BLACK SEA OCEANOGRAPHY Understanding BLACK SEA 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 water 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 oceans. In particular, research on deep ventilation could facilitate understanding of similar controlling processes in the paleocean when the ocean’s conveyor belt was shallower. Be- cause water and salt balances are easily controllable and the scales are smaller than in the global ocean, this basin is a useful test region 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, PO Box 1931, Rockville, MD 20849-1931, USA. Oceanography Vol.18, No.2, June 2005 57 INTRODUCTION waters, 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 Sea of Marmara and cal environment (see Konovalov, this is- models are given in the papers of Stanev the Sea of Azov by the narrow Bosporus sue). The cold intermediate water (CIW) (1990), Oguz and Malanotte-Rizzoli and Kerch Straits, respectively. Its catch- mass, with temperatures lower than the (1996), Staneva et al. (2001), Stanev et ment area covers large parts of Europe mean annual temperature of the surface al. (2003) and Kara et al. (2005a). This and Asia (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 (river run- winter cooling followed by spreading of opments and perspectives. off, plus precipitation 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 ob- 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 salinity 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 main 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 Baltic Sea), 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 Bulgaria, 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, Germany. 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 Atlantic 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 eddy-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.