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BLACK OCEANOGRAPHY

Understanding DYNAMICS

An Overview of Recent Numerical Modeling

BY EMIL V. STANEV

he importance of the Black Sea extends far beyond its regional role as a mixing body T where Mediterranean is diluted. This sea’s marine environment acts as a small- scale laboratory for investigating processes that are common to different areas of the world’s

. In particular, research on deep ventilation could facilitate understanding of similar

controlling processes in the paleocean when the ’s conveyor belt was shallower. Be-

cause water and balances are easily controllable and the scales are smaller than in the

global ocean, this basin is a useful test for developing models, which can then be ap-

plied to larger scales. Moreover, studying outputs from numerical models is an important

complement to sparse observations and extends our knowledge. The major purpose of this

paper is to demonstrate this possibility, using Black Sea physical oceanography examples

based on numerical modeling results.

Th is article has been published in Oceanography, Volume 18, Number 2, a quarterly journal of Th e Oceanography Society. Copyright 2005 by Th e Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 56 Oceanography Vol.18, No.2, June 2005 copy machine, reposting, or other means without prior authorization of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or Th e Oceanography Society, Box 1931, Rockville, MD 20849-1931, USA. Oceanography Vol.18, No.2, June 2005 57 INTRODUCTION , limiting the vertical exchange and about the individual applications as The Black Sea is a nearly enclosed basin creating a unique chemical and biologi- well as about the strengths of different connected to the and cal environment (see Konovalov, this is- models are given in the papers of Stanev the Sea of by the narrow sue). The cold intermediate water (CIW) (1990), Oguz and Malanotte-Rizzoli and Straits, respectively. Its catch- mass, with temperatures lower than the (1996), Staneva et al. (2001), Stanev et ment area covers large parts of mean annual temperature of the surface al. (2003) and Kara et al. (2005a). This and (Figure 1), providing a total and deep layers, is a consequence of ex- paper does not summarize all major re- freshwater supply of 3 x 102 km3 per tremely stable stratifi cation. This cold sults of these reviews and original pub- year1. Although evaporation exceeds pre- intermediate layer (CIL) is formed by lications, rather it reports on new devel- cipitation, the freshwater fl ux ( run- winter cooling followed by spreading of opments and perspectives. off, plus minus evapora- CIW throughout the sea in a layer ap- There are many examples in oceanog- tion) remains large in comparison to ba- proximately 50 to 100 m below the sur- raphy of fundamental processes that had sin volume (~5.4 x 105 km3), making the face. It acts as a boundary between sur- been fi rst described by theory and later Black Sea a typical estuarine basin. Be- face and deep waters. supported by observations. Numeri- cause of the large freshwater fl ux and the The Black Sea is where most kinds of cal modeling also provides a powerful narrow opening in the strait of Bospo- numerical models based on primitive tool for estimating fundamental ocean rus, the exchange between the Black and equations have been used to simulate cir- characteristics that cannot be directly Marmara Sea is asymmetric: the volume culation and transport of matter (see Box measured. The aim of this paper is to of water transported by the outfl owing 1). This sea provides optimal possibilities, demonstrate that studying outputs from surface current is two times larger than using easily manageable models and - numerical models is an important com- the infl owing deep counter-current, thus servational data to address a wide spec- plement to sparse observations and pro- the Black Sea’s surface is about trum of processes observed in the ocean. vides important scientifi c guidance. half that of the Mediterranean’s. The Black Sea also has unique oceano- CIRCULATION Numerical modeling made it possible, for ...studying outputs from numerical models is an the fi rst time, to quantify the individual important complement to sparse observations and impact of mechanical and thermohaline forcing in the Black Sea (Stanev, 1990; provides important scientific guidance. Oguz and Malanotte-Rizzoli, 1996), re- vealing wind as the driving force in creating a cyclonic general circulation. Unlike other large estuarine basins graphic conditions maintained by strong The haline buoyancy anomalies at the sea (e.g., the ), the Black Sea is a stratifi cation, river runoff, and infl ows. surface enhance the cyclonic circulation deep basin (maximum depth of ~2200 There are more than 25 peer-review because most of the freshwater enters the m) with a large shelf. A distinct vertical papers in English dealing with different sea in the coastal area. The circulation is layering is created between the surface aspects of modeling Black Sea circula- structured usually in two connected gyre waters in the upper 100 m and the deep tion. Extended references on modeling and observations are provided in sev- Emil V. Stanev ([email protected]fi a.bg) 1In view of the great interannual variability during eral reviews (Stanev, 1990; Stanev et al., is Professor, Department of Meteorol- the last 50 to 100 years (Stanev and Peneva, 2002; Oguz paper 2, this issue), errors of observations and 2002; Kara et al., 2005a). In this paper ogy and Geophysics, University of Sofi a, non-steady state conditions, these numbers, as well we will refer only to well-documented , also at the Institute for Chemistry as some other budget numbers in this paper, give a (and in most cases freely available) mod- and Biology of the Marine Environment, very rough estimate of the climatic state. els applied to the Black Sea. More details University of Oldenburg, .

58 Oceanography Vol.18, No.2, June 2005 56°N

46°N

36°N 10°E 20°E 30°E 40°E

Figure 1. Th e Black Sea system is coupled with the atmospheric and hydrological systems over Europe and Asia Minor. Th e excess freshwater at the sea surface, along with the basin shape and topography and meteorological forcing, make the Black Sea a good overall integrator of the various kinds of processes that act over large continental areas. Th ere is a clear correlation between North Oscillation and sea-level variability (Stanev and Peneva, 2002; Oguz, paper 2, this issue). Th e orography of the Black Sea catchment area (m) is plotted in this fi gure with brighter colors. Streamlines illustrate the annual mean surface wind computed from the ERA-40 data.

