided by marginal shear. Rapid loss of lateral drag may result in an increase in longitudinal tension, through down-glacier acceleration, to balance forces near the calving front. This geometric effect could magnify the mechanical feedback and may help explain the increasing rates of thinning, acceleration and retreat observed at low elevations on large outlet glaciers.
DE: 0720 Glaciers
DE: 0726 Ice sheets
DE: 0728 Ice shelves
DE: 0758 Remote sensing
DE: 0774 Dynamics
SC: Cryosphere [C]
MN: Fall Meeting 2005