Description and validation of the ice-sheet model Nix v1.0

Abstract

We present a physical description of the ice-sheet model Nix v1.0, an open-source project intended for collaborative development. Nix is a two-dimensional (flowline combined with a vertical dimension) thermomechanical model written in C and C++ that simultaneously solves for the momentum balance equations, mass conservation and temperature evolution. Nix's velocity solver includes a hierarchy of Stokes approximations: Blatter–Pattyn, depth-integrated higher order and shallow shelf. The grounding-line position is explicitly solved by a moving coordinate system that avoids further interpolations. The model can be easily forced with any external boundary conditions. Nix has been verified for standard test problems, showing versatility from regular machines (lightweight memory allocation) to high-performance computing (multi-threading capabilities). Resolutions below 0.1 km are attainable even with minimal computational resources: Nix's serial run finalizes within hours on a single CPU. Here we show results for a number of benchmark experiments from the Marine Ice Sheet Intercomparison Project (MISMIP) and assess grounding-line migration with an overdeepened bed geometry. Lastly, we further exploit the thermomechanical coupling by designing a suite of experiments where the forcing is a physical variable, unlike previously idealized forcing scenarios where ice temperatures are implicitly fixed via an ice rate factor. That is, we use atmospheric temperature and oceanic temperature anomalies to assess model hysteresis behaviour with active thermodynamics. Our results show that hysteresis in an overdeepened bed geometry is similar for atmospheric and oceanic forcings. Notably, the classical hysteresis loop is widened for both forcing scenarios (i.e. atmospheric and oceanic) if the ice sheet is thermomechanically active as a result of the internal feedback among ice temperature, stress balance and viscosity. These results show that a temperature-dependent ice viscosity provides inertia and stability to the ice sheet, regardless of the particular external forcing applied. In summary, Nix combines rapid computational capabilities with a Blatter–Pattyn stress balance fully coupled to a thermomechanical solver, not only validating against established benchmarks but also offering a powerful tool for advancing our insight into ice dynamics and grounding-line stability.