Emil V. Stanev

A seasonal model of the Mediterranean Sea general circulation

This paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The forcing is composed of climatological monthly mean atmospheric parameters, which are used to compute the heat and momentum budgets at the air-sea interface of the model. This allows heat fluxes to be determined by a realistic air-sea interaction physics. The Strait of Gibraltar is open, and the model resolution is in the horizontal and 19 levels in the vertical. The results show the large seasonal cycle of the circulation and its transient characteristics. The heat budget at the surface is characterized by lateral boundary intensifications occurring in downwelling and up welling areas of the basin. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream which enters from Gibraltar and assumes the form of different boundary current subsystems is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested not only by a change in amplitude and location of the gyres but also by the appearance of seasonally recurrent gyres in different parts of the basin. Distinct westward propagation of these gyres occurs, together with amplitude changes. For the first time a Mersa-Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air-sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. Deep water renewal does not occur, probably due to the climatological forcing used.

Regional sea level response to global climatic change: Black Sea examples

Abstract The sensitivity of Black Sea level to variations in the global forcing is studied here using tide gauge and satellite altimeter data, as well as hydro-meteorological data for the fresh water flux components. The consistency between satellite and sea borne data is analyzed and the characteristics of variability with monthly to interannual time scales are revealed. The analysis of 6-year-long data series of TOPEX/Poseidon altimeter shows that the first EOF accounts for 85% of the total variance and is associated with the water cycle, the latter forced by the air–sea exchange, continental hydrological budgets and straits outflow. This result is a demonstration that the Black Sea level integrates the variations of global forcing over vast catchment area, thus making them quite distinguishable. The second EOF describes the seasonal variability of circulation. The third and higher EOFs describe synoptic and basin-oscillations, and the corresponding principal components are characterized by strong interannual variability. By analyzing the correlation of sea level and water balance in the last 70 years, we quantify the response to the external forcing. The mean sea level trend during 1993–1997, derived from the TOPEX/Poseidon data, of ∼12 cm is much lower than the largest trends of this type observed in the last 120 years, which are associated with interannual-to-decadal variability and reach ∼20–30 cm. The correlation between the sea level and NAO index, starting from 1870s, is well pronounced, suggesting that future variations of sea level could be predicted using global climate indices. It is shown that the long-term changes of water balance are strong enough to substantially affect the exchange between Black Sea and Mediterranean Sea. This in turn might result in changing the conditions of water mass formation in the Aegean Sea and motivates further studies on the prediction of extreme events of deep water mass formation in the Eastern Mediterranean Sea as function of the global and regional water cycles.

Numerical simulation of the interannual variability of the Mediterranean Sea upper ocean circulation

Numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the Mediterranean. From January 1980 to November 1988, the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents. The model prediction of the interannual fluctuations of the Sicily Strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region. The current anomalies persist for many months after a Winter atmospheric anomalous disturbance has occurred over the basin. The Eastern Mediterranean basin is the area where the interannual ocean response is most pronounced.

On the mechanisms of the Black Sea circulation

Abstract Bryans (1989) model is used in this study to simulate the circulation in the Black Sea. Wind stress, heat exchange, precipitation and evaporation at sea surface are prescribed from climatic data. River inflow and the inflow from the Mediterranean Sea are taken from observations. Two types of models are discussed: GCM with horizontal resolutionΔλ = 1°, Δφ = 0.5° and 12 levels; and EGCM with horizontal resolutionΔλ = 20′,Δφ = 10′ on the same levels. GCM is designed to study the role of the wind and buoyancy fluxes for the establishment of circulation. Model data shows that in the baroclinic experiments kinetic energy is less than in the barotropic experiment. This is a consequence of the formation of a strong halocline in the upper layer, where a substantial part of the energy input transforms into potential energy. The specific vertical stratification in the Black Sea restricts the penetration of seasonal signal into deep layers, and preconditions the establishment of a cold intermediate layer. Model data from EGCM shows that eddies contribute to some change in the transport mechanisms, compared to those in GCM. Seasonal forcing affects substantially the variability of the main Black Sea gyre. Comparisons between model and observational data indicate that models resolve the main features of the Black Sea circulation. They illustrate correctly physical processes in strong stratified geophysical fluids, and particularly in semi-enclosed basins, where precipitation and runoff exceeds evaporation.

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.

