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Modelling the transition zone of marine ice sheets.

Nowicki, S.M.J.; (2007) Modelling the transition zone of marine ice sheets. Doctoral thesis , University of London. Green open access

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An investigation of the flow in the transition zone of marine ice sheet is con ducted, and is motivated by recent satellite observations of rapid changes in the West Antarctic Ice Sheet. This region may undergo prolonged retreat as a consequence of changes in sea level or climate. To gain insight into ice flow from a grounded ice sheet into a floating ice shelf, this thesis uses scale anal ysis and perturbation methods to solve analytically idealized ice sheet and ice shelf flows. In the transition zone, this 'shallow ice' approximation does not hold and cannot be used to simplify the governing equations. This transition zone problem does not appear to have an analytical solution, so a numeri cal analysis is necessary for gaining an understanding of the behavior of this region. The thesis presents a numerical model developed to solve the full steady-state Stokes equations on a two-dimensional domain, and to evaluate the position of the free surfaces. In the finite element model, ice density and temperature are treated as spatially homogeneous, ice rheology is considered linear, and the position of the flotation point is fixed. The hypothesis for ma rine ice sheet instability rests upon sea level controlling mass discharge at the flotation point. To test this hypothesis, we seek steady-state solutions for a range of mass fluxes, sea levels, and flow constrictions or back pressure. We demonstrate that for a given sea level, the number of steady-state solutions can be restricted by verifying that the simulations satisfy two contact inequal ities. The contact conditions reflect that for physically acceptable solutions, the compressive normal stress at the base of the grounded ice should exceed water pressure, and that the shelf surface should not get into contact with the bedrock. Violation of either conditions would result in grounding line migra tion. We find that when ice slides over the bedrock and for a given sea level, only one combination of mass flux, grounding line thickness and back pressure, satisfy the contact conditions. When ice is frozen to the bedrock, however, a range of mass fluxes, grounding line thicknesses and back pressures satisfy the contact conditions for a particular sea level. These results suggest that mass discharge at the grounding is an unique single valued function of sea level for sliding ice sheets, but this need not be the case for a frozen bed. To the extent that our simulations are applicable to real marine ice sheets, we conclude that sliding is the factor that affects their stability, and that Weertman's instability hypothesis holds when basal sliding occurs.

Type: Thesis (Doctoral)
Title: Modelling the transition zone of marine ice sheets.
Identifier: PQ ETD:593050
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by Proquest
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/1445726
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