systems encompassing the basin (the Rim Similar to many oceanic systems, Black Different physical controls are thus pos- Current) (Figure 2). Simulated velocities Sea dynamics is controlled by topogra- sible depending upon the geometric and of ~0.5 m/s, and greater than 1 m/s in phy, where the largest slope of sea level physical scale. The former is measured some areas (Staneva et al., 2001), com- is located along the continental slope. by the width of slope area, the latter by pare well with the observations of Oguz However, the topographic slope is gentle the Rossby radius (this physical scale and Besiktepe (1999). in the northwestern and western Black depends upon vertical stratifi cation and The Rim Current is associated with Sea, where the width of slope area is planetary rotation and is used in ocean- a difference of ~0.2 m between sea level ~80 to 160 km, and abrupt along the ography as a measure of the size of ocean in the coastal and open sea (Figure 2). southern and eastern coasts (Figure 1). eddies). In the area of abrupt slope, the

Oceanography Vol.18, No.2, June 2005 59 BOX 1: NUMERICAL MODELS AND THEIR APPLICATION IN THE BLACK SEA

Th e hydrodynamic equations solved by the has a good introduction to ocean models. ocean models are discretized vertically and Th e very early version of the -permit- horizontally in a fi nite number of grid elements ting Black Sea model Stanev (1990) used the H with diff erent shapes. Th e horizontal grid reso- Modular Ocean Model (MOM) (see http:// lution has to be suffi ciently fi ne to be able to www.gfdl.noaa.gov/~smg/MOM/MOM.html resolve all of the important processes. How- for more information), which is a z-coordinate a ever, using a very-fi ne-resolution grid over large model based on the so-called “box-concept.” z-Coordinate Model areas is not possible because of the limitation Th is study has been followed by a few other of computational resources. At a minimum, eddy-resolving ocean general circulation mod- horizontal resolution has to be fi ne enough to els (OGCMs). One of them is the Princeton suffi ciently resolve ocean eddies (“eddy-resolv- Ocean Model (POM) (see http://www.aos. ing models”), which have spatial scales of sev- princeton.edu/WWWPUBLIC/htdocs.pom for H eral tens of kilometers. more information), which uses σ-coordinates. Diff erent vertical resolutions are used when Its application to the Black Sea is documented b modeling diff erent ocean regimes (e.g., surface, by Oguz and Malanotte-Rizzoli (1996). Th e Di- σ coastal, or deep ocean). Th is fi gure represents etrich Center for Air Sea Technology (DieCAST) -Coordinate Model four diff erent examples of vertical resolution. (see http://www.ssc.erc.msstate.edu/DieCAST/ Undisturbed sea level is schematically given by for more information) model was set up for the green line. Th e free ocean surface and sur- the Black Sea by Staneva et al. (2001). Th e high- faces between model layers are plotted with red order numerics employed in this model made H lines, H is ocean depth. (a) Th e simplest vertical it possible, for the fi rst time, to resolve all im- resolution is given if horizontal surfaces follow portant mesoscale features of Black Sea circu- geopotential lines, which are almost horizontal. lation. In the study of Beckers et al. (2002) the c In these z-coordinate models, the surface layer’s GeoHydrodynamics and Environment Research Layered/Isopycnal-Coordinate Model vertical resolution is usually fi ner in order to (GHER) (see http://modb.oce.ulg.ac.be/GHER/ resolve fi ne-scale, upper-ocean processes. (b) In Beckers.html/publications/gherm.pdf for more σ-coordinate models (terrain-following mod- information) model (horizontal resolution h els) there are always an equal number of layers of 5 km) demonstrated, for the fi rst time, the between the ocean surface and bottom. Th us, potential of nested modeling. Th e fi nest avail- the vertical resolution is better in the shallower able resolution basin wide has been reached by ∞ shelf and coastal ocean, but not very good in the modeling eff orts of Kara et al. (2005a) and d the deep ocean. (c) In isopycnal-coordinate Peneva and Stips (2005). Th e former authors Reduced Gravity-Coordinate Model models, vertical discretization follows isopycnal set up, for the fi rst time in the Black Sea, the surfaces, where density plays the role of verti- Hybrid Coordinate Ocean Model (HYCOM) cal coordinate. (d) Th ere are also simpler and (see http://hycom.rsmas.miami.edu/ for more number of eff orts towards operational ocean- computationally effi cient models consisting of information) with 3.2-km resolution. Th e hy- ography in the Black Sea (Korotaev et al., 1999). only: surface one with thickness h and motion- brid coordinates allow the model to behave One such eff ort is due to Besiktepe (2003), who less deep layer with infi nite thickness. Th e den- like a conventional sigma (terrain-following) made extensive use of the Harvard Ocean Pre- sity diff erence between two layers reduces the model in very shallow oceanic , like a diction System (HOPS) (see http://oceans.deas. gravity with respect to the interface, therefore, z-level coordinate model in the mixed layer or harvard.edu/HOPS/HOPS.html for more infor- these types of models are known as reduced other unstratifi ed regions, and like an isopycnic mation). Th is is a system of integrated software gravity-coordinate models. Substantial decrease coordinate model in stratifi ed regions. Peneva for multidisciplinary oceanographic research of computations could be reached if one as- and Stips (2005) set up, for the fi rst time, the and forecasting that is built on the Geophysical sumes that sea surface is motionless (horizontal General Estuarine Transport Model (GETM) Fluid Dynamics Laboratory primitive equation green line in the fi gure). Th is fi gure is modifi ed (see http://www.bolding-burchard.com/html/ model. Most models use rich physical param- from the Computational Science Education GETM.htm for more information) with ~ 3.7- eterizations (e.g., turbulent schemes, bottom Project fi gures available online at http://csep1. km resolution for the coupled system Black layers, overfl ows, upper layer physics, and ra- phy.ornl.gov/CSEP/OM/OM.html. Th is site also Sea-Azov Sea. Recently, there have been a diation schemes).

