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Eddy Processes in Semienclosed Seas: a Case Study for the Black Sea

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Eddy Processes in Semienclosed Seas: a Case Study for the Black Sea AUGUST 1997 RACHEV AND STANEV 1581 Eddy Processes in Semienclosed Seas: A Case Study for the Black Sea NICKOLAY H. RACHEV AND EMIL V. S TANEV Department of Meteorology and Geophysics, University of So®a, So®a, Bulgaria (Manuscript received 14 December 1994, in ®nal form 29 November 1995) ABSTRACT The enclosed boundaries and small scales of some seas lead to the formation of speci®c physical balances, which motivates the oceanographic interest in studying the dynamics of semienclosed ocean basins. The focus in the paper is on the speci®c appearances of eddy processes when the basin scales and the ones of the topographic features are comparable with the baroclinic radius of deformation. The Black Sea is used as a test basin. Eddy variability is analyzed using simulation results and compared with existing observations. The Bryan±Cox model with horizontal resolution Dw 5 1/108 and Dl 5 1/68 is forced with annual-mean wind stress data. Buoyancy ¯ux at the sea surface is proportional to the deviation of the model density from the annual-mean climatological data. Sensitivity studies on different forcing and on the topographic control are carried out. Synoptic periods are estimated to be about 0.5 yr. Eddies form in the eastern Black Sea and propagate westward with a speed of about 3 cm s21. The narrow section of the Black Sea, between the Crimea Peninsula and the Turkish coast, strongly affects eddy propagation. Dissipation increases in the western basin, where eddies slow down and their scales become small. This process is dependent on topography, which is dominated by a large shelf area in the western basin. Eddy kinetic energy exceeds the kinetic energy of the mean motion over large areas. Energy transfer between external and internal modes shows that the topographic control and the nonlinear transfer almost compensate each other. Energy spectra indicate that an inverse cascade may occur. 1. Introduction to the very small exchange with the Mediterranean Sea. Experiments and theory have shown remarkable The freshwater input from rivers and the net balance progress in the past two decades, resulting in elucidation between precipitation and evaporation tend to decrease of some important physical processes, which govern the salinity in surface layers. The input of more saline water mesoscale/synoptic eddies. A large amount of knowl- through the Strait of Bosphorus compensates the salinity edge already exists for the Gulf Stream, Kuroshio, Ant- de®cit at the sea surface, and an extremely stable strat- arctic Circumpolar, and equatorial currents. However, i®cation is formed down to 200±300 m. It tends to de- the impact of large-scale turbulence and baroclinic in- crease the vertical mixing and favors the unique envi- stability on the circulation in semienclosed seas is not ronment of the Black Sea, which is manifested by the well understood. Regional studies could be of general existence of an anoxic layer occupying 90% of its vol- oceanographic interest, showing speci®c physical bal- ume. ances in areas where the Rossby radius of deformation The Black Sea can be regarded as a two-layer basin is comparable to the basin scales. Decreasing resolution in which the deep layer is much thicker than the upper to about 1/108, or less, and integrating models for suf- one (Fig. 2). Synoptic scales are larger than the corre- ®ciently long times, are realistic with present computers sponding scales in the neighboring Mediterranean Sea for entire (enclosed) basins if their size is small enough. [eddy diameters could reach 200 km (Latun 1990)] and In the present study we use the Black Sea as a test basin. may approach the width of the narrow section between Therefore, a short introduction to the physical ocean- the Crimea and Sinop (later in the text we refer to this ography of this sea is given below with a focus on the simply as the Black Sea narrow section). The growth topics discussed in the paper. of instabilities in this small basin is sometimes limited The Black Sea (Fig. 1a) is a typical example of an by the basin scales, which could give more relative ocean basin where the vertical strati®cation is dominated weight to the basin or subbasin mode oscillations. by salinity. This is due to the speci®c water balance and The Black Sea has a relatively simple coastal line in contrast to the Mediterranean Sea. The con®guration of the deep sea is geometrically even simpler, and the deep bottom topography is rather smooth. The continental Corresponding author address: Prof. Emil V. Stanev, Department