Emin Özsoy

On the Physical Oceanography of the Turkish Straits

The Bosphorus and the Dardanelles Straits and the Sea of Marmara constitute a system through which exchange of Mediterranean and the Black Sea waters takes place. The two layer flow regime displays temporal and spatial variability on a wealth of scales. An assessment of the volume fluxes for the various elements of the system, based on recent hydrographic investigations, shows that a major portion of the Mediterranean flow entering through the Dardanelles is transported back to the Aegean Sea due to upward mixing induced by internal hydraulic adjustments of the exchange flow in the straits and by wind in the Sea of Marmara proper. The jet-like Bosphorus outflow in the exit region of the Marmara Sea also has a substantial contribution to the overall upward mixing. A mesoscale anticyclonic eddy to the right of the outflow off the Thracian coast is a quasi-permanent feature of the system. Hydraulic controls in the Bosphorus strait result in a maximal exchange, while a submaximal exchange exists in the Dardanelles. The Mediterranean inflow enters the Black Sea on an essentially continuous basis, with only few, short interruptions.

General Circulation of the Eastern Mediterranean

Abstract A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

Oceanography of the Black Sea: A review of some recent results

Abstract A new synthesis of the Black Sea oceanography is presented, primarily based on studies carried out in the southern Black Sea, as well as on some recent work covering the entire basin, obtained in a new era of increasing cooperation between the riparian countries. A review of the physical environment is given. Seasonal and interannual climatic variability of the system are discussed in relation to its hydrology. Water mass variability and formation are studied, with emphasis on the inflow of Mediterranean waters, pycnocline variability, shelf and internal mixing, and double diffusive convection. The general circulation of the basin, and the roles of stratification, topography and coastline variations in determining the behaviour of the rapid, unstable boundary currents and upwelling along the coast are discussed, based on hydrographic data and satellite observations. Impacts of the physical processes on the ecosystem are discussed.

The circulation and hydrography of the Marmara Sea

Abstract A comprehensive set of data collected during 1986–1992 reveal seasonal and interannual variability in the circulation and hydrography of the Marmara Sea. Waters, which have contrasting properties and originate from the adjacent basins, supply the two-layer stratified flows in the Sea of Marmara. Turbulent entrainment into the upper layer in the exit region of the Bosphorus jet, and wind-stirring in winter, both contribute equally to the basin vertical mixing. The upper layer circulation of the Marmara Sea is determined from ADCP measurements and from dynamical calculations based on hydrographic data. The mean upper layer circulation is anti-cyclonic, mainly driven by the southward flowing Bosphorus jet in the enclosed domain. The Bosphorus inflow is well defined, except during the periods of low discharge in autumn and winter, when the jet becomes weaker and tends to become attached to the west coast near the exit. Mediterranean water, entering from the Dardanelles, supply the suhalocline layer. The negatively buoyant plume of well-oxygenated water is the only means of renewal of the deep waters, partially compensating for the oxygen consumed by the degradation of organic matter sinking from the upper layer into the lower layer. Yet the subhalocline waters remain permanently deficient in oxygen, as a result of the internal balances of diffusion, advection and consumption. The depth to which the plume penetrates is a function of the seasonal characteristics of the inflow density (modified in the Strait) and the weak interior stratification.

Hydrographic properties and ventilation of the Black Sea

Using hydrographic data collected by CTD during five cruises of the 1988 Black Sea Oceanographic Expedition, from 16 April to 29 July 1988, we describe the distribution of potential temperature (θ), salinity (S), and potential density (σθ) throughout the water column. The salinity and density increase rapidly with depth, while temperature decreases to a minimum at 50 m in the cold intermediate layer (CIL). All three variables increase slowly with depth in the deep water. The hydrographic properties of the upper 200 m varied little over the duration of the 1988 expedition. Significant differences are observed when the 1988 data are compared with the 1969 Atlantis II data set. All of the 1969 data are warmer at a given salinity than the 1988 data to a depth with a salinity of about 21.0%o. Possible causes for these changes are increased heat loss to the atmosphere and decreased freshwater input. The most distinctive feature in the deep water is a homogeneous benthic bottom layer that extends from about 1700 m to the bottom. There is a single pronounced step in all hydrographic properties at the top of this layer. Vertical transport across the upper boundary may be controlled by double diffusion driven by geothermal heat flow. The predicted double-diffusive heat flux agrees with geothermal heat flow to within a factor of 5. A simple box model with surface, entrainment and deep-water reservoirs is used to model the entrainment process and the residence time of deep water in the Black Sea. The results suggest that the Bosporus inflow entrains water with properties of the CIL. The ratio of entrainment to Bosporus inflow is 3.3. Assuming a Bosporus inflow of 312 km3 y−1, the resulting residence time of the deep water is 387 years. A total CO2 balance is used to calculate the flux of carbon into the deep water and Δ14C balance is used to calculate the pre-nuclear value of Δ14C = −200%o in the entrainment water. This highly depleted value would have resulted in an apparent age of 1400 years for the CIL and, probably, the surface water as well. If the carbon flux of biological origin was depleted to the same extent this may account for some of the differences in sedimentary chronology based on 14C dates and varve counts.

