Temel Oguz

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.

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.

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.

Simulation of annual plankton productivity cycle in the Black Sea by a one‐dimensional physical‐biological model

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one-dimensional vertically resolved physical-biological upper ocean model, coupled with the Mellor-Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place sometime between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December–January and emerges as the winter bloom, or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January–March period. These bloom structures are similar to the year-to-year variabilities present in the data.

Modeling the response of top‐down control exerted by gelatinous carnivores on the Black Sea pelagic food web

Recent changes in structure and functioning of the interior Black Sea ecosystem are studied by a series of simulations using a one-dimensional, vertically resolved, coupled physical-biochemical model. The simulations are intended to provide a better understanding of how the pelagic food web structure responds to increasing grazing pressure by gelatinous carnivores (medusae Aurelia aurita and ctenophore Mnemiopsis leidyi) during the past 2 decades. The model is first shown to represent typical eutrophic ecosystem conditions of the late 1970s and early 1980s. This simulation reproduces reasonably well the observed planktonic food web structure at a particular location of the Black Sea for which a year-long data set is available from 1978. Additional simulations are performed to explore the role of the Mnemiopsis-dominated ecosystem in the late 1980s. They are also validated by extended observations from specific years. The results indicate that the population outbreaks of the gelatinous species, either Aurelia or Mnemiopsis, reduce mesozooplankton grazing and lead to increased phytoplankton blooms as observed throughout the 1980s and 1990s in the Black Sea. The peaks of phytoplankton, mesozooplankton, Noctiluca, and gelatinous predator biomass distributions march sequentially as a result of prey-predator interactions. The late winter diatom bloom and a subsequent increase in mesozooplankton stocks are robust features common to all simulations. The autotrophs and heterotrophs, however, have different responses during the rest of the year, depending on the nature of grazing pressure exerted by the gelatinous predators. In the presence of Mnemiopsis, phytoplankton have additional distinct and pronounced bloom episodes during the spring and summer seasons. These events appear with a 2 month time shift in the ecosystem prior to introduction of Mnemiopsis.

Wind and thermohaline circulation of the Black Sea driven by yearly mean climatological forcing

Using an eddy-resolving ocean circulation model endowed with active thermodynamics and a turbulence closure parameterization, a hierarchy of numerical experiments is carried out to investigate the relative contributions of the wind forcing, the surface thermohaline fluxes, the river runoff, and the Bosphorus inflow/outflow in driving the yearly mean circulation in the Black Sea. The model accommodates a topographic and boundary-fitted curvilinear coordinate system and resolves steep topographical changes around the periphery of the basin using O(10 km) grid spacing and 18 stretched vertical levels. Model experiments show that topography, wind forcing, and buoyancy forcing are all first-order contributors to the primary circulation of the Black Sea. If any of these features are neglected, significant elements of the model circulation do not reproduce observations. Subbasin scale gyres are caused by both wind and thermohaline forcing. Annual mean wind stress is sufficient to produce the major interior cyclonic gyres. Stronger winds produce more defined interior flow than the weaker winds of the Hellermann and Rosenstein (1983) fields. Heat flux is an important contributor to subbasin scale cyclonic circulation. However, the annual mean heat flux with spatial structure given by the climatology produces unrealistic features in the interior circulation. The latter ones disappear when including the seasonal heat flux variability. This results strongly suggests that the seasonal cycle of the wind stress is much less crucial than the heat flux seasonal cycle in producing a realistic basin circulation. The Rim Current is locked to the steep topographic shelf slope, regardless of the forcing mechanism. Without including the strong topography the Rim Current is absent for all forcings. Mesoscale variability arises from the dynamic evolution of the Rim Current. This variability is enhanced by the Danube inflow and the Bosphorus inflow/outflow, demonstrating the importance of these buoyancy sources in enhancing the mesoscale. The deep-layer circulation is controlled by the barotropic pressure gradient and is insensitive to the magnitude, seasonality, or strength of the surface forcing. A transition zone separates the surface and deep-layer circulation patterns. Its circulation is mainly driven by the slope of the pycnocline developed as a response to the surface forcing.

Modeling distinct vertical biogeochemical structure of the Black Sea: Dynamical coupling of the oxic, suboxic, and anoxic layers

A one-dimensional, vertically resolved, physical-biogeochemical model is used to provide a unified representation of the dynamically coupled oxic-suboxic-anoxic system for the interior Black Sea. The model relates the annual cycle of plankton production in the form of a series of successive phytoplankton, mesozooplankton, and higher consumer blooms to organic matter generation and to the remineralization-ammonification-nitrification-denitrification chain of the nitrogen cycle as well as to anaerobic sulfide oxidation in the suboxic-anoxic interface zone. The simulations indicate that oxygen consumption during remineralization and nitrification, together with a lack of ventilation of subsurface waters due to the presence of strong stratification, are the two main factors limiting aerobic biogeochemical activity to the upper ∼75 m of the water column, which approximately corresponds to the level of nitrate maximum. The position of the upper boundary and thus the thickness of the suboxic layer are controlled by upper layer biological processes. The quasi-permanent character of this layer and the stability of the suboxic-anoxic interface within the last several decades are maintained by a constant rate of nitrate supply from the nitrate maximum zone. Nitrate is consumed to oxidize sinking particulate organic matter as well as hydrogen sulfide and ammonium transported upward from deeper levels.

A physical-biochemical model of plankton productivity and nitrogen cycling in the Black Sea

A one-dimensional, vertically resolved, physical—biochemical upper ocean model is utilized to study plankton productivity and nitrogen cycling in the central Black Sea region characterized by cyclonic gyral circulation. The model is an extension of the one given by Oguz et al. (1996, J. Geophys. Res. 101, 16585—16599) with identical physical characteristics but incorporating a multi-component plankton structure in its biological module. Phytoplankton are represented by two groups, typifying diatoms and flagellates. Zooplankton are also separated into two groups: microzooplankton (nominally (200 lm) and mesozooplankton (0.2—2 mm). The other components of the biochemical model are detritus and nitrogen in the forms of nitrate and ammonium. The model incorporates, in addition to plankton productivity and organic matter generation, nitrogen remineralization (ammonification) and ammonium oxidation (nitrification) in the water column. Numerical simulations are described and compared with the available data from the central Black Sea. The main seasonal and vertical characteristics of phytoplankton and nutrient dynamics inferred from observations appear to be reasonably well represented by the model. Fractionation of the biotic community structure is shown to lead to increased plankton productivity during the summer period following the diatom-based early spring (March) bloom. The annual nitrogen budget for the euphotic zone reveals the substantial role of recycled nitrogen in the surface waters of the Black Sea. ( 1999