EGU2020-6021
https://doi.org/10.5194/egusphere-egu2020-6021
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Impacts of initialisation of coupled ice sheet-ocean models forecasting.

Daniel Goldberg1, Paul Holland2, and Mathieu Morlighem3
Daniel Goldberg et al.
  • 1University of Edinburgh, School of GeoSciences, Edinburgh, United Kingdom of Great Britain and Northern Ireland (dan.goldberg@ed.ac.uk)
  • 2British Antarctic Survey
  • 3Earth System Science, University of California-Irvine

In recent years, there have been great advances in coupled ice sheet-ocean modelling, to the point where ice-ocean interactions can be represented in global climate models — with potential to greatly improve forecasting of marine ice-sheet loss and sea level rise in the coming century and beyond. However, initialisation of coupled ice sheet-ocean models has not yet been properly examined; and initialisation approaches applied to ocean and coupled atmosphere-ocean models may not be appropriate due to the long time scales inherent in dynamic ice sheets. Moreover, as ocean melt rates and ice-shelf geometry strongly influence each other, nonphysical transients in incorrectly initialised coupled ice-ocean models may persist for longer than in ice-sheet models alone.

In this work, two approaches to coupled initialisation are considered using a synchronously coupled ice-ocean model. The two approaches are based on two commonly used approaches to ice sheet model initialisation: “snapshot” calibration, where ice-sheet basal and internal parameters are configured to optimise fit with observed surface velocity; and “transient” calibration, where these parameters are configured to jointly optimise fit with velocity and geometry change; however, the transient calibration makes use of the ocean component to ensure the ice model is not subject to “initialisation shock” from ocean melting. The approaches are applied to Smith Glacier, a small but fast-thinning glacier in West Antarctica, and the model is forced under ocean warming scenarios in multidecadal runs. Initially there is much faster retreat seen in the Snapshot-calibrated simulation, but this difference decays over several decades, and ultimately the Transiently-calibrated model sees more retreat.

The experiments further suggest that Smith Glacier is not likely to exhibit Marine Ice Sheet instability in the next century. But the methods discrepancy has strong implications for glaciers which are susceptible to this instability.

How to cite: Goldberg, D., Holland, P., and Morlighem, M.: Impacts of initialisation of coupled ice sheet-ocean models forecasting., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6021, https://doi.org/10.5194/egusphere-egu2020-6021, 2020

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Importance of cavity geometries?, Olaf Eisen, 05 May 2020

    Wondering about this statement: "need to improve models of cavity circulation" p19. To improve models of cavity circulation you need to know bathymetry (=observations), which is lacking for most ice shelves - or only very coarsly know ... how useful are improvements of models/assimilation/inversion before the cavity geometries are known to a reasonable extent? The question is basically on the sensitivity of the ice shelf component to the cavity geometry, e.g. from synthetic analyses with various cavity geometries.

    • AC1: Reply to CC1, Dan Goldberg, 05 May 2020

      Thank you Olaf this is a good question and apologies for not responding to it in the session. I think that observations of sub-ice shelf bathymetry are steadily improving even if still not there yet; 3 years ago I could not have run these simulations as there was no record, incomplete or not, of bathymetry under Crosson ice shelf that was publicly available. I would argue that aerogravity records (Millan et al, 2017, GRL; Wei et al, 2020, Cryosphere) are giving us a first-order representation of bathymetry under strongly thermally-forced ice shelves; these products should perhaps be refined through more in-depth geophysical study but we also need a better idea of how melt is sensitive to these refinements. It is possible ocean models can play this role; the paper 

      Heimbach, P., & Losch, M. (2012). Adjoint sensitivities of sub-ice-shelf melt rates to ocean circulation under the Pine Island Ice Shelf, West Antarctica. Annals of Glaciology, 53(60)

      carried out comprehensive assessments of sensitivities to boundary conditions, for instance. Knowing how sensitive melt rates are to bathymetry, and where these sensitivities are strongest, could perhaps help direct more focussed observations...?