The Danube River role in the Black Sea dynamics

The aim of this study is to investigate the mechanism of the estuarine water exchange and to offer a proper and useful representation of the riverine release into mesoscale-resolving ocean models.

In particular, this study focuses on the effects of the Danube River release (in terms of volume flux, temperature, and salinity) in the Black Sea circulation and dynamics, thus contributing to upgrade the current representation of the riverine release in the framework of the Copernicus Black Sea forecasting system (https://marine.copernicus.eu/about/producers/bs-mfc).

The mesoscale-resolving ocean models, that is few kilometer as horizontal resolution, cannot solve the estuarine dynamics because they cannot represent the estuary geometry due to their low resolution. However, they may produce a reliable representation of the river plumes if they are forced by a realistic estuarine water release.

A 2-layer box model, named CMCC EBM (https://www.estuaryboxmodel.org), is proposed as an intermediate-complexity solution encompassing the main estuarine dynamics processes averaged over the diurnal tidal cycle. The model solves the estuarine water exchange by two conservation equations for volume and salt fluxes and uses two parametric equations for the SWI length and the horizontal mixing along the estuary.

Minimal calibration and short CPU time make the EBM a powerful tool for coupling with ocean models and hydrology models to produce both operational forecasts and climate scenarios.

Verri et al. 2020; 2021 investigated the estuarine processes of the Po river delta system and the CMCC EBM is found to offer reliable estimates of runoff and salinity at the river mouths and to improve the representation of the buoyancy plume in coupled mode with a mesoscale ocean model over the Adriatic Sea based on NEMO code.

Here we propose a couple of twin experiments by the NEMO code over the Black Sea during 2020-2021: Experiment 1 considers observed river discharge for the five Danube branches coming from NIHWM dataset at daily frequency, and monthly climatological salinity from SeaDataNet; Experiment 2 replaces the discharge at the Danube mouths with the results of the EBM running over the same range 2020-2021 and forced by the daily observed runoff applied at the estuary heads and the entering salinity and volume flux at the river mouths provided by the NEMO model.

Comparison of the twin experiments is discussed to point out the added value of coupling the NEMO model with the EBM.

Moreover this study aims at investigating how the temperature gradient at the river mouths may affect the Regions Of Freshwater Influence (i.e., shelf areas adjacent to estuaries influenced by wind stress, the surface heating-cooling and the tidal currents) and the whole basin dynamics. Thus an additional conservation equation for the heat flux is added to the CMCC EBM and we evaluate the role of the temperature gradient at the river mouths.

Finally the Danube river-estuary-sea continuum is intended to be solved with a 3D finite element model in order to represent the estuarine dynamics in a seamless way and to provide a benchmark for validating the EBM+NEMO system.