60 Oceanography Vol.18, No.2, June 2005 Figure 2. Snapshot of sea level and surface streamlines simulated by the DieCAST model (for more details, see Staneva et al., 2001). Note the existence of the eastern and western gyres, coastal eddies (most of them anticyclonic), the major sub-basin gyres (known as and eddies), and the bifurcation of the Rim Current east of Cape . Th is plot compares well with the scheme of the Black Sea surface circulation based on a synthesis of dynamic height derived from hydrographic observations by Oguz et al. (1993). Th e names of major coastal eddies as known from observations (usually, associated with geographic names, for example, capes and towns) are also given.

Rossby radius (~20 to 30 km in the Black demonstrate that for very steep slopes, rine basins (e.g., ), strong stratifi - Sea) is comparable to the width of the the current interacts with the bottom cation dynamically isolates deep layers slope area. Thus, an interesting phenom- slope zone approximately as a vertical from surface and intermediate ones. enon occurs, which is well traced both wall, and the eddy fi eld becomes highly Numerical simulations of Stanev (1990) in simulations (Figure 2) and in obser- unstable and meandering. A similar in- demonstrated that in strongly strati- vations (Figure 3): the mean position crease in instability (Figure 4) had been fi ed geophysical fl uids, the joint effect of Rim Current follows the continental fi rst observed in the numerical experi- of baroclinicity (density is a function slope in the northwestern and western ments of Stanev and Staneva (2000), of more than just pressure) and relief parts of the Black Sea and is displaced which were designed to demonstrate the is substantially reduced. This is a typi- seawards elsewhere, revealing that very “competition” between controls of the cal case in the Black Sea, and possibly steep topography does not effi ciently planetary and topographic β-effects (i.e., in the paleooceans when the freshwater control the current. The above results the slope of topography acts in a similar fl uxes were larger and the stratifi cation from modeling and observations are way as the change of Coriolis parameter was much stronger than today. Data consistent with the laboratory experi- with latitude [the β-effect]). from the above-cited numerical simula- ments of Zatsepin et al. (in press), which In the Black Sea, like in other estua- tions demonstrated that currents in the

Oceanography Vol.18, No.2, June 2005 61 deep layers reach several cm/s, revealing modeling consistently supported the damental question about whether posi- a highly barotropic structure (very small idea that there was no reversal; however, tive buoyancy provided by the Bosporus change of currents with the depth). This there were theories based on labora- plume, or wind, is more important in result was supported recently by obser- tory experiments (e.g., Whitehead et al., driving deep Black Sea circulation. The vations with profi ling fl oats (Gennady 1998), which demonstrated that buoy- answer: wind is the major control on the Korotaev, Marine Hydrophysical Insti- ant plumes originating from straits dominant Black Sea cyclonic circulation tute of the Academy of Science, could drive anticyclonic circulation in throughout the water column (including Ukraine, pers. comm., 2005) showing the deep sea. Very recent observations the deep layers). that currents in the barotropic layers can from profi ling fl oats (Gennady Korotaev, Although horizontal circulation, reach ~5 cm/s. Marine Hydrophysical Institute of the which can be reconstructed from ob- There have been long-lasting de- Ukraine Academy of Science, Ukraine, servations, has been studied for many bates among Black Sea oceanographers pers. comm., 2005) clearly support the years, the equally important vertical cir- about whether or not circulation in the numerical simulations—circulation culation was completely unknown until deep layers is opposite that of the sur- is not reversed in the deep layers. This recently. In particular, the quantitative face layer. In the last decade, numerical conclusion helps us to answer the fun- characteristics of vertical circulation

46.00

44.00

42.00

28.00 30.00 32.00 34.00 36.00 38.00 40.00 Figure 3. Numerical simulations (Figure 2) and Lagrangian drifter observations (Zhurbas et al., 2004) show that, depending on the trajectory, it takes about three to six months for surface currents to make one around the basin. Th e looping trajectories are best pronounced in the area of Batumi and Sevastopol eddy. Th is fi gure shows trajectories of Lagrangian (SVP, SVPBD2), quasi-Lagrangian (SVPBD2TC), and non-Lagrangian (XAN) drifters launched in the Black Sea in 1999-2004. Figure courtesy of Sergey Motyzhev, Marine Hydrophysical Institute NASU, Sevastopol, Ukraine.

62 Oceanography Vol.18, No.2, June 2005 were unknown because they cannot be Figure 4. Th e Rim Current easily measured. Numerical simulations is accompanied by a series of anticyclonic mesoscale (Stanev, 1990; Stanev et al., 2002) dem- eddies between the conti- onstrated that vertical circulation (~105 nental slope and the coast m3/s) is much weaker than horizontal (typical radius between 50 and 100 km). Th eir growth 6 3 circulation (~5 x 10 m /s). The verti- gives rise to large meanders cal circulation cell includes the coast- that could either detach and propagate in the open sea ward transport of surface waters due to or stagnate for some time in cyclonic wind stress and compensating coastal areas feeding sub-ba- inward transport in the deep layers. This sin-scale eddies (Batumi and Sevastopol eddy). Th is fi gure cell is closed in the vertical by upward shows simulations with the motion in the basin interior and down- DieCAST model in the area ward motion in the coastal regions. off the coast. Col- ors give the relative vorticity The intensity of vertical circulation normalized by the Coriolis estimated from numerical models is parameter and multiplied comparable with the amount of water by 10. In vast areas, relative vorticity is only ~ 5 times entrained by the Mediterranean plume smaller than planetary vor- in Black Sea surface and intermediate ticity. Th e evolution of ed- dies compares well with the water. These models indicate that the observations of vortex di- infl ow from the Bosporus Strait plays (Zatsepin et al., 2003), an important role in controlling vertical including lifetimes (several months), defl ection of Rim exchange. What has not been understood Current to the southwest, until very recently is that the outfl ow and entrainment of coastal from the Bosporus Strait does not only water into the open sea (see also Figure 9 of Korotaev et propagate into a fi xed ambient water col- al., 2003). umn, but also changes the water’s verti- cal stratifi cation. By unifying downward motion in the coastal zone (caused by slope currents and coastal anticyclones) and upward motion in the basin interior (associated with wind forcing), vertical circulation regulates the impact of dif- ferent dynamic controls on stratifi cation (Stanev et al., 2004). The Black Sea’s general circulation is subject to pronounced seasonal varia- tions. Sea level responds to meteorologi- cal forcing of a different kind; its season- al amplitudes are ~10 to 20 cm, and the amplitudes of interannual variations are ~5 to 10 cm (Stanev and Peneva, 2002). The seasonal amplitude of the difference between sea-level height in the coastal