Sea level variations and their dependency on meteorological and hydrological forcing: Analysis of altimeter and surface data for the Black Sea

TOPEX/Poseidon (T/P) altimeter data in the Black Sea are analyzed for almost 5 years in parallel with available hydrological and meteorological data with the aim of studying the water balance and the dependency of sea level oscillations on meteorological and hydrological forcing. This forcing induces seasonal variations of mean sea level with oscillations of ∼10–15 cm. The consistency between satellite and tidal gauge data is demonstrated in several coastal locations, and a mean ascending trend of ∼3 cm yr−1 is found in the two data sets. The variability in all components of water balance, including the Bosphorus outflow calculated as the difference between the fresh water flux and the time rate of sea level change estimated from altimeter data, is analyzed. The T/P data give very clear signals in the patterns of amplitudes of oscillations at intraannual, seasonal, and interannual timescales that help in understanding the variability of circulation. The intraannual variations are well pronounced on the continental slope and shelf and reach highest amplitudes in the areas of Sevastopol and Batumi quasi-permanent eddies. The clearest representation of oscillations with seasonal periodicity exists in the area of Batumi Eddy. This variability is associated with the transition between states with intense cyclonic circulation in winter and weaker (sometimes anticyclonic) circulation in summer-fall period. The Sevastopol Eddy is not clearly resolved in the seasonal variability. The interannual variability has the strongest signature in the area of western gyre and southeastern Black Sea. The analysis of satellite data supports some earlier studies on the circulation based on dynamic computations and numerical modeling. They make it possible to estimate the amount of water exchanged between coastal and open ocean areas caused by the time variability in the Ekman drift. The good quality of altimeter data and the high level of signals could ensure more accurate numerical simulations by means of data assimilation.

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.

Reconnaissance of the main Black Sea's ecohydrodynamics by means of a 3D interdisciplinary model

Abstract A 3D interdisciplinary model has been used to test the sensitivity of the Black Seas ecosystem to physical processes. The hydrodynamical model of the general circulation has been built up, using the GHER primitive equation model. A model with 15 km horizontal resolution and 25 vertical levels is used to compute the typical seasonal cycle. The model is forced by climatological monthly mean fields of temperature, salinity and wind stress at the air–sea interface; the river discharges of the Danube, Dnestr and Dnepr are taken into account. An ecosystem model at basin scale is then defined by a nitrogen cycle considering several phytoplankton and zooplankton sizes and including the microbial loop. The ecosystem model is embedded on-line into the 3D hydrodynamical model with a superimposed cycle for the light intensity. This model must be regarded rather as a first tool for testing the coupling of hydrodynamic and ecosystem submodels, while acquiring some preparatory assessment of the effect of physical processes on the ecodynamics. The results display a highly three-dimensional aspect with important horizontal and vertical variations, obviously imparted to the system by the physical processes (horizontal and vertical advection, vertical mixing and diffusion, upwelling…) associated with light limitation at depth and sinking of dead organisms. In this paper, the results are described emphasizing the effects of the hydrodynamic constraints on the space–time distribution of the primary and secondary production.

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.

Water transport in the Bosphorus Straits estimated from hydro-meteorological and altimeter data: seasonal to decadal variability

Abstract TOPEX/Poseidon (T/P) altimeter data for the period 1993–1997 and their correlation with available precipitation, evaporation and river runoff data are analysed with the aim to study the variability of sea level and water balance in the Black Sea. The difference between the time rate of change of basin volume estimated from altimeter and fresh water flux data is used as a measure of the vertically integrated transport through the Bosphorus Strait. The errors in the transport resulting from the use of tide gauge data when estimating the variations of basin volume are objectively quantified using sea level from T/P data as reference measurements. Then, from the long-term tide gauge and hydro-meteorological data series for the period 1923–1997, we derive the variability of basin volume and transport in the Bosphorus Strait. It is found that the main signal in the transport is the seasonal one, with maximum values in March–April and minimum in August. The amplitude of this signal is ∼2/3 of the amplitude of the net fresh water flux. In low frequency range, the oscillations with periods 10 and 4 years have significant amplitudes. However, the ratio between the magnitude of oscillations in the forcing (fresh water flux) and the response (transport in the strait) tends to unity. This indicates that the resistance of strait to climatic variability with interannual and decadal time scales is negligible. It is demonstrated that the available data present a valuable source of information when calibrating parameterizations of straits transport based on theoretical developments.