**5. Conclusions**

In this study, we investigated the importance of wave-currents interaction in the Black Sea for the first time. A reciprocally forced numerical system has been implemented using the ocean circulation model NEMO v4.0, which is now including the most important wave-currents physics, and the third-generation wave model WaveWatchIII. The coupling consists of providing Sea Surface Temperature and surface currents to the wave model, computed by the hydrodynamic model and returning sea-state dependent momentum flux, Surface Stokes Drift and wave dissipated energy to ocean vertical mixing. Even if our main focus was the assessment of tracers, a positive effect has been found also on waves.

The inclusion of wave-currents interaction in the Black Sea hydrodynamics, determined reduction of the RMSE for SST (≈−3.5%) and the upper ocean from 7.5 to 200 m water depths (≈−3%). The main differences between the forced and free runs are related to the uppermost part of the water column (depth < 35 m). The strongest impact on the vertical profile caused by the for is related to the sea-state dependent momentum flux, while the Stokes–Coriolis force and the sea-state dependent vertical mixing have negligible effects. On average, the coupling produced greater benefits in Winter and Spring, which are characterised by intense wave activity and low vertical stratification. According to our validation, the forced run was found to perform better than the free run for moderate wave heights. In general, the forced run demonstrated a slightly warmer water temperature than the free one.

The forced experiment had a positive impact even on salinity, with a reduction of ≈−10% in BIAS and ≈−6.5% in RMSE on the uppermost averaged 200 m.

The main impact of waves on currents concerned the reduction of direction BIAS, without improvement/worsening for the velocity module. Anyway, we noticed a wider dynamic in direction and speed for the currents field in the fully-forced experiment.

We inferred that the physical process which has been improved in forced hydrodynamics is related to the vertical mixing, with larger mixing during Winter and lower mixing during Spring–Summer with respect to free-run experiments, as showed in Section 4.1.3.

Coupling also improved the wave model performance, which slightly better represents the Hs satellite observations, in forced configuration. The results indicate that the improvement was mainly related to the better representation of the effect of air-sea temperature differences on the wave growth, while the usage of the surface currents plays

a minor role, as already shown in [28]. Unfortunately, the lack of wave buoys in the basin prevented the analysis of the coupling impact on wave period and direction.

In conclusion, this preliminary coupling configuration produced a modest but clear improvement in the simulations of temperature, salinity, currents and waves in the Black Sea. Future works could investigate this setup-up over a longer time scale to evaluate its impact on climatological time scale as in [19,104,105]. The use of an external coupler to conduct online field exchange is an important technical development that can be considered in further studies.

**Author Contributions:** S.C. designed the forced system and coordinated the work in collaboration with S.A.C., G.C. and P.L., E.C. participated in the scientific discussion sharing the theoretical approach as implemented in the Mediterranean Sea. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Copernicus Marine Environment and Monitoring Service for the Black Sea Monitoring and Forecasting Centre, contract n. 72.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
