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Thursday, August 21, 2008
1:00 PM - 2:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Seminar

Grounding line movement and ice shelf buttressing in marine ice sheets: an adaptive approach

Dan Goldberg
Courant Institute of Mathematical Sciences, New York University

The West Antarctic Ice Sheet is marine in nature, meaning most of its base is below sea level. At the sheet's margin (known as its grounding line), its outlet streams flow into large floating ice shelves, which are thought to play a large role in the dynamics of the ice sheet. Gravity applies stress to an ice shelf, and this stress is both taken up by the stiff margins of the shelf (a phenomenon known as buttressing) and transmitted back to the grounding line. The amount of stress felt at the grounding line determines flux within the area just upstream (known as the transition zone), which in turn controls the evolution of both the thickness and grounded extent of the ice sheet. In the case of a foredeepened bedrock, this can lead to the Marine Instability predicted by Weertman (1973). However, Thomas (1979) and others pointed out that this instability may be lessened by buttressing within the ice shelf.

One of the obstacles to the numerical modeling of a marine ice sheet is the difficulty in representing grounding line movement in such a model, especially when conditions are such that the transition zone of such a sheet is small compared to the overall length scale. This is due to sharp transitions in stress near the grounding line, necessitating very high grid resolution. We have developed a numerical model that makes use of grid adaption in order to provide the resolution needed near an evolving grounding line, while still being computationally tractable. The model has two modes of adaption: a moving mesh scheme and an adaptive refinement (so-called "h-adaption) scheme.

We show that our model overcomes the problems previously observed in models that move the grounding line, such as strong dependence on mesh size and on initial conditions, and that it is in good agreement with quasi-analytic results (those of the Marine Ice Sheet Intercomparison, or MISMIP). We also investigate the ability of buttressing to counter the marine instability of a sheet resting on a fore-deepened bed, and how this ability is affected by the bottom slope, the width of the shelf, and the basal strength of the sheet.

Host: William Lipscomb, T-03