Atmospheric interactions with the sea also play a key role in determining water circulation in the Mediterranean Sea. This circulation is complex because the Mediterranean is large enough to be subject to the same dynamics that characterize global ocean circulation (Robinson et al. 1992: 285, 2001: 1). The Mediterranean Sea is open only to the Atlantic Ocean at the Straits of Gibraltar and to Black Sea waters through the Dardanelles straits. The Mediterranean basin is divided into roughly equal western and eastern sub-basins, with the effective boundary between them lying at the Sicilian straits. Sea circulation is forced by water exchanges with the Atlantic Ocean and Black Sea, winds, buoyancy effects of different water masses at the surface due to water density (temperature and salinity) contrasts (thermohaline circulation), and topographic features including islands, coasts, narrows, and bathymetry. There are three types of water masses defined by depth: surface waters, intermediate waters, and deep and bottom waters extending to the sea floor. On an annual basis, evaporation, particularly intense in summer, exceeds the total input of rain and river outflows into the Mediterranean, resulting in water fluxes from the Atlantic Ocean and Black Sea. Most of the deficit is rectified by Atlantic water through Gibraltar (71%), with much smaller contributions from rivers (25%) and the Dardanelles (4%; Agouridis 1997: 3). Thus, water exchanges with the atmosphere drive the influx of lower-density ocean waters, which in turn determines the thermohaline circulation of water and many of the characteristics of the marine ecosystem in each basin (Zervakis et al. 2004: 1846).
Water from the Atlantic enters as a coherent surface stream because of its lower salinity, and thus low density, relative to the Mediterranean water. The flux is most intense in summer when evaporation over the Mediterranean is highest, creating a current of six knots or more. This buoyant stream flows eastward, becoming denser and less coherent as it mixes with other water masses and is affected by evaporation and convection. Along the way, it exhibits instabilities as it interacts with powerful gyres (see below), and bifurcates into multiple pathways, yet it reaches the Levantine coast as a distinct stream (Fig. 4.4).
By contrast, the volume of Black Sea Water (BSW) that enters the Mediterranean through the Dardanelles is much smaller and produces significant effects only in the Aegean Sea (Kourafalou 2007; Kourafalou and Tsiaras 2006; Fig. 4.5). The BSW brings less saline and thus less dense surface water into the Aegean to spread over the warmer and more saline intermediate waters. The general cyclonic (counterclockwise) flow of water around the Aegean is promoted by the north - and westward trajectory of the powerful plume of BSW exiting the
4.4 Mediterranean currents and water circulation. Drawing by Felice Ford after Roussenov etal. 1995: 13,516, fig. 1.
4.5 General sea-surface circulation flow in the Aegean. After Papageorgiou 2009: 209, fig. 3.
4.6 Typical positions of major cyclonic and anticyclonic gyres in the Aegean. Data from Olson et al. 2007; Sayin et al. 2011.
Narrow Dardanelles Strait. Driven by buoyancy, winds, and the complicated topography of the Northern Aegean, the BSW is deflected to the west along the rim of the northern Aegean coast, and subsequently driven by strong northerly winds down the eastern coast of Greece. Following seasonal patterns, the current is partially deflected to the Cycladic islands and the central Aegean, and partially pushed southward along the eastern coast of the Peloponnese, where it meets the eastward flow of Mediterranean waters from the Ionian Basin and is entrained by these and by the land masses of the Cretan Arc to flow east into the southern Aegean. Finally, the flow joins the Asia Minor Current (AMC), carrying warm, highly saline Levantine waters north along the eastern coast of the Aegean. When the AMC meets the outflow of the BSW, an intense thermohaline front is formed, which is partly responsible for the strength of the Dardanelles current. In this way the general cyclonic circulation around the Aegean rim is completed.
At sub-basin scale, Aegean currents are constantly affected by gyres and eddies. Gyres are oceanic surface currents driven by the interaction of the strong Dardanelles outflow with the topography of the Aegean sub-basin, including narrow straits and underwater basins, ridges, and plateaus; the complex shape of the shoreline; and the prevailing northerly winds (Fig. 4.6). Their spiral form is due to the interaction of pressure gradients and the Coriolis effect, which deflects water flow to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Sub-basin gyres typically have diameters in the range of 200 to 350 kilometers. They can be cyclonic (rotating counterclockwise) or anticyclonic (rotating clockwise) and their duration can be characterized as permanent, recurrent, or transient. They may also exhibit strong seasonal, interannual, or multiannual behaviors. They are distinct from, but analogous to, atmospheric cyclones and anticyclones. The current temperature and sea surface pressure generated by gyres may exert measurable effects on regional temperature and rainfall as they interact with atmospheric forces.
Similar hydrospheric interactions give rise to mesoscale eddies throughout the Aegean, and it is these that most directly affect local — including coastal — currents. Eddies are small-scale currents of water moving against the main current with a circular motion; in the Mediterranean they have diameters in the range of 10 to 14 kilometers, about one-fourth the size of oceanic eddies (Robinson et al. 2001: 5). Like gyres, they can be cyclonic or anticyclonic, and they may be transient or longer lived. They are fairly evenly distributed throughout the Aegean, but anticyclonic eddies occur with greater frequency around the edges of the basin, while cyclonic eddies are more prevalent toward the basin interior.
Eddies are spawned where currents are interrupted by topographic features, or where differences in pressure, temperature, or density exist. Fields of transient eddies form along the border swirl flow of sub-basin-scale gyres; they may then break off to meander in the open sea (Fig. 4.7). In the Aegean, the inflow of BSW at the Dardanelles sets into motion a series of conditions in which eddies emerge, beginning with strong anticyclonic eddies that form in response to the high pressure of the narrow outflow channel and the thermohaline front between the buoyant BSW and the denser Aegean waters. These anticyclones then spread out along the path of the BSW as effects of density gradients. Along the way, the irregular features of the shoreline, for instance headlands or peninsulas and deep inlets, modify the path and motion of currents, spawning eddies that can ride along linear coastlines and fill bays. Donald Olson and colleagues (2007) describe the effects of eddies in the large Thermaic Gulf (with Thessaloniki located at its head). Some of the BSW flow enters the gulf, creating a dense pattern of eddies, dominated by cyclones at the mouth of the bay and anticyclones within it. The anticyclonic rotation forms a rