The impact of the baroclinic eddies and basin oscillations on the transitions between different quasi-stable states of the Black Sea circulation

Model data from a 7-year integration of an eddy-resolving circulation model are analyzed to study the impact of sub-basin scale eddies on the transitions between different quasi-stable states in the Black Sea circulation. The data are produced using the Bryan–Semtner–Cox modular ocean model with grid intervals of 1/9°×1/12° in the longitude/latitude. The model is driven by realistic time variable wind and buoyancy forcing. The time variability simulated in the model is dominated by basin oscillations, which are strongly modified by baroclinicity and topography. The simulations support the concept that the anticyclonic circulation in the Black Sea is observed between the jet-current and the coast, where meanders and eddies with changing amplitudes are spontaneously formed. The baroclinic instability results in pronounced quasi-periodic changes of circulation. The meanders increase, and spectacular intrusions of coastal water into the open sea are simulated. In the extreme cases, the gyre breaks into sub-basin scale eddies. Altimeter data support the model simulations, particularly the characteristics of time variability. The baroclinic instability is strongly suppressed under coarse resolution, and the seasonal variability is characterized by almost constant amplitudes of the oscillations repeating every year. On the contrary, in the eddy-resolving model, the natural variability is well pronounced leading to interannual changes.