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Coupling between the Antarctic ice flow, subglacial regimes and regional climate conditions


Bernales,  Jorge
1.3 Earth System Modelling, 1.0 Geodesy, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Bernales, J. (2018): Coupling between the Antarctic ice flow, subglacial regimes and regional climate conditions, PhD Thesis, Berlin : Freie Universität, xxxiv, 100 Seiten, Seite A1-A8 p.
URN: http://nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:kobv:188-fudissthesis000000106198-8

Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_2911894
The Antarctic ice sheet is part of an intricate feedback system that includes the solid Earth, the atmosphere, and the ocean. A deep understanding of the interactions between these sub-systems would path the way for improved reconstructions of the Antarctic ice dynamics during past periods, particularly when global climate conditions were similar to those expected in the upcoming centuries. However, the acquisition of observational data needed to strip some of the key ice sheet processes, such as basal ice sliding modulated by the presence of water and soft earth materials, has proven to be difficult due to the particular remoteness and harsh climate conditions of the Antarctic continent. This thesis interconnects three scientific papers to demonstrate that uncertainties in subglacial regimes may explain commonly large discrepancies between the model-based and observed dynamical states of the present-day Antarctic ice sheet and that model-based reconstructions of these regimes can be used to reveal biases in the external forcing. Until now, most of ice sheet modelling studies have either relied on simplified representations of basal sliding that assume homogeneous bedrock conditions or employed inferences from previous studies as boundary conditions. Using an automated calibration technique of a continental-scale ice-sheet model, the first scientific paper within this thesis deciphers subglacial sliding regimes under the Antarctic ice sheet and shows that they are likely highly heterogeneous across the Antarctic continent. They also appear sensitive to differences in model formulations implying that a direct transfer of such reconstructions from a different model is ill-fated. Ice shelves respond strongly to the thermal regime of the Southern Ocean that modulates iceberg calving and sub-shelf melting, with the latter being the largest source of ice loss from the Antarctic ice sheet at present. Thus, an accurate representation of ice-shelf basal melting regimes is key to realistic modelling of the Antarctic ice sheet. The second scientific paper uses a combination of an ice sheet model and observations to derive the spatial distribution of melting and freezing rates at the base of the entire Antarctic ice shelf system. This novel technique captures the complexity of the observation-based basal mass balance of ice shelves well, including high ice-shelf melt rates near grounding lines and along the East Antarctic coasts and extensive accretion zones under the largest ice shelves. These estimates appear largely insensitive to uncertainties in the internal model parameters, but respond strongly to changes in the model grid resolution and external forcing. In particular, this study demonstrates that the basal mass balance of ice shelves required by ice sheet models is similar to that inferred from observational studies, and far from the values suggested by commonly utilised parameterisations. High sensitivity of the reconstructed subglacial regimes of the Antarctic ice sheet to external forcing has motivated the third and last scientific paper included in this thesis. Using ensemble simulations driven by a wide range of atmospheric data sets, this study shows that large biases in the atmospheric forcing can be erroneously compensated through a calibration of highly uncertain ice-sheet model parameters. This error cancellation is difficult to detect if the modelled ice sheet state is analysed in terms of the resulting ice sheet volume and ice distribution only. However, an inclusion of the observed surface flow velocity and ice shelf basal regimes in the validation procedure helps unfolding biases in the climate model outputs. This thesis reveals that a complementary use of the reconstructed subglacial sliding conditions and sub-shelf melting rates can significantly reduce the discrepancies between the modelled and observed ice geometries and flow patterns over large tracts of the grounded and floating Antarctic ice sheet sectors. Such model-based reconstructions contain large-scale features that are rarely taken into account by modelling experiments of the Antarctic ice sheet, demonstrating that the complexity of the subglacial regimes required by ice sheet models is similar to observed, and far from the inferences of commonly utilised parameterisations. Furthermore, these subglacial regimes can complement observational data sets during the evaluation of climate model reconstructions across Antarctica which traditionally provide external forcing to numerical simulations of future ice sheet changes.