Joanna Staneva

Rim current and coastal eddy mechanisms in an eddy resolving Black Sea general circulation model.

Abstract The DieCAST ocean model is applied to a study of the circulation in the Black Sea, using 1/12° horizontal resolution and with 20 vertical layers. Boundary forcings are monthly wind stress, evaporation minus precipitation, air–sea heat flux, freshwater influx from 11 rivers and exchange with the Mediterranean Sea through the Bosphorus Strait. The model reproduces fundamental physical features of the Black Sea: seasonal fluctuations in the quasi-permanent cyclonic Rim Current, numerous anticyclonic meanders and eddies lying between the Rim Current and the coast, Rossby waves propagating westward across the basin, coastally trapped waves, and the annual cycle of vertical mixing. Model results shed light on the mechanisms affecting such features. These include interactions of the Rim Current with coastal bathymetry abutments, leading to recirculations that pinch off vortices as in island wakes, and possible baroclinic instability of the Rim Current; these are modulated by the large annual stratification cycle above a relatively shallow and strong pycnocline, as is the Rim Current itself. The resulting wake eddies often merge into major coastal circulation features such as the seasonal Batumi and Sevastopol eddies. These anticyclonic eddies play a fundamental role in coastal and open-sea exchange processes. Hydrographic data from sampling cruises and recent Topex–Poseiden (T/P) altimeter data strongly supports our analysis.

Impact of Density Gradients on Net Sediment Transport into the Wadden Sea

Abstract This study tests the hypothesis that horizontal density gradients have the potential to significantly contribute to the accumulation of suspended particulate matter (SPM) in the Wadden Sea. It is shown by means of long-term observations at various positions in the Wadden Sea of the German Bight that the water in the inner regions of the Wadden Sea is typically about 0.5–1.0 kg m−3 less dense than the North Sea water. During winter this occurs mostly because of freshwater runoff and net precipitation; during summer it occurs mostly because of differential heating. It is demonstrated with idealized one-dimensional water column model simulations that the interaction of such small horizontal density gradients with tidal currents generates net onshore SPM fluxes. Major mechanisms for this are tidal straining, estuarine circulation, and tidal mixing asymmetries. Three-dimensional model simulations in a semienclosed Wadden Sea embayment with periodic tidal forcing show that SPM with sufficiently high se...

Oceanic response to atmospheric forcing derived from different climatic data sets. Intercomparison study for the Black Sea

Abstract Available climatic and atmospheric analysis data have been used to prepare forcing functions for the Black Sea numerical model, based on the Bryan-Semtner-Cox Modular Ocean Model and including parameterizations for the atmosphere-ocean exchange, inflow through the strait of Bosphorus and the Mediterranean plume. Atmospheric data from different sources are compared and the drawbacks of some of them illustrated. A new wind stress data set, based on ship observations, is prepared. Compared to the existing wind stress estimates, the present ones use additional data and more accurate parameterization of the boundary layer physics. The intercomparison between forcing data sets is focused on the heat flux and freshwater flux at the sea surface. The model simulates adequately vertical stratification, seasonal variability and horizontal patterns. Five data sets for heat flux, freshwater flux and wind stress are used in different combinations to study the model response. The large differences between the simulations, forced by different wind stress and identical thermohaline forcing, justify the computation of the new wind stress. Though the forcing data used are perhaps close to the best available at the moment for the Black Sea, the model simulations range in large intervals and some of them are very poor. The model responses to forcing functions of different origin give rough estimates on the possible errors in present-day simulations. Some inconsistencies give clear indications that further verifications, improvements of the forcing functions, and intercomparisons between the responses simulated by the ocean circulation models are needed.

Sea response to atmospheric variability. Model study for the Black Sea

Abstract A general circulation model for the Black Sea, based on the Bryan and Cox primitive equation model, is used to study the seas response to atmospheric variability. The forcing is based on well-known bulk-formulas, model simulated sea surface temperatures and on atmospheric analysis data for temperature, relative humidity and winds. The climatology of the forcing and the model response are analyzed in different frequency ranges. Model data give strong indications that interannual variability can be easily traced in the behavior of the modelled averaged characteristics, particularly in the core of the intermediate layer, which is extremely sensitive to convection. Experiments with smoothed and non-smoothed atmospheric data show that short-period atmospheric variability, related to the atmospheric cyclones, is of paramount significance not only for the ocean convection, which has relatively short time scales, but also for some permanent and very important climatic characteristics, such as the water masses.

On the Black Sea water mass formation. Model sensitivity study to atmospheric forcing and parameterizations of physical processes

Abstract A numerical model of the Black Sea circulation is developed using the Bryan-Cox-Semtner modular ocean model. Some available climatic and atmospheric analysis data are used to force the model. The analysis of the simulations gives a strong motivation to construct new sea surface forcing functions for this basin. It is shown that they provide heat and water fluxes that are consistent with the observations and with some independent balance estimates. The improved model forcing results in simulating of a realistic circulation. The inflow through the Strait of Bosphorus is parameterized using physical concepts, based on theoretical models and observational data, relating the strait exchange to the net fresh water flux at sea surface. A new parameterization is included to simulate the Mediterranean plume. The model sensitivity to driving forces, physical processes, and different parameterizations is studied. The results are used to tune the model to simulate adequately the water mass formation. The most sensitive elements of the vertical stratification, as the cold intermediate layer and the deep temperature intrusions, are used in this tuning. The model circulation, ventilation of the deep and intermediate layers, some characteristic time and length scales show a good agreement with the observations. The cooling capacity of the intermediate water is analysed as dependent on the fresh water flux at sea surface. The model simulations give indications that the changes in thermal stratification in the intermediate depths are indicative not only for how severe the preceding winter was. These changes are also dependent on the variability in the fresh water. In a long run, they could illustrate climate changes in the catchment area of the Black Sea rivers.

