Discussion concerning the current circulation along the Romanian Black Sea coast

Abstract. An integrated approach was used to understand how wind and freshwater input influence the water circulation along the Romanian Black Sea coast. Field data from different ADCP (Acoustic Doppler Current Profiler) surveys in the area of the delta were combined with a 3D finite element model. The model SHYFEM (Shallow Water HYdrodynamic Finite Element Model), developed at the Institute of Marine Sciences (ISMAR – National Research Council, Venice, Italy), was used on the entire Black Sea, with higher resolution near the Romanian coast and the Danube River distributaries. Several theoretical simulations were carried out, varying the wind velocity and river discharge, according to the available long-term data and using measured data as a reference for model validation. The simulated currents have the same order of magnitude as the measured ones. The results show that the Danube discharge influences the circulation along the entire Romanian Black Sea coast, generating a longshore current, mainly localized in the surface layers. This current occurs even at low river discharges and is found both at low and high wind velocity. In this study we compared the results of the simulations for two main wind directions, NE and SE, but the wind from SW (generally inland) was also considered since it is opposite to the downdrift buoyant flow propagation. Maps of the current field and the fluxes, calculated on several sections perpendicular to the coast, were analyzed for all these forcings. We made the simulations both for the cold and warm season, by applying different temperature and salinity fields. The results show that, for both cold and warm seasons, the constantly blowing wind becomes a significant factor in the formation of coastal currents. Calculated fluxes on cross-sections versus Danube discharge usually show important differences between the cold and warm seasons, which are due to the distribution of temperature and salinity. Those seasonal differences are less significant in the case of increased wind velocity. Thus, at higher velocity, wind becomes a dominant factor, controlling the overall circulation. This study is a first step towards the development of a fully operational oceanographic model for the northwestern part of the Black Sea. Future developments will include the use of real meteorological forcing, modelling of the waves and sediment transport