Circulation in the surface and intermediate layers of the Black Sea

Abstract Circulation features of the Black Sea are presented based upon a basin-scale survey carried out in September–October 1990. The circulation pattern for the upper 300–400 dbar consists of a cyclonically meandering Rim Current, a series of anticycloniceddies confined between the coast and the Rim Current, and a basin-wide, multi-centered cyclonic cell in the interior of the basin. In contrast to prior investigations, although the currents are much weaker as compared with those in the upper layer, the intermediate depth (defined here between 500 and 1000 dbar) circulations reveal considerable structural variability. This involves counter-currents, shift of eddy centers, coalescence of eddies, and associated sub-basin-scale recirculation cells separated by the meandering Mid-Basin Current system. A descriptive synthesis of the upper layer circulation, combining the present results with earlier findings, identifies the quasi-permanent and recurrent features even though the shape, position, strength of eddies and meander pattern, and the bifurcation structure of currents vary.

The diffusive regime of double-diffusive convection

The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.

Modeling of Hydraulically Controlled Exchange Flow in the Bosphorus Strait

Abstract Recent hydrographic observations obtained in the Bosphorus Strait illustrate several features of the flow that may be related with the internal hydraulics. A two-layer numerical model indicates that the two-way exchange flow may indeed be subject to a series of internal hydraulic adjustments along the strait due to morphological features such as sills, a contraction and abrupt expansion of the width of the strait. The model identifies three distinct regions of the supercritical flow. The lower-layer flow of the Marmara Sea origin is directed to the north towards the Black Sea in a progressively thinning layer and is controlled by the sill located near the Black Sea entrance of the strait. The upper-layer water of the Black Sea origin flows in the opposite direction and is controlled upon reaching the constricted region located about 10–12 km away from the Marmara end of the strait. The upper-layer flow is then matched to the subsequent subcritical conditions by undergoing an internal hydraulic ju...

Observations of the Mediterranean inflow into the Black Sea

Abstract The Mediterranean inflow issuing from the Bosphorus Strait has been documented to enter the Black Sea essentially confined in a 10-km long channel which is a continuation of the strait over the adjacent shelf. The width of the channel is between 500 and 1000 m. A 3.5-km-long sill, with a depth of 60 m, is situated in the channel at its beginning, just north of the end of the strait. The Mediterranean water flows into the Black Sea essentially on a continuous basis throughout the year, but it may be interrupted for short durations under unusually strong and persistent winds. After exiting from the channel, the Mediterranean inflow spreads in a thin layer above the bottom and continues in a generally northerly direction towards the shelf break.

A synthesis of the Levantine Basin circulation and hydrography, 1985-1990

Abstract The Levantine Basin circulation derived from recent data consists of a series of sub-basin-scale to mesoscale eddies interconnected by jets. The basin-scale circulation is masked by eddy variability that modulates and modifies it on seasonal and interannual time scales. Long-term qualitative changes in the circulation are reflected in the bifurcation pattterns of ther mid-basin jets, relative strengths of eddies and the hydrographic properties at the core of these eddies. Confinement within the Basin geometry strongly influences the co-evolution of the circulation features. Surface measurements, satellite images and the mass field indicate an entire range of scales of dynamical features in the region. The complexity of the circulation is consistent with the basin-wide and mesoscale heterogeneity of the hydrographic properties. The interannual variability of LIW (Levantine Intermediate Water) formation in the region appears correlated with the changes in the circulation. Wintertime convective overturning of water masses reach intermediate depths and constitute a dominant mechanism of LIW formation, especially in anticyclonic eddies and along the coasts of the northern Levantine Basin.