Oceanography Vol.18, No.2, June 2005 63 and open ocean, estimated from nu- (2000) that the transition between sum- AIRSEA EXCHANGE merical simulations and altimeter data mer and winter circulation is controlled Numerical simulations provide valu- (Stanev et al., 2003), reaches half of the by baroclinic eddies and is well revealed able information about air-sea exchange, annual mean value (see Figure 2 for the by the changing balance between vortic- which cannot easily be measured over sea-level slope). This sea-level informa- ity in the open ocean and coastal sea. vast ocean areas. To enable such diag- tion demonstrates that horizontal trans- Because potential vorticity (i.e., vorticity nostics, one needs outputs from coupled port almost doubles in winter, which is of a water parcel normalized by depth or ocean-atmosphere models, or at a mini- the season of more intense circulation. layer thickness) depends upon stratifi - mum to be able to specify surface forcing During this season, the circulation is or- cation, the transition between different in ocean models through a combination ganized in one gyre system encompass- circulation states (one-gyre circulation of data from atmospheric analyses, mod- ing the entire basin (the Batumi sub- in winter and multiple sub-basin-scale el-simulated sea surface temperature, and basin gyre is absent). In the warm part eddies in summer) is enhanced by the specifi c parameterizations called bulk of the year, anticyclonic sub-basin-scale large seasonal stratifi cation cycle above aerodynamic formulae. The diagnostics eddies are more pronounced (Stanev, a relatively shallow and strong pycno- of numerical simulations (Stanev and 1990). It follows from the numeri- cline as in the Rim Current (Staneva et Staneva, 2001) demonstrate that the larg- cal simulations of Stanev and Staneva al., 2001). est heating of ~2 x 102 W/m2 is reached in June and the largest cooling of ~-3 x 102 W/m2 is reached in early winter. - thermore, the amplitude of the seasonal thermal buoyancy fl ux exceeds that of the Wind Stress (Pa) haline buoyancy fl ux by an order of mag- 0.03 Figure 5. Temporal nitude (Figure 5). However, the net ther- variability (mean year) Model ECMWF of (a) wind stress, (b) 0.02 mal buoyancy fl ux is about four times thermal buoyancy fl ux, smaller than the net haline fl ux. Because and (c) haline buoy- 0.01 buoyancy fl uxes control the density fi eld, ancy fl ux. Th ese results are diagnosed from a we conclude that the annual mean strati- simulations carried 0.00 2 4681012fi cation in the Black Sea is dominated by out with the 5-minute- dilution of surface waters by , while resolution Black Sea Thermal Buoyancy Flux (10-8 m2/s3) Modular Ocean Model 9 the seasonal variability is created mostly (MOM) (Stanev et 6 Model by air-sea heat exchange. The contribu- al., 2003). Also shown ECMWF 3 are the corresponding tion of Ekman pumping (which mea- curves calculated from 0 sures the vertical velocity resulting from the ECMWF-reanalyzed -3 the wind stress curl) is comparable to data (stands for Euro- -6 b pean Centre for Me- the one of haline buoyancy fl ux (Stanev dium-Range Weather 246 81012 et al., 2003). This specifi c combination Forecasts) using aero- Haline Buoyancy Flux (10-8 m2/s3) of different forcing mechanisms tends dynamic bulk formu- lae, air and sea surface 0.5 Model to create a stable salinity stratifi cation, temperature, relative ECMWF which changes very little through time; humidity, and winds. 0.0 however, there is an extremely high vari- -0.5 ability in the upper ocean thermal struc- -1.0 ture. The formation and character of the c CIL is one of the main consequences of 2 4 6 8 10 12 Months the present-day balances of buoyancy.

64 Oceanography Vol.18, No.2, June 2005 MIXING AND WATER MASS Temperature at 31.5°E FORMATION 13.2 In contrast to the neighboring Marmara 13.5 Sea, which has relatively young water, the 13.8 deep water in the Black Sea is character- 14.1 ized by slow renewal (longer than 10,000 14.4 years). It is commonly accepted that the 14.7 15.0 Black Sea’s water was formed over a long 15.3 time period, and perhaps under differ- 15.6 ent condition than today’s climate. These 15.9 a stagnant conditions are well illustrated 16.2 by the fact that temperature signals in 41°N 42°N 43°N 44°N 45°N 46°N the numerical simulations (Figure 6) and observations cannot be traced much deeper than 200 to 400 m. Furthermore, numerical simulations and the limited Salinity at 31.5°E number of observations reveal the slow penetration rates of passive tracers (Fig- 13.5 ure 7): the concentration of CFC-12 at 13.8 400 m in 1988 is less than 1 percent of 14.1 14.4 surface values, which is not the case in 14.7 the ocean, nor in the neighboring Mar- 15.0 mara Sea, where the vertical exchange is 15.3 much faster. This unique feature of the 15.6 Black Sea is explained by the fact that 15.9 16.2 b vertical mixing in strongly stratifi ed fl u- ids tends to molecular values (Stanev, 41°N 42°N 43°N 44°N 45°N 46°N 1990; Gregg and Özsoy, 1999). It follows from the numerical simu- lations of Stanev et al. (2004) that the Figure 6. Meridional cross sections of simulated (a) temperature and (b) salinity in den- sity coordinates at 31.5°E during April 1993. Th e simulations have been done with the 5- horizontal contrasts in concentration minute-resolution Black Sea Modular Ocean Model (MOM). Its setup, forcing, and other of CFCs as seen on density surfaces are technical details are described by Stanev and Staneva (2001) and Stanev et al. (2003). Th e ~70 times smaller than the contrasts on upper panel represents the CIL. Th e larger “depth” of CIL in the interior Black Sea seen in isopycnical coordinates is due to upwelling (causing shallower isopycnals). Th e bottom z-surfaces (Figure 7). This trend, which 3 panel demonstrates, that below σt = 15.5 (density layer in kg/m ), stratifi cation is entirely has also been found for other geochemi- dependent on salinity, and isohalines coincide with the isopycnals. cal tracers in the Black Sea, is consistent with the theory of homogenization of potential vorticity, indicating that tracers tend to align themselves along isopycnal surfaces (Figure 6; see also Box 1). Although the diapycnal mixing is quite small, it is very important to Black Sea thermodynamics and its origin needs

