Marine ice-sheet experiments with the Community Ice Sheet Model using the MISMIP+ experimental framework.

Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law, and to the treatment of stresses and melt rates near the grounding line. We present the effects of these numerical choices on marine ice-sheet dynamics in the Community Ice Sheet Model (CISM). In the experimental framework of the Marine Ice Sheet Model Intercomparison Project (MISMIP+), we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice-sheet geometry, forcing, or model numerics. In the MISMIP+ framework, the ice flow is also sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of ∼2 km (arguably 4 km) when the connectivity is low or moderate, and ∼1 km (arguably 2 km) when the connectivity is strong.