Comparing biases in the earth system model ICON-ESM-ER with its predecessor MPI-ESM-ER

The resolution of climate models significantly influences their ability to simulate physical processes and reduce biases, especially in oceanic and atmospheric systems. The Eddy-Rich Earth System Models (EERIE) project focuses on developing next-generation Earth System models at kilometer-scale resolution. In this study, we compare the control simulations of one of the EERIE models, the ICOsahedral Non-hydrostatic Earth System Model (ICON-ESM-ER), with those of its eddy-rich predecessor, the Max Planck Institute Earth System Model (MPI-ESM-ER). The ICON-ESM-ER features a 5 km ocean resolution coupled with a 10 km atmospheric resolution, while the MPI-ESM-ER employs a 10 km ocean resolution and a 100 km atmospheric resolution. Additionally, the ICON-ESM-ER uses an unstructured icosahedral grid, whereas the MPI-ESM-ER is based on a tripolar curvilinear grid. As models gradually move to finer spatial resolution, we naturally expect to improve simulations of atmospheric and oceanic flows. However, things become particularly interesting when new thresholds are crossed, as it enables the explicit simulation of previously unresolved phenomena. This can also introduce new complexities and challenges. The analysis reveals distinct differences in biases between the two models. For instance, focusing on the Southern Ocean, ICON-ESM-ER exhibits overall warmer biases than its predecessor MPI-ESM-ER and shows very large positive dynamic sea level biases. Additionally, ICON-ESM-ER produces large positive zonal surface wind biases in this region. On a more positive note, the sea surface salinity biases in the South Atlantic and Indian Ocean are negligible in ICON-ESM-ER. The ICON-ESM-ER does not outperform MPI-ESM-ER and, in some cases, introduces larger biases in key climate variables. Understanding these biases, particularly in comparison to its predecessor, is essential to guide future model development and improve the representation of critical processes in the Earth system.