Oceanography Vol.18, No.2, June 2005 65 to be explained. Breaking internal waves in the biogeochemistry of the Black Sea, better studied than deep mixing because are one of the candidates, as demon- suggesting that most of the important at the sea surface, signals are stronger, strated in the numerical simulations of interactions and mixing over the con- there are more observational data, and Stanev and Beckers (1999) where the os- tinental slope and shelf edge are due to numerical modeling has been success- cillations of produce mixing oscillations of the pycnocline and the ful. The Black Sea once again provides in the area of shelf edge, enhance the en- signals (“mixtures”) propagate along iso- an interesting case study—this time in trainment of surface water into the inter- pycnal surfaces into the open sea. These ventilation of the pycnocline. As dem- mediate layers, and affect vertical stratifi - processes are still not well explored, and onstrated in the numerical study of cation. This mechanism is reminiscent of there is a need for dedicated observations Stanev et al. (2003), ventilation occurs the “manganese pump.” The “manganese to support the above concepts. only during certain parts of winter, un- pump” hypothesis is a fundamental idea Mixing originating at the sea surface is like the ocean, where seasonal variability is manifested by the north-south excur- sions of outcropping isopycnal surfaces during the whole year. The above consid- erations become clearer if we remember that in the Black Sea, the ratio of the net 0 heat fl ux through the ocean surface to its seasonal amplitude is a small number 50 (~10-2) (Stanev et al., 2003). Therefore, ventilation of the intermediate layer is 100 maintained by the seasonal signal (not by 150 permanent structures of isopycnal surfac- es), which pumps cold water periodically 200 into the CIL (Figure 8). The replenish- 29.1°E, 41.6°N ment time of the CIW of about fi ve years 250 29.0°E, 41.7°N 29.3°E, 41.8°N estimated by the model supports the esti- 30.3°E, 41.9°N mates based on sparse observations. Depth (m) Depth 300 30.0°E, 42.2°N Ocean mixing is intimately related 350 31.3°E, 43.0°N to water mass formation, the latter be- 32.8°E, 42.2°N ing generally associated with air-sea ex- 32.8°E, 42.1°N 400 change. There is a long-lasting debate in 32.9°E, 42.0°N the Black Sea oceanography community 450 34.0°E, 43.1°N about the relative importance of differ- 500 ent mechanisms contributing to water -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 mass formation: cooling of shelf waters CFC-12 (pmol k/g) and their advection, or convection in the Figure 7. CFCs are passive tracers giving a valuable information about pathways of water open sea enhanced by mesoscale eddies. masses (Stanev et al., 2004). Here, we show vertical profi les of CFC-12 in the Black Sea Using results from numerical simula- (area mean and one standard deviation) as simulated by the Black Sea Modular Ocean Model (MOM). Th e accuracy of simulations in replicating the penetration of surface tions we show that the main issue here is, signals is supported by comparisons with fi eld observations. Data from the R/V Knorr instead, to what extent air-sea exchange 1988 cruise (for more details see Stanev et al., 2004) are plotted by symbols. Th e legend (Figure 9) is dominated by dynamics. gives the correspondence between symbols and station numbers. Th e outlier at 200 m is from measurements very close to the strait, thus, this value is higher, displaying deep It follows from the results of numeri- ventilation by the buoyant plume. cal simulations that not much CIW in-

66 Oceanography Vol.18, No.2, June 2005 trudes the pycnocline south of Kerch Figure 8. Simulated (a) tem- Strait (Figure 10) where air-sea exchange perature, (b) salinity, and (c) depth of isopycnic surfaces is strongest in winter (Figure 9). The ex- during 1991-1995 plotted in planation of this “discrepancy” is that at- time-density coordinates. Th e mospheric cooling in this area is almost coordinates of the location are 42.5°N and 31°E, which completely compensated by advection is in the interior of the west- of warm water by the Rim Current, thus ern basin. Th e ventilation in the Black Sea is confi ned in a dynamics reduces the water mass forma- very thin surface layer and is tion rates. Just the opposite situation is manifested by the outcrop- observed in the western Black Sea, where ping isopycnal surfaces in winter, which completely the Rim Current transports cold water submerge in summer as they originating from the northern shelves are overlain by light surface over the continental slope. Mixing with water. Note also the haline- dominated density in (b) and more saline open ocean water enhances the up-and-down excursions convection rates. of isopycnal surfaces (c). What was not realized in the earlier studies was that water mass formation is localized over a very narrow band along the shelf edge of the western Black Sea. This oversight could have been due to the coarse space/time resolution of the climatic data, precluding detection of such small-scale features. As demon- strated by the numerical experiments of Stanev and Staneva (2001) small-scale processes govern the ventilation regime not only in the bottom layer, but also at the fringe of the mesoscale eddies. Entrainment of the CIW by the in- fl owing Bosporus plume is another key mixing mechanism that enables a mixture of surface and intermediate waters (in much larger quantities than the infl ow from the strait) to penetrate the pycnocline. Consistent with this general idea, the simulations presented by Stanev et al. (2004) demonstrate that most of the CFCs penetrating the deep layers (Figure 7) have their source at the sea surface within the Black Sea rather than in the Marmara Sea! This penetra- tion occurs in the area of the abrupt continental slope, where dilution of the

Oceanography Vol.18, No.2, June 2005 67 Heat Flux (September-February) SST (September-February)

Figure 9. Surface thermal forcing (as diagnosed from the 5-degree-resolution Black Sea Modular Ocean Model (MOM) averaged during the cold part of the year. Left panel: heat fl ux. Right panel: sea surface temperature (SST). Th is fi gure would suggest that the northern Black Sea is the dominating area of water mass formation, the area south of the providing the largest contribution. However, mixing in the surface layer could bias the estimates based on heat fl ux data alone.