of Meteorology and Geophysics, University of So®a, 5, J. Bourchier slope is steep in the southern and eastern Black Sea, Street, 1126 So®a, Bulgaria. and rather ¯at between the Crimea and the Bulgarian E-mail: [email protected]®a.bg coast (Fig. 1b), where depths less than 100 m dominate. q1997 American Meteorological Society Unauthenticated | Downloaded 09/26/21 06:08 AM UTC 1582 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 27 FIG. 1a. The Black Sea. Isobaths for 100, 500, 1000, and 2000 m are plotted. The straight zonal line across the sea and the points A, B, and C give the position of the cross section and of the isolated points where model data are analyzed. The thick lines show the sections where bottom pro®les are shown in Fig. 1b. Thus, regional conditions for the synoptic processes are Dynamic computations initiated early this century quite different over large areas in the deep sea than over (Knipovich 1932; Filippov 1968; Bogatko et al. 1979) the continental shelf. This motivates us to address in established the concept of a basinwide circulation gyre the paper the penetration of the synoptic eddies from (named recently in some studies Rim Current), with two the eastern to the western part of the sea, focusing on centers in the eastern and western basin, named ``Kni- the impact that the basin shape and bottom topography povich spectacles.'' Much less is known about the space have on their mobility. and time variability of the Black Sea circulation with FIG. 1b. Bottom pro®les along the thick section lines in Fig. 1a, plotted with different vertical discretization. The capital letters at each pro®le correspond to the section line from the coast to the open sea location (A, B, or C) in Fig. 1a. To more clearly illustrate the differences in the topographies, which are signi®cantly larger on the shelf and on the continental slope, the section lines are limited to the isobath 1400 m. Full lines correspond to coarse-resolution, dash lines correspond to ®ne-resolution (details on the resolution of the topography are given in section 2b). Unauthenticated | Downloaded 09/26/21 06:08 AM UTC AUGUST 1997 RACHEV AND STANEV 1583 velopment and analyses. In this paper, our interest is focused on the temporal dynamics, which is one of the most important issues of the Black Sea oceanography. We hope that elucidation of the regional dynamics could be of general oceanographic interest, showing possible results that may be expected in other basins whose scales are comparable to the Rossby radius of deformation. In the next sections we present a description of the model, analyses on the model phenomenology and on its phys- ics, followed by discussion. 2. Description of the model a. The numerical model and boundary conditions The model is based on the primitive equation nu- merical model of Bryan (1969) in the version docu- mented by Cox (1984). The momentum equations, the equation of quasistatics, and the equation of continuity, written in spherical coordinates (l, w, z) are FIG. 2. Mean vertical density pro®le (s units) for the Black Sea; t ]U ¹p solid line corresponds to initial density, dash line corresponds to h n 1 L(Uhh) 1 f 3 U 52 1A hhDU density at the end of the integration. ]t r0 1AvhU, (1) synoptic scales, though large surveys were carried out zz recently (Oguz et al. 1994) with a very dense coverage pz 52rg, (2) of the entire basin. Numerical modeling has resulted in some progress in L(1) 5 0. (3) the study of the Black Sea eddy ®eld too, but much still In the above equations U 5 (u, y,w) is the velocity remains to be done. The horizontal resolution was ®rst vector, Uh its horizontal component, f 5 2V sin(w)k, k reduced to 20 km for the entire Black Sea in the non- 5 (0, 0, 1), p and r are the pressure and density, n 5 linear diagnostic model of Bulgakov and Korotaev 1 for Laplacian mixing, and n 5 2 for biharmonic mix- (1987), which is rather coarse for eddy resolution in this ing. The advection operator L(m) and the Laplacian Dm basin. The Bryan and Cox GCM with horizontal reso- are de®ned as lution Dw 5 1/68 and Dl 5 1/38 was later applied by 21 Stanev (1988, 1989b, 1990) to study the variability of L(m) 5 {sec(w)a [(um)lw1 (ym/sec(w)) ] the circulation. Forcing functions with seasonal oscil- 1(wm) } (4) lations (wind stress, heat ¯ux, precipitation minus evap- z 21 21 oration, river runoff, and exchange through the Strait D(m) 5 sec(w)a {sec(w)mll1 [m wsec (w)]w}, (5) of Bosphorus) were used in different combinations to study the response of the model sea to external forcing. where m is any scalar quantity, a is the radius of the Some of the topics, addressed in these studies, to be earth, and subscripts l, w, and z denote differentiation.
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