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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.

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.

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.

The upper layer circulation of the Black Sea - Its variability as inferred from hydrographic and satellite observations

Quasi-synoptic hydrographic data and satellite imagery are used to describe the circulation and the structural variability of the Black Sea with particular emphasis on the Turkish coast. The circulation is indicated to involve a variable cyclonic circulation with no apparent central locus and a well-defined cyclonic “Rim Current” containing meanders and interacting eddy fields confined to the shelf slope. Interspersed between the coastal eddies are filaments and intense jets, often with dipole eddies at their termina. The extension of these features across the shelf-slope into the central basin offshore waters implies important dynamical processes related to the shelf-deep basin exchanges. These features are often steered by the topography and evolve continuously through the mixed baroclinic-barotropic instability of the Rim Current.

Circulation and hydrography of the Levantine Basin. Results of POEM coordinated experiments 1985–1986

Abstract A brief review of the meteorological setting, hydrography and the circulation in the Levantine Basin of the Eastern Mediterranean is given. The recent high resolution data obtained in POEM coordinated experiments of 1985–1986 are then used to optimally estimate the circulation in the basin in two different seasons and to describe the water mass distributions. Some of the features observed during the experiments support the historical knowledge on the locations of sub-basin scale gyres and the general circulation, in addition to which some new features are established. Details of the circulation such as the intensity, the multiple scales and the three dimensional structure of the various vortices and the Central Levantine Basin Current are displayed extensively. A variety of sub-basin, meso- and sub-mesoscale vortices occur with highly assymmetric (baroclinic) vertical structures. Some eddies split into multiple centres with depth, interpreted as indicating possible coalescences. A number of long-lived eddies were persistent in both surveys. The sub-surface Atlantic Water (AW) is advected by and entrapped within the eddy field. The Levantine Intermediate Water (LIW) at intermediate depths is shown to be maintained throughout the year in the northeastern sector of the Levantine Basin and along the periphery of the Rhodes gyre. The Levantine Intermediate Water also has a patchy distribution, owing to the advection and trapping by the eddy field which it helps to generate through adjustment processes.

The evolution of Mediterranean water in the Black Sea: interior mixing and material transport by double diffusive intrusions

Abstract After its entry into the Black Sea from the Bosphorus Strait, Mediterranean Water first evolves by mixing with the Cold Intermediate Water on the shelf region, and later sinks along the continental slope, reaching the halocline in the form of cold anomalies. The intrusion of the modified waters drives a series of intermediate depth nepheloid layers spreading from the southwestern margin into the interior of the Black Sea basin. In many cases, the temperature, salinity, suspended matter and other properties of the intruding layers in the interior can be traced back to the southwest shelf region. The opposing effects of the existing temperature and salinity gradients on the stability of the interior density stratification, and the anomalous temperature and salinity of the intrusions themselves, result in a unique mechanism of double diffusive convection at intermediate depth. The mixture of Mediterranean and Cold Intermediate Water is injected at a wide range of intermediate depths below the anoxic interface, leading to time-dependent, filamented and layered structures in the interior. The net entrainment and convective mixing produced by the intrusions are significant in determining the overall boundary transport and mixing across the permanent halocline. The time-dependent penetration of the sinking water implies a statistical control of the Black Sea interior stratification, and may explain the peculiar features of the permanent halocline. Local shelf dynamics and coherent structures of the Black Sea circulation modify the transport by the intrusions and their mixing with the interior waters. Boundary currents transport the anomalous properties east along the shelf, and interactions of the currents with shelf topography near Sakarya Canyon lead to suspension and spreading of shelf sediments.