The next generation sea-ice model neXtSIM, version 2

Abstract. While many large-scale sea ice models can represent regional to global sea ice evolution, their representation of sea ice dynamics varies little between models. This is because they all use rheologies based on the hypothesis that sea ice behaves as a visco-plastic solid. This works reasonably well for several quantities (e.g. sea ice volume) but fails to capture sea ice deformation features at coarse and moderately high resolutions (i.e. coarser than about 5 km resolution). This may be problematic since these deformations result in the formation of leads and ridges, which likely play an essential role in ice-atmosphere-ocean interactions, and because these are the resolutions at which sea ice models run in coupled models such as Earth System Models. An alternative is to use brittle rheologies that better capture these features independently of the resolution. The neXtSIM model has been at the core of the effort by its developers and users to explore the usage of brittle rheologies and new modelling approaches in geophysical scale simulations of sea ice. Here, we document neXtSIM, now in version 2 of its development, to foster its use for the sea ice community and release a public version of the model. We describe the sea ice dynamics and the core of the model in detail and give insights into the parameters specific to the brittle rheologies included in neXtSIM. We also document the model's specificity associated with its Lagrangian framework and how it affects the coupling with other components of Earth system models. We hope that the insights provided in this study and the public release of the model will trigger innovative research in the sea ice modelling community.