Advancing Ocean Biogeochemistry in the ICON-XPP Earth System Model for CMIP7 Contribution

We present an evaluation of ocean biogeochemistry simulations in a newly developed Earth System model configuration: ICON XPP. ICON XPP is an outcome of a national effort to develop an Earth system model configuration within the ICON architecture, serving as a baseline for next-generation climate predictions and projections. This model will underpin the German contribution to CMIP7. 

The ocean biogeochemistry in ICON XPP is represented by the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013), with novel developments over its predecessor in the MPI-ESM CMIP6 version. HAMOCC has already been implemented in the previous configurations of the ICON-based models (Jungclaus et al. 2022; Hohenegger et al. 2023). Recent key advancements include integrating a prognostic calculation for marine aggregate sinking speeds (Maerz et al. 2020), providing an improved distribution of particulate organic carbon fluxes critical to the biological pump. Additionally, the model incorporates an enhanced nitrogen cycle, enabling a more comprehensive representation of nutrient dynamics and N2O fluxes.

Here we discuss the HAMOCC model performance within the higher-resolution ICON XPP configuration, capturing finer-scale oceanographic processes. The framework features horizontal grid spacing of approximately 80km with 130 vertical levels in the atmosphere, and 20km with 72 vertical levels for the ocean, running in concentration-driven mode. We detail the ocean biogeochemistry model tuning steps and show that the simulated present-day distributions of biogeochemical fields compare well with observations, demonstrating the robustness of the ICON XPP configuration for next-generation Earth system modeling.