Geosci. Model Dev., 19, 543-578, 2026
https://doi.org/10.5194/gmd-19-543-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
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Geoscientific & ( G -
Model Development 8 EGU
Evaluation of coupled and uncoupled ocean-ice-atmosphere
simulations using icon-2024.07 and NEMOv4.2.0
for the EURO-CORDEX domain
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Vera Maurer‘, Wibke Düsterhöft-Wriggers”, Rebekka Beddig?, Janna Meyer”, Claudia Hinrichs’,
Ha Thi Minh Ho-Hagemann?, Joanna Staneva?, Birte-Marie Ehlers”, and Frank Janssen”
!Deutscher Wetterdienst, Offenbach, Germany
’Bundesamt für Seeschifffahrt und Hydrographie, Hamburg/Rostock, Germany
‘Institute of Coastal Research, Helmholtz-Zentrum Hereon, Geesthacht, Germany
Correspondence: Vera Maurer (vera.maurer@dwd.de)
Received: 15 July 2025 — Discussion started: 8 September 2025
Revised: 5 December 2025 — Accepted: 17 December 2025 — Published: 15 January 2026
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 com-
ponents (ICON-CLM and NEMO-NBS, respectively) are
analyzed and compared with various observational datasets.
ROAM-NBS complements atmosphere-only climate projec-
ons 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 (SS'T) bias is, on av-
erage, about + 0.5 K. Differences in fluxes and precipitation
over the ocean between the coupled and uncoupled simu-
lations are largely related to SST differences. However, the
nean influence on the land areas is negligible. The evalua-
tions of ocean varlables indicate a strong agreement between
ROAM-NBS and NEMO-NBS. Compared to observations,
both simulations overestimate sea ice concentration and ex-
cent. 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 ma-
rine heat wave (MHW) evaluation against observations in the
North and Baltic Sea demonstrates that the simulations cap-
ture 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 pro-
jections 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.
1
Introduction
In climate modeling and research, the results of re-
gional coupled or ocean models are still underrepresented
compared to global ones. While the regional down-
scaling of the atmosphere is coordinated through the
CORDEX initiative (Giorgi et al., 2009), data from re-
gional ocean models remain sparse. Only in 2025, the
CORDEX "Task Force on Regional Ocean Climate Pro-
jections (https://cordex.org/strategic-activities/taskforces/
task-force-on-regional-ocean-climate-projections; last
access: 25 September 2025) was established. Since stan-
Published by Copernicus Publications on behalf of the European Geosciences Unian.
