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Figure 3. Bias of winter (DJF) and summer (JJA) seasonal mean SST time series for NEMO-NBS and ROAM-NBS to Copernicus observa- 
tion data for the whole NEMO-NBS domain (a), Open Atlantic (b), the North Sea (c) and the Baltic Sea (d). 
which could be a hint at an underestimation of Atlantic on- 
shelf transport (Ricker and Stanev, 2020) into the North Sea. 
In Fig. 2b, the difference between the seasonal mean SST of 
ROAM-NBS and NEMO-NBS is shown. The coupled sim- 
ulation is generally slightly warmer than the uncoupled one, 
especially in summer (JJA). The tuning of the cloud cover 
scheme in ICON-CLM and ROAM-NBS reduced a positive 
surface shortwave radiation bias over land and ocean, so that 
it lies between +10 Wm”? (see Fig. Al) in the NBS re- 
gion compared to CERES data. This reduction of the radi- 
ation bias contributed to a decrease in the positive SST bias 
in summer. However, it was not possible to reduce it further 
by a tuning of the atmospheric part. Eddy diffusivity in the 
eastern Atlantic was parameterized to be an order of mag- 
nitude higher than in the western Atlantic, which may have 
contributed to the cold bias observed near the French and 
Portuguese coasts. 
Time series of the spatial mean seasonal SST biases 
against the Copernicus observations are shown for different 
regions in Fig. 3. The outlines of these regions (Baltic Sea, 
North Sea, Open Atlantic, and the whole domain) are shown 
in Fig. 1. As for the bias maps, points with ice cover in the 
observations were masked out for the calculation of the bi- 
ases. For the whole domain, the area-averaged bias is about 
-0.5K for both simulations in winter (DJF, Fig. 3a, upper 
panel). In summer (JJA, Fig. 3a, lower panel), the bias is 
slightly larger for NEMO-NBS (about —0.75 K), but smaller 
for ROAM-NBS. However, this smaller bias for ROAM-NBS 
in summer is due to the higher warm bias in the Atlantic 
https:/doi.ore/10.5194/esmd-19-543-2026 
(Fig. 3b, lower panel), combined with a negative bias in the 
Baltic Sea (Fig. 3d). The magnitudes of the biases for the 
Open Atlantic region and the North Sea are similar to those 
for the whole domain, or even smaller. In these regions dur- 
ing the summer season, ROAM-NBS is warmer than NEMO- 
NBS by about 0.3 to 0.4 K. In the Baltic Sea region, the SST 
biases in both simulations fluctuate around zero in winter 
while reaching —0.75 to —1K during summer. As the spa- 
tial averaging of biases may cancel out positive and negative 
values, the time series of the RMSE are additionally shown in 
Fig. A4. Especially in the Baltic Sea, the RMSE is, with val- 
ues of about 1.5 K in summer, higher than the absolute values 
of the mean bias. In the Open Atlantic and the North Sea, it 
is comparable to or slightly higher than the mean bias, with 
about 0.75 K in all seasons. The present SST biases in DJF 
and JJA (Fig. 3) as well as the RMSE (see Fig. A4) in all sea- 
sons and areas remain reasonably stable during the evolving 
simulation. Thus, no accumulation of errors takes place. The 
time series for the absolute area-averaged SSTs (see Fig. A4) 
demonstrate that the year-to-year variability as well as warm- 
ing trends in the North and Baltic Sea are well reproduced in 
both simulations. 
3.2 Meteorolosical conditions 
3.2.1 Surface and near-surface temperatures 
Over the ocean, the seasonal mean surface temperature dif- 
ferences between ROAM-NBS and ERA5 (Fig. 4a) mainly 
Geosci. Model Dev... 19. 543-578. 2026