Description and validation of the ice sheet model Nix v1.

We present a physical description of the ice-sheet model Nix v1.0, an open-source project intended for collaborative development. Nix is a 2D thermomechanical model written in C/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, shallow-shelf and shallow-ice. 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, including those of stochastic nature. Nix has been verified for standard test problems. Here we show results for a number of benchmark tests from standard intercomparison projects 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 idealised forcing scenarios where ice temperatures are implicitly fixed via an ice rate factor. Namely, we use atmospheric temperatures 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. We find that not only the particular sub-shelf melting parametrisation determines the temperature anomaly at which the ice sheet retreats, but also the particular value of calibrated heat exchange velocities. Notably, the classical hysteresis loop is narrowed for both forcing scenarios (i.e., atmospheric and oceanic) if the ice sheet is thermomechanically active as a results of the internal feedback among ice temperature, stress balance and viscosity. 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 on ice dynamics and grounding line stability.