By the nature of the computational effort imposed by solving the Stokes equations in connection with the strong shear thinning viscosity of ice, the shallow ice approximation (SIA) and shallow shelf approximation (SSA) as well as a combination of both are the common choice for ice-sheet simulations exceeding the century scale. This comes with the caveat that they are of limited accuracy for certain parts of an ice sheet, which would rise motivation for the deployment of full-Stokes (FS) computations coupled to these approximations over such regions. In this new article the authors report on a novel way of iteratively coupling FS and SSA that has been implemented in Elmer/Ice and applied to conceptual marine ice sheets. Applied to MISMIP type of experiments, the FS–SSA coupling appears to be very accurate; the relative error in velocity compared to FS is below 0.5% for diagnostic runs and below 5% for prognostic runs. Results for grounding line dynamics obtained with the FS–SSA coupling are similar to those obtained from an FS model in an experiment with a periodical temperature forcing over 3000 years that induces grounding line advance and retreat. The rapid convergence of the FS–SSA coupling shows a large potential for reducing computation time, such that modelling a marine ice sheet for thousands of years should could become feasible. Despite inefficient matrix assembly in the current implementation, computation time is reduced by 32% in a 3-D ice shelf setup.
van Dongen, E. C. H., Kirchner, N., van Gijzen, M. B., van de Wal, R. S. W., Zwinger, T., Cheng, G., Lötstedt, P., and von Sydow, L., 2018. Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3). Geosci. Model Dev., 11, 4563-4576. doi:10.5194/gmd-11-4563-2018