Iceberg calving parameterisations currently implemented in ice sheet models do not reproduce the full observed range of calving behaviours. For example, though buoyant forces at the ice front are known to trigger full-depth calving events on major Greenland outlet glaciers, a multi-stage iceberg calving event at Jakobshavn Isbræ is unexplained by existing models. To explain this and similar events, we propose a notch-triggered rotation mechanism, whereby a relatively small subaerial calving event triggers a larger full-depth calving event due to the abrupt increase in buoyant load and the associated stresses generated at the ice–bed interface. We investigate the notch-triggered rotation mechanism by applying a geometric perturbation to the subaerial section of the calving front in a diagnostic flow-line model of an idealised glacier snout, using the full-Stokes, finite element method code Elmer/Ice. Different sliding laws and water pressure boundary conditions are applied at the ice–bed interface. Water pressure has a big influence on the likelihood of calving, and stress concentrations large enough to open crevasses were generated in basal ice. Significantly, the location of stress concentrations produced calving events of approximately the size observed, providing support for future application of the notch-triggered rotation mechanism in ice-sheet models.
Read more: Trevers M., A.J. Payne, S.L. Cornford and T. Moon, 2019. Buoyant forces promote tidewater glacier iceberg calving through large basal stress concentrations, The Cryosphere, 13, 1877-1887 DOI: 10.5194/tc-13-1877-2019
On the Greenland Ice Sheet (GrIS), ice flow due to deformation and sliding across the bed delivers ice to lower- elevation marginal regions where it can melt. We measured the two mechanisms of motion using a three- dimensional array of 212 tilt sensors installed within a network of boreholes drilled to the bed in the ablation zone of GrIS. Unexpectedly, sliding completely dominates ice motion all winter, despite a hard bedrock sub- strate and no concurrent surface meltwater forcing. Modeling constrained by detailed tilt observations made along the basal interface suggests that the high sliding is due to a slippery bed, where sparsely spaced bed- rock bumps provide the limited resistance to sliding. The conditions at the site are characterized as typical of ice sheet margins; thus, most ice flow near the margins of GrIS is mainly from sliding, and marginal ice fluxes are near their theoretical maximum for observed surface speeds.
Read more: Maier, N., N. Humphrey, J. Harper and T. Meierbachtol, 2019. Sliding dominates slow-flowing margin regions, Greenland Ice Sheet. Sci. Adv. 5, DOI: 10.1126/sciadv.aaw5406
The purpose of this new paper published in GMD is to assess the sub-shelf melting parameterisations that are more or less commonly used in Antartic ice-sheet modelling. In West Antarctica, the floating ice shelves fringing the continent are currently thinning, mainly because of increasing sub-ice-shelf melting coming from an oceanic origin. The consequence is less ice-shelf buttressing undergone by the upper part of the ice sheet that is thus accelerating and discharging more ice to the ocean, therefore increasing sea level rise. Representing sub-ice-shelf melting in ice-sheet models is thus crucial to future sea-level projections. This can be done by coupling your favourite ice sheet model, say Elmer/Ice, to your favourite ocean model, say NEMO, but the computational cost is likely to be too large to model the whole Antarctic ice sheet over the next century. To deal with computational cost, a solution is to parameterised the relation between oceanic properties (temperature and salinity) and sub-ice-shelf melting. So far, various parameterisations have been proposed in the litterature but they have never been compared to (i) each other and (ii) to the results of a coupled model, which is what we have done in this paper.
The melting parameterisations range from simple scalings with far-field thermal driving to emulators of box and plume models, which are more complex parameterisations. In total we have assessed 19 types of melting parameterisations, forced by 6 ocean temperature and salinity scenarios over the next century, using an ideal ice-sheet/ice-shelf system resembling the Pine Island Glacier in West Antarctica. All these simulations have been compared to a small ensemble (4 members) of the NEMO-Elmer/Ice coupled model, for which different aspects of the ocean model have been changed. Figure shows a clear difference between the melting patterns obtained from the coupled simulations in the one hand and the parameterisations in the other hand. For instance, the coupled model yields relatively more melting at the grounding line and less melting when getting closer to the calving front where the ice is thinner. It is however difficult to anticipate the different responses of the ice sheet to ocean scenarios, because we don't really know how it is affected by ice-shelf buttressing distribution. Thus, one of the best ways to evaluate the parameterisations is to use the time evolution of ice loss.
To quickly sum up the results of the study, the plume parameterisation underestimates the contribution to sea level when forced by the warming scenarios. The box parameterisation compares fairly well to the coupled results in general and gives the best results using five boxes. For simple scalings, the comparison to the coupled framework shows that a quadratic dependency to thermal forcing is required, as opposed to linear. In addition, the quadratic dependency is improved when melting depends on both local and nonlocal, i.e. averaged over the ice shelf, thermal forcing. The results of both the box and the two quadratic parameterisations fall within or close to the coupled model uncertainty.
Read More: Favier L., N.C. Jourdain, A. Jenkins, N. Merino, G. Durand, O. Gagliardini, F. Gillet-Chaulet and P. Mathiot, 2019. Assessment of sub-shelf melting parameterisations using the ocean–ice-sheet coupled model NEMO(v3.6)–Elmer/Ice(v8.3). Geosci. Model Dev., 12, 2255-2283, doi:10.5194/gmd-12-2255-2019