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Theses

Influence de l'endommagement et du frottement basal sur la dynamique de la ligne d'échouage

Abstract : The Antartic ice sheet represents the world's largest potential contributor to sea level rise. Over 80 % of Antarctica's grounded ice drains through its fringing ice shelves which surround close to 45 % of the continent's shore. Because of Archimède' principle, the contribution of the ice to sea level is accounted for as soon as it flows through the grounding line, which defines the limit beyond which ice grounded on the bedrock starts floating on the ocean. Therefore, realistic modelling of grounding line dynamics is crucial to produce trustworthy projections of future sea level rise. This dynamics is affected by a number of physical processes, some of which are not properly represented in current ice flow models. This PhD thesis focuses on two of these processes: damage of ice on the one hand and basal friction related to basal hydrology on the other hand.Damage accounts for the degradation of ice mechanical properties due to the presence of fractures and crevasses, commonly observed at the surface of glaciers. Damage affects ice flow by lowering ice viscosity. The evolution of damage is governed by a pure advection equation, the numerical resolution of which requires stabilisation methods. We show that, for numerical resolutions associated to acceptable calculation times, grounding line dynamics is sensitive to the choice of this method, which seriously complicates the modelling of damage processes.Ice flow models account for basal friction through the use of friction laws, i.e. the mathematical relationship between basal drag and sliding velocities. Several formulations of these laws have been proposed over the last decades based on theoretical arguments. Some of these formulations explicitly include the effect of basal water which is present in the subglacial drainage system and the pressure of which eases basal motion. Unfortunately, the temporal and spatial scales at stake in glaciology make it impossible to validate these different formulations in situ and large-scale ice flow models usually make use of the simplest one, the Weertman law. The effect of basal water pressure is then accounted for in an implicit fashion via a friction coefficient, the spatial distribution of which is inferred through the use of inverse methods. Because the temporal evolution of this coefficient is poorly constrained, it is usual to keep it stationary. This lead to an unphysical discontinuity of friction at the grounding line when the latter retreats. First of all, we show that grounding line dynamics is sensitive to the way this discontinuity is treated numerically. Then, we demonstrate on a synthetic case that the fact of explicitly accounting for the effect of basal water pressure on basal friction leads to positive feedback phenomeno which implies larger ice losses. Finally, these conclusions are extended to a real case, the Amundsen basin in West Antarctica, by showing a significant sensitivity of grounding line dynamics to the chosen friction law as well as to the values given to some of the parameters involved in the tested friction laws.
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https://tel.archives-ouvertes.fr/tel-01730867
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Submitted on : Tuesday, March 13, 2018 - 4:15:30 PM
Last modification on : Friday, September 25, 2020 - 11:00:08 AM
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Julien Brondex. Influence de l'endommagement et du frottement basal sur la dynamique de la ligne d'échouage. Sciences de la Terre. Université Grenoble Alpes, 2017. Français. ⟨NNT : 2017GREAI092⟩. ⟨tel-01730867⟩

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