Convective Cooling effl uent is quite strong. The large values of turbulent kinetic energy observed by Gregg and Özsoy (1999) indicate that in this area, entrainment becomes dominant, supporting the simulations of Stanev et al. (2001).

HEAT AND SALT CONVEYOR BELT Slope (or gravity) currents originating from the Bosporus Strait present a chal- Figure 10. Simulations with the 5-minute-resolution Black Sea Modular Ocean lenge for numerical simulations. Slope Model (MOM) help to estimate the rates of water mass formation by convec- currents contribute to the lateral venti- tive cooling (Stanev et al., 2003). Th e results here are presented as the thickness of water column (m) intruding the pycnocline every year. Most cold water lation of the Black Sea because they are penetrates the CIL along the northwestern slope area. Th e advective cooling of the origin of the massive intrusions of the water column in these areas preconditions the penetration of surface cool- -enriched water that extend to ing down to the CIL. Furthermore, the mixing of cold shelf water with saltier open seawater increases instability and triggers convection along the conti- ~200 km from the coast. This situation is nental slope. Th e area of most effi cient cooling acts as a small (compared to highlighted by the outlier seen in Figure the basin surface) “throat” where the model predicts maximum penetration of 7, and in the more detailed observations cooled water into the CIL. by Konovalov et al. (2003). An explana- tion for these intrusions is given by the numerical simulations of Stanev et al. (2001) (Figure 11). In a motionless ocean, the “mean for the basin” suboxic layer (a biogeochem-

68 Oceanography Vol.18, No.2, June 2005 O2 (Plume) ical transition zone between oxic surface Figure 11. Stanev et al. (2001) and sulfi dic deep waters) (Murray et al., developed a reduced-gravity model for the buoyant plume this issue; Konovalov et al., this issue) originating from the Bosporus would intersect the bottom along a very Strait with 600 m horizontal resolution. Unlike the case narrow area, which is illustrated by the shown in the Box 1, Figure d, red strip in the bottom panel of Figure the bottom layer moves and 11. Numerical simulations demonstrate the surface layer is motionless. Stratifi cation of the motion- that the Bosporus plume displaces the less ambient fl uid (including ambient water reaching depths of ~250 chemical tracers) is prescribed m (upper and middle panel of Figure from observations. Th e model is coupled with a simple chemi- 11). Thus, it provides the positive (rela- cal model simulating the oxida- tive to ambient water) oxygen anoma- tion of H2S by O2. Th e upper lies in the layer 15 to 16 σ (a constant panel gives the distribution of t O in the plume, and the mid- 3 2 density surface, measured in kg/m ) (see dle panel shows the diff erence between O in the ambient also Konovalov et al., 2003). Because O2 (Ambient-Plume) 2 fl uid and plume. Th e bottom the ambient water is motionless in the panel gives an idea about the reduced gravity model, the high oxygen projection of the suboxic layer signatures extend only to ~50 km from on the bottom represented by the product between H S and the coast. In the real ocean, oxygenated 2 O2 (low values are due to either water is entrapped by coastal eddies a negligibly small concentration of H S in surface layers, or O (remember the large dispersion of drift- 2 2 in deep layers.) Th e narrow red ers in the Rim Current) (Figure 3) and strip follows a depth of ~120 propagates far from the strait. m, which is approximately the The above results lead to a hypoth- depth of sulfi de onset in the open sea (Konovalov et al., esis about the driving forces behind the 2003). As seen in the upper Black Sea heat and salt conveyor belt, de- and middle panels, the plume reaches a depth of ~250 m. Th e rived from comparing modeling results plotted area extends between and observations. As seen in Figure 10, 28.8°E and 29.51°E and 41.18°N the heat conveyor belt starts and ends and 41.6°N. Depth contours down to 100 m are plotted at the Black Sea surface (Stanev et al., H2S*O2(Ambient) with dashed lines with a con- 2003); however, only its origin is rela- tour interval of 20 m, the last tively easily detectable by the large values dashed line is isobath 500 m. of CIW intrusions. Because the rate of CIW intrusions is very sensitive to cli- mate change, the amount of CIW can be substantially reduced after several warm winters. This is highlighted in Figure 12, where the good correlation between simulations and observations gives us the confi dence to speculate that the ther- mal conveyor belt gives an integrated response of Black Sea to climate change (see Oguz, paper 2, this issue).

