On the genesis and development of coastal submesoscale eddies in the Northwestern Black Sea shelf

Submesoscale features, characterized by intense horizontal and vertical exchanges, significantly influence the transport of heat, salt, and suspended matter, as well as upper-layer productivity. This study investigates submesoscale features at the coastal boundary on the Northwestern Black Sea shelf under the influence of Danube Delta waters. High-resolution simulations with an unstructured-grid model provide insight into the mechanisms governing submesoscale dynamics. We demonstrate that increased model resolution is crucial for accurately capturing submesoscale features, with particular attention to the vertical resolution required to represent the mixed-layer depth. Our results reveal that barotropic processes dominate the initial generation of submesoscale eddies, particularly in areas of abrupt coastline changes, such as Sacalin Island (Romania) and Cape Kaliakra (Bulgaria). In its turn, the baroclinic conversion is more significant in their subsequent growth and maintenance along the coast. The interaction between the Danube plume and submesoscale eddies is twofold. The Danube plume enhances the formation of eddies by creating shear and density gradients. On the other hand, the eddies facilitate the along-shore transport of brackish waters and contribute to restratification. These findings contribute to understanding the interplay between mixing and eddy processes, thus shedding more light on the dynamics of the coastal boundary layer.