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We used PISM (55, 56) to carry out regional simulations of the WAIS at a horizontal resolution of 4 km and a minimum vertical resolution of 7 m. The model applies a superposition of the shallow ice approximation (35) and the shallow shelf approximation (36, 37) of the full-Stokes stress balance (38). This hybrid scheme ensures a smooth transition between different ice sheet flow regimes and allows for stress transmission across the grounding line. A linear interpolation of the freely evolving grounding line and, accordingly, interpolated basal friction enable realistic grounding line motion also at medium or low resolution (40). Basal friction was calculated using a nonlinear Weertman-type sliding law (38) with a sliding exponent of 3/4 in combination with a Mohr-Coulomb model for plastic till (57, 58) that accounts for the effect of evolving ice thickness and the associated change in overburden pressure on the basal till (59). The till friction angle was parameterized with bed elevation, as in (60), and the basal pore water pressure was limited to a maximum fraction of 0.97 of the overburden pressure. This friction scheme ensures a continuous transition from quasi–nonslip regimes in elevated regions to the marine areas where basal resistance was low. We used a kinematic first-order calving law (61, 62), with a prescribed proportionality factor of 1017 m·s and a minimum ice thickness at the calving front of 200 m. The physically motivated calving law takes into account the eigenvalues of the horizontal strain-rate tensor, allowing for geographically confined ice shelves and dynamic calving front positions. A physical stress boundary condition was applied at the calving front to close the equations of the shallow shelf approximation (56).

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