Oceanography Vol.18, No.2, June 2005 69 The functioning of the salt conveyor fi eld of theory and observations of the 1. Coupled Atmosphere-Ocean belt is different. Salinity fl uxes (in con- Strait’s control. So far, it is known that Models trast to heat fl uxes) are more localized the sea-level difference between basins Most ocean models are driven by data (in the river run-off areas and Bospo- controlled by water balance, regional cir- based on either ocean observations or rus Strait), showing a smaller seasonal culation, and direction of wind strongly atmospheric analysis data (“uncoupled” buoyancy signal and large annual mean affects the two-layer exchange; however, models). These models cannot resolve values. The deep branch of the Black modulation of these connected basins’ some very important ocean processes Sea conveyor belt is coupled with the climate by Strait’s dynamics and feed- resulting from air-sea exchange because surface water; the back mechanisms are still unclear. the atmosphere is predetermined. In ad- surface branch exports low-salinity Black dition to the inaccuracies caused by lack Sea water into the Mediterranean Sea. FURTHER PERSPECTIVES of interaction between the atmosphere The two branches “meet” in the interior Although recently much has been and ocean in uncoupled models, another Black Sea. The processes discussed in learned about Black Sea dynamics, there problem is the insuffi cient horizontal this paper reveal the multiple mecha- are still many questions and exciting is- resolution of available atmospheric data. nisms closing the loop of conveyor belts. sues to be addressed. We mention four The dominant processes in the coastal There is still much to be studied in the of them. ocean have small scales, which are usu- ally sub-grid scales in global atmospheric models. The question then is this: do simulations from numerical models forced by coarse-resolution data provide reliable estimates for the coastal ocean? One of the problems with using coarse- 50 resolution atmospheric analysis data 40 (Figure 1) to force fi ne-resolution ocean models (see Kara et al., 2005b) is associ- 30 ated with the errors close to the coasts, which can be attributed to the misrepre- 20 sentation of the land/sea mask (a matrix that defi nes which points in the hori- 10 zontal grid are dry and which are wet). 0 However, with the present-day resolution of available products, this problem can -10 be solved only partially even if the masks are corrected. More important is that in

CIL cold water content (m*°C) content CIL cold water -20 areas of large orographic slopes, coarse- resolution atmospheric models give large -30 Observations errors, highlighted by the fact that in Simulations these models, wind crosses steep topogra- -40 1976 1978 1980 1982 1984 1986 1988 1990 phy along the southern coast (Figure 13, Years lower panel), which is not very plausible. Figure 12. Temporal variability of the cold water content between isopycnal levels Using regional atmospheric models could σt =14.4 and 15.6. Observations and simulations with the Black Sea Modular Ocean Model (MOM) are shown. Th is variability is triggered by large-scale atmospheric forc- eliminate errors in regions with com- ing (Stanev and Peneva, 2002; Oguz, paper 2, this issue). plex orography (Figure 13, upper panel).

70 Oceanography Vol.18, No.2, June 2005 RegCM2 Wind Speed (m/s) and Curl (10-5/s) July 1992 48°N

47°N

46°N

45°N

44°N

43°N 5

42°N 4 3 41°N 2 26°E 28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E 1 0 ECMWF Wind Speed (m/s) and Curl (10-5/s) -1 July 1992 -2 48°N -3 47°N -4 -5 46°N -6 45°N

44°N

43°N

42°N

41°N

26°E 28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E

Figure 13. Th e regional atmospheric model RegCM2 with a horizontal resolution of 30 km has recently been set up by E. Peneva (manuscript in preparation) for an area extending from the Adriatic to the (the Black Sea is almost in the middle of the area). Th e model is driven by the ECMWF reanalysis data with 1-degree resolution. Th e problems with the wind crossing steep orography (lower panel) are removed in the regional simulations. Note the small-scale curl in the wind fi eld (upper panel) emerging in the coastal zone.

Oceanography Vol.18, No.2, June 2005 71 Furthermore, comparing these models In the Black Sea-Mediterranean Sea modeling and data together (e.g., assimi- with Figure 2 demonstrates that the wind cascade, a large amount of freshwater lation of data into numerical models), patterns resolved by the fi ne-resolution accumulated on land is exported towards it results in the best use of theory and regional atmospheric model have hori- the Mediterranean. It is natural to ex- observations, (3) operational models zontal scales that are comparable with pect that long-term variations of water produce valuable data sets, analysis of the scales of the coastal anticyclones. One transport through the straits may be which contributes to scientifi c develop- could expect that comparable oceanic key to understanding climatic controls ment (this possibility has been largely demonstrated in meteorology by the quality of recently available atmospheric Th e Black Sea is where most kinds of numerical models analyses and reanalyses, for example, based on primitive equations have been used to simulate ERA-40 data). Efforts in the fi eld of data assimilation in the Black Sea (Stanev, circulation and transport of matter... 1994; Korotaev et al., 1999) have already contributed to studying the Black Sea system and also towards developing op- and atmospheric scales in the coastal dominating the European river catch- erational capabilities (Besiktepe, 2003; zone could enhance coupled modes in ment area and their interaction with the Korotaev et al., 2003). The interest is the regional atmosphere and ocean. The regional and global . These particularly strong in the coastal areas, next step towards understanding fi ne- transports have undergone dramatic where most of operational products are scale coastal dynamics is coupling ocean evolution caused by global sea-level needed. Because the Black Sea is a typical and regional atmospheric models. change. Thus, the Black Sea-Mediterra- coastal sea with many specifi c features, it nean Sea cascade provides one more case can provide an important test region for 2. Cascading Basins study, this time in the fi eld of long-term developing operational models. The system of European coastal climatic transitions. This case study is We demonstrated in this paper that consists of several cascades, the biggest actually unique because there were situ- available numerical models for the Black of which includes Azov, Black, Marmara ations in the past when transport in the Sea are well developed and validated and Mediterranean Seas. (Here we speak Bosporus Strait was unidirectional, and (see also the references). Furthermore, about cascades, keeping in mind that the other times when it had ceased. These the merged products from satellite net transport in the straits is comparable transitions resulted in dramatic transi- altimetry provide very valuable data to to the transports in each layer, which is tions between different states of Black be assimilated in operational models, not the case in the Strait, for and Mediterranean Sea’s physical and with high level and quality of physical example.) The two-layer exchange in the biogeochemical systems, another exiting signals making it possible to accurately connecting straits modulates short- and problem to be addressed with the help of reconstruct both temporal variability long-term climate variability in each numerical modeling. and mesoscale circulation. When as- basin, introducing a number of feed- similated in numerical models, these back mechanisms, which are still largely 3. Black Sea Operational data ensure low errors in simulated unknown. Numerical modelling seems Oceanography fi elds as shown by comparisons with to provide a powerful tool to address Although operational oceanography has independent observations (Korotaev et dynamics of coupled basins, however specifi c practical goals, it can contribute al., 2003). The harmonization between straits are very narrow, and ultra-high- to further understanding ocean dynam- modeling (theory) and monitoring (ob- spatial resolution is needed—a big chal- ics because (1) operational oceanogra- servations) efforts seems to become a lenge for numerical models (Figure 14) phy needs and develops knowl- major challenge in the Black Sea ocean- (see also Peneva and Stips, 2005). edge of natural systems, (2) by bringing ography in the years to come.

