biogeodyn-MITgcmIS (v1): a biogeodynamical tool for exploratory climate modelling

Abstract. Modelling the climate system is challenging when slow-response components, such as the deep ocean, vegetation and ice sheets, must be evolved alongside fast-response ones. This is crucial for investigating, for example, climate tipping elements and their interactions on the global spatial scale over multimillennia timescales. While Earth system models, such as those used in the Coupled Model Intercomparison Project (CMIP), are too computational expensive for simulations spanning thousands of years, simplified parameterizations and coarse resolutions in Earth Models of Intermediate Complexity (EMICs) can significantly affect the nonlinear interactions among climate components. Here, we describe a new tool, biogeodyn-MITgcmIS, which has a complexity level intermediate between EMICs and CMIP-class models. The core of biogeodyn-MITgcmIS is a coupled MITgcm setup that includes atmosphere, ocean, thermodynamic sea ice, and land modules. To this, we have added offline couplings with a vegetation model (BIOME4), a hydrological model (pysheds), and a new global-scale ice sheet model (MITgcmIS). The latter is implemented on the same cubed-sphere grid as MITgcm, using the shallow-ice approximation, as well as MITgcm outputs and a modified Positive Degree Day method to estimate the ice-sheet surface mass balance. Here, we describe in detail the new ice sheet model and the coupling procedure. We evaluate biogeodyn-MITgcmIS using simulations for the pre-industrial period and the 1979–2009 period. These two experiments allow us to assess the model's performance against CMIP-class models, as well as a combination of reanalyses and observations. biogeodyn-MITgcmIS successfully reproduces the large-scale climate and its major components, with results comparable to those of two CMIP models with dynamical vegetation. We discuss its potential applications and future developments.