The application of radiotracers to a study of Black Sea circulation: Validation of numerical simulations against observed weapons testing and Chernobyl 137Cs data

In this paper, we use the distribution of the artificial radionuclide, 137Cs, to investigate mixing and ventilation in the Black Sea. Timeseries data of vertical radionuclide distributions are combined with model simulations in order to follow the magnitude and depth of penetration of surface oxic water into intermediate depths, below the oxic/anoxic interface. Simulated data are produced by a three-dimensional circulation model that includes a new parameterization of the Bosphorus inflow/plume to simulate the effects of Mediterranean water on internal mixing. A radioactive tracer model for pre-Chernobyl 137Cs (weapons testing 137Cs) and Chernobyl 137Cs is coupled to the circulation model. The model results are compared to field data collected between 1986 and 1992. The main output from the simulations is the identification of the contribution of entrainment in the mixing of surface waters into subsurface layers. The trend of tracer penetration into the deeper layers following isopycnal surfaces is well demonstrated in the model and is consistent with the known circulation and physics of the Black Sea. The correlation between the activities of radionuclides and salinity, found in the field data, is supported by the simulations. These model results illustrate that the time-space abundance of the existing field data is sufficient for reconstructing the distribution of tracers in space and time, provided reliable estimates of Black Sea circulation exist.

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

Abstract 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.

The sensitivity of the heat exchange at sea surface to meso and sub-basin scale eddies : Model study for the Black Sea

The heat exchange between the atmosphere and ocean (a function of the difference between the sea surface temperature and atmospheric temperature) is differently accounted for in numerical models, depending on their horizontal resolution. This could result in large differences in the rates of formation of water masses in coarse resolution and eddy resolving models. We address this issue in the present paper comparing simulations carried out with the modular ocean model (MOM), which is set-up with coarse (1/4°) and eddy (1/12°) resolution. Both models are forced by atmospheric analysis data. The Black Sea is used as a test area, since it has well constrained heat balance. The absence of large open boundaries, which is not usually the case in most ocean models, precludes the dependency of the results on poorly known open boundary conditions and makes possible to accurately evaluate the model heat fluxes associated with mesoscale processes. We find many indications that the transport and mixing associated with mesoscale eddies provide an important mechanism for the penetration of cold surface water into the pycnocline. Their contribution is illustrated on the example of heat exchange with the atmosphere and the resulting intermediate water mass formation. Two areas are investigated in more detail: the shelf edge/continental slope area, where the time averaged patterns of cooling are realistically simulated in eddy resolving models, and the area of anticyclonic circulation between the coast and the main current the latter following approximately the continental slope. It is shown that the winter convection is enhanced at the periphery of coastal anticyclones. Lateral intrusions of coastal waters into the pycnocline associated with eddies govern the characteristics of cold intermediate water and the amplitude of annual signal at these depths. Statistical characteristics quantifying the contribution of mean versus eddy part of the heat flux at sea surface in the two models are analyzed.

Heat balance estimates using atmospheric analysis data: A case study for the Black Sea

The heat balance for the Black Sea is estimated, based on classical bulk formulas, using the U. S. National Meteorological Center atmospheric analysis data and different types of sea surface temperature data. The main focus in the study is on the sensitivity of the heat fluxes to the input data. The contribution of the short-period variability to the heat flux components is evaluated. It is shown that the use of monthly averaged atmospheric data produces substantial inconsistencies in the heat flux estimates due to inaccurate calculation of the nonlinear terms in the bulk formulas. Inventory of the past estimates for the Black Sea heat flux components and intercomparison with the ones calculated in this paper are carried out. Error estimates for experiments using independent sea surface temperature data are analyzed.

A comparison of modelled and measured Chernobyl 90Sr distributions in the Black Sea

Strontium-90 data are used to verify the mixing and ventilation patterns simulated in a circulation model of the Black Sea. We trace physical processes using 90Sr which was delivered after the Chernobyl accident, primarily via riverine discharges in the Northwest shelf region. The results used for the verification of our model simulations were obtained from samples collected between 1986 and 1992. A three-dimensional circulation/transport model coupled with a tracer model simulates the transport of 90Sr and its penetration into the intermediate and deep layers. Though most of the 90Sr is introduced in the model at the sea surface by river runoff in the NW shelf area, the concentration patterns suggest that a large amount of this signal penetrates the halocline in the Bosphorus area and along the southern coast. This fundamental spatial characteristic of mixing in the Black Sea is associated with the entrainment of surface and intermediate waters in the vicinity of the Bosphorus strait, and with vertical exchanges along the southern coast resulting from the anticyclonic circulation patterns. Another important fraction of the river water penetrates intermediate layers at the shelf edge in the NW Black Sea. Higher model resolution and more elaborated forcing functions would be needed in order to more accurately define mesoscale features in this basin.