Circulation and Mixing Dynamics of the Black Sea

This project centers on modeling the circulation and thermodynamics of the Black Sea, the dominant Rim Current and its ubiquitous mesoscale meanders and anticyclonic eddies strung around the coastal margin. The effects of coastal abutments, shelf width variations, slope topography, winds, air-sea exchange and riverine buoyancy fluxes on the generation, evolution and dissipation of the coastal anticyclones are being investigated.

Surface streamlines and sea surface elevation (cm) for day 6120 (summer) from the DieCAST model. Most of the major coastal eddies can be identified (clockwise from top): Crimea, Caucasus, Batumi, Kizilirmak, Sinop, Sakarya, Bosphorus and Sevastopol.

The transport and mixing associated with these mesoscale eddies provide an important scale-dependent mechanism for the penetration of cold surface water into the pycnocline. Two areas are being investigated in detail: the shelf edge/continental slope area, and the area of anticyclonic circulation between the coast and the Rim Current. It is thought that winter convection is enhanced at the edges of coastal anticyclones. Lateral intrusions of coastal waters into the pycnocline associated with these eddies then govern the characteristics of cold intermediate water and the amplitude of the annual signal at depth.

Photograph of a simulation of Black Sea surface circulation using DieCAST ocean model. The color represents the geosrophically balanced contours of sea surface height. click here to view the simulation

The DieCAST ocean model is being applied at very high resolution (~ 4 km). DieCAST has already been used in the Black Sea at coarser resolution with considerable success. The model has the advantage of running at very low dissipation (vertical viscosity approaching molecular in the strongly stratified halocline) with 4th order numerics and unsmoothed bathymetry which allows a more realistic simulation of Black Sea mesoscale features than hitherto possible.

Photograph of SeaWiFS image (color enhanced to increase contrast) taken in June 2000 showing the interaction between the circulation and biological production, particularly at the mesoscale.

A series of parameter varying experiments is being run to illuminate the processes responsible for the generation of the coastal anticyclonic eddies. Model results (i.e., structure and time variations of surface circulation, dynamic topography, SST, and water column properties) are being compared with existing hydrographic data and satellite thermal IR imagery. Previous estimates of heat exchange between the atmosphere and the Black Sea using ocean circulation models have been found to depend on their horizontal resolution, resulting in large differences in estimates of the rates of formation of water masses between coarse resolution and eddy-resolving models. The scale-dependent sensitivity of air-sea heat exchange across mesoscale and sub-basin eddies is investigated by running the DieCAST model at two resolutions (1/12o and 1/24o) and comparing results with recent output from the Modular Ocean Model (MOM) run at coarser resolution.