The water circulation near the Danube Delta and the Romanian coast modelled with finite elements

A numerical model, based on a finite element discretisation technique, was used to study water circulation in the Black Sea׳s north-western shelf, particularly near the Danube Delta and the Romanian coast. The numerical grid covers the entire Black Sea and the model resolution is gradually increased versus the Romanian coast to resolve both mesoscale and microscale hydrodynamic features. Sea level, water temperature and CTD profiles, collected in the north-western shelf, were used to validate the model. The seasonal and daily coastal hydrodynamics, in 2009, were studied using 3-D water current and salinity fields. Moreover, different numerical tracers were released on the Danube׳s arms to characterise the Danube׳s plume extension. Results show that near the Danube Delta the strong salinity stratification confines most of the wind momentum input to the surface water layer, while the subsurface current is mostly influenced by the open sea circulation. In the southern Romanian coast, the vertical salinity gradient is weaker and the action of the wind can reach deeper layers. The numerical tracers show a net predominance of the Kilia branch and how coastal anticyclonic eddies can trap the river freshwater, carrying it away from the coast. This happens especially in spring, when the Danube׳s discharge is high and a large eddy is active in front of the Kilia branch.