72 Oceanography Vol.18, No.2, June 2005 Sea Surf. Height Year 0.90 (Dec 11) [10.0H]

Sea Surf. Height Year 0.31 (May 10) [10.0H]

Figure 14. Th ree cascading basins. In the recent joint eff ort of the author and H. Hurlburt, J. Metzger, B. Kara, and A. Wallcraft (all at Naval Research Laboratory), the Hybrid Coordinate Ocean Model (HYCOM) was set up for a triple cascade with a resolution of ~ 6 km. Th e fi gure shows two snapshots of the simulated sea level: winter (less devel- oped coastal anticyclones) and spring (initiation of anticyclonic circulation). Although the resolution is very coarse for reproducing realistic exchange between basins, the results are encouraging and motivate developing downscaling strategies, which can be applied to similar cases in other areas.

Oceanography Vol.18, No.2, June 2005 73 4. Coupling Between Physical and well preserved due to the anoxic bottom REFERENCES Biogeochemical Systems conditions. This information is not only Beckers, J.-M., M.L. Gregoire, J.C.J. Nihoul, E. Because the mixed layer depth is very of regional interest, but can contribute Stanev, J. Staneva, and C. Lancelot. 2002. Modelling the -infl uenced North- shallow, especially in summer, an accu- to revealing important changes in the western of the Black Sea. I: rate treatment of insulation penetration system. The modeling of the bio- Hydrodynamical processes simulated by 3-D is needed. The HYCOM, which includes geochemical process is quite advanced and box models. Estuarine Coastal and Shelf an up-to-date parameterization of opti- (Oguz et al., 2002), there are promising Science 54:453-472. cal processes in the upper layer Kara et results in the fi eld of nitrogen cycling in Besiktepe, S.T. 2003. Development of the re- al. (2005b,c), proved that the sea surface the shelf-continental slope area (Gregoire gional forecasting system for the Black Sea. temperature in the Black Sea is very sen- and Friedrich, 2004), and coupling with Pp. 297-306 in Oceanography of the and Black Sea: Similarities and sitive to water turbidity. This result is advanced circulation models can facili- Differences of Two Interconnected Basins, A. consistent with the evidence that shows tate the quantifi cation of biogeochemi- Yilmaz, ed. Proceedings on the Second Inter- that changes of water turbidity can cal functioning, in particular if national Conference on Oceanography of the change upper-layer stratifi cation (Gen- dynamics is addressed properly. Eastern Mediterranean and Black Sea held nady Korotaev, Marine Hydrophysical October 14-18, 2002, in , . Institute of the Ukraine Academy of Sci- ACKNOWLEDGMENTS TUBITAK Publishers, Ankara, Turkey. Gregg, M.C. and E. Özsoy. 1999. Mixing on the ence, Ukraine, pers. comm., 2005). Be- The author thanks all of his co-authors Black sea shelf north of Bosporus. Geophysi- cause turbidity in this sea is large, highly who contributed to the results in this cal Research Letters 26:1869-1872. variable, and controlled by external forc- paper, as well as to G. Korotaev, T. Oguz, Gregoire, M. and J. Friedrich. 2004. Nitrogen ing and biological processes, an interest- S. Besiktepe, D. Dietrich, and B. Kara Budget of the north-western Black Sea shelf ing possibility exists that in this basin, for their encouragement to prepare this as inferred from modeling studies and in-situ the feedback mechanisms between physi- review and for their useful comments. benthic measurements. Marine Ecology Prog- cal and biological systems could play an Many thanks to A. Zatsepin and S. Mo- ress Series 270:15-39. Kara, A.B., A.J. Wallcraft, and H. Hurlburt. important role in the overall Black Sea tyzhev, who made Figure 3 available and 2005a. A new solar radiation penetration scheme for use in ocean mixed layer studies: An application to the Black Sea using a fi ne- Th e Black Sea is one of the most interesting cases worldwide resolution Hybrid Coordinate Ocean Model (HYCOM). Journal of Physical Oceanography where one detects high-quality, short-time signals, as well as 35:13-32. signals of its paleo-evolution that are well preserved due to the Kara, A.B., A.J. Wallcraft, and H. Hurlburt. 2005b. Sea surface Temperature Sensitivity anoxic bottom conditions. to Water Turbidity from Simulations of the Turbid Black Sea Using HYCOM. Journal of Physical Oceanography 35:33-54. dynamics. Furthermore, the authors sug- for the comments of A.Z. about the con- Kara, A.B., A.J. Wallcraft, and H. E. Hurlburt. gest that if the Black Sea turbidity is en- trol of continental slope on the position 2005c. How does solar attenuation depth tirely or largely due to biology, a lack of of Rim Current. Figure 1 was plotted affect the ocean mixed layer? Water turbid- ity and atmospheric forcing impacts on the nutrients (or another cause for a loss of by G. Georgievski. This study has been simulation of seasonal mixed layer variability biomass) will have a signifi cant impact supported by the European Community in the turbid Black Sea. Journal of Climate on the overall circulation. (EC) contract EVK3-CT-2002-00090 and 18:389-409. The Black Sea is one of the most inter- by Offi ce Of Naval Research Internation- Konovalov, S.K., G.W. Luther, III, G.E. Fried- esting cases worldwide where one detects al Field Offi ce (ONRIFO) grant to the erich, D.B. Nuzzio, B.M. Tebo, J.W. Murray, high-quality, short-time signals, as well author during his visit to Naval Research T. Oguz, B. Glazer, R.E. Trouwborst, B. Clem- ent, K. J. Murray, and A.S. Romanov. 2003. as signals of its paleo-evolution that are Laboratory (NRL).

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