SISIPHUS: Scalable Ice-Sheet Solvers and Infrastructure for Petascale, High-Resolution, Unstructured Simulations

SISIPHUS is a model and software infrastructure for climate simulations related to the dynamics of ice sheets -- glaciers that straddle
land and sea as ground ice slides into the ocean. Understanding the dynamics of ice sheets is crucial for accurate predictions of long-term
sea levels changes, as the melt of the ice near its grounding line is expected to be the main contributor to the rise of global sea levels.

The dynamics of ice sheets unfolds on the timescale of decades or even centuries, but at the same time, can undergo sudden changes
when the glaciers protruding from land into the ocean disintegrate in a matter of weeks. These sudden changes are conjectured
to be related to instabilities in the ice sheets associated with the geometry of their grounding lines. Thus, accurate predictions of the future
state of ice sheets and their contribution to the rise of global sea levels necessitates long-term simulations
of the ice sheet dynamics, calculations of its (quasi)stationary states and their bifurcations.

Ice sheet models describe a component of the global climate system and are coupled to other models describing the atmosphere and the ocean.
These other models evolve on much smaller timescales making the global system very stiff because of the presence of rapidly-moving gravity waves
in the ocean and other fast modes. Thus, in order to be able to span decade- and century-long scales of ice sheet evolution, we propose an implicit
formulation of ice sheet dynamics, which allows us to side-step the prohibitively short stiff time scales and still to yield accurate long-term predictions.
Rapid changes in the state of ice sheets can be identified via a stability analysis of their quasi-stationary states and the attendant bifurcations.

Nonlinear Stokesian model of ice sheets

Ice sheets are modeled as an incompressible Stokesian fluid with a nonlinear power law rheology (rendering it non-Newtonian) under a free-surface
evolution, constrained by the nonlinear slip boundary condition at the bedrock boundary, as well as by the atmosphere and ocean models. [Details of the model to follow.]

Scalable software infrastructure

The simulation infrastructure is based upon scalable linear and nonlinear solvers encapsulated by the PETSc library
(Portable Extensible Toolkit for Scientific Computation) and the ITAPS collection of services ​https://trac.mcs.anl.gov/projects/ITAPS,
delivering scalable meshing, mesh (re)distribution and mesh data coupling services.
The interrelationship between the different components of the SISIPHUS simulation infrastructure are illustrated in Figure 1 below

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