The evolution of Mediterranean water in the Black Sea: interior mixing and material transport by double diffusive intrusions

Abstract

After its entry into the Black Sea from the Bosphorus Strait, Mediterranean Water first evolves by mixing with the Cold Intermediate Water on the shelf region, and later sinks along the continental slope, reaching the halocline in the form of cold anomalies. The intrusion of the modified waters drives a series of intermediate depth nepheloid layers spreading from the southwestern margin into the interior of the Black Sea basin. In many cases, the temperature, salinity, suspended matter and other properties of the intruding layers in the interior can be traced back to the southwest shelf region. The opposing effects of the existing temperature and salinity gradients on the stability of the interior density stratification, and the anomalous temperature and salinity of the intrusions themselves, result in a unique mechanism of double diffusive convection at intermediate depth. The mixture of Mediterranean and Cold Intermediate Water is injected at a wide range of intermediate depths below the anoxic interface, leading to time-dependent, filamented and layered structures in the interior. The net entrainment and convective mixing produced by the intrusions are significant in determining the overall boundary transport and mixing across the permanent halocline. The time-dependent penetration of the sinking water implies a statistical control of the Black Sea interior stratification, and may explain the peculiar features of the permanent halocline. Local shelf dynamics and coherent structures of the Black Sea circulation modify the transport by the intrusions and their mixing with the interior waters. Boundary currents transport the anomalous properties east along the shelf, and interactions of the currents with shelf topography near Sakarya Canyon lead to suspension and spreading of shelf sediments.