Black Sea Intrapycnocline Lenses according to the Results of a Numerical Simulation of Basin Circulation

Abstract

The exact scenario of the propagation of highly saline Marmora Sea waters in the Black Sea basin is not clear. Together with river runoff, they form the prevailing haline stratification of the basin. One regime of their propagation can be the formation of lenses similar to highly salted and warm lenses of the Mediterranean Sea water in the Atlantic Ocean basin. The purpose of this study is the numerical modeling and subsequent analysis of these dynamic structures that form in the Black Sea due to water exchange through the Bosphorus strait. In this case, the main difficulty is the need for high spatial resolution in the strait. The paper provides a brief description of the NEMO ocean-modeling framework used and the features of configuration that make it possible to reproduce the circulation in the basins of the Azov, Black, and Marmara seas with a spatial resolution of 1.16 km. Fields from the ERA5 atmospheric reanalysis are used as the atmospheric forcing. The numerical experiment carried out by us allows reproducing a typical structure of the seasonal variability of the Black Sea rim current and the features of the vertical structure of the basin waters for the period of 2008–2009. The spatial resolution of the results of numerical modeling made it possible to accomplish a high-quality reconstruction of the saltwater dynamics in the basin and at the exit of the Bosphorus strait. Five lenses formed near the strait and captured by the rim current jet were simulated for the period of 2008–2009. They are well-pronounced in the seawater salinity field for the period of 1–2 months. Using one of the lenses as an example, it is shown that its core is located at a depth of about 75 m. The vertical size of the structure is almost 40 m and its horizontal scale is 15 km. The simulation results demonstrate that lens formation events are associated with the gain in the saltwater inflow after storms in the Sea of Marmara basin. Verifying the results of the study can be done based on a specialized field experiment in the Bosphorus region during the periods of the most probable formation of the Black Sea intrapycnocline lenses.