Evaluation of coupled and uncoupled ocean–ice–atmosphere simulations using icon-2024.07 and NEMOv4.2.0 for the EURO-CORDEX domain

Abstract. Evaluation results from the reanalysis-driven (ERA5/ORAS5) simulation for the years 1979–2021 with a regional coupled ocean–atmosphere model (ROAM) are presented. The coupled setup portrayed here is one of the first regional climate modeling systems to couple the ICON atmosphere model in climate limited-area mode (CLM) with the ocean model NEMO for the North and Baltic Sea (NBS), using a flux-based OASIS3-MCT coupling approach. Along with the simulation using the coupled model configuration ROAM-NBS, the simulations with the uncoupled components (ICON-CLM and NEMO-NBS, respectively) are analyzed and compared with various observational datasets. ROAM-NBS complements atmosphere-only climate projections with the same atmospheric model and setup, which will all be published in accordance with EURO-CORDEX specifications. Climate projections by ROAM-NBS will enrich the data available to support the German Strategy for Adaptation to Climate Change (DAS), especially for our target region, which are the German national waters. In general, the mean model climate is well represented by all setups. The sea surface temperature (SST) bias is, on average, about ±0.5 K. Differences in fluxes and precipitation over the ocean between the coupled and uncoupled simulations are largely related to SST differences. However, the mean influence on the land areas is negligible. The evaluations of ocean variables indicate a strong agreement between ROAM-NBS and NEMO-NBS. Compared to observations, both simulations overestimate sea ice concentration and extent. Mean temperature and salinity profiles in the Baltic Sea are generally reproduced by both simulations, with biases in the deeper layers. Major inflow events are captured but underestimated. Sea surface height and storm surge highly coincide with observational data, with NEMO-NBS slightly outperforming ROAM-NBS in terms of correlation. The marine heat wave (MHW) evaluation against observations in the North and Baltic Sea demonstrates that the simulations capture the inter-annual variability of MHW characteristics. Overall, the coupled simulation demonstrates adequate performance for both the atmosphere and the ocean, and the setup will be used to produce coupled regional climate projections for Europe. However, bias correction for the deeper Baltic layers remains necessary for further applications, and future work will focus on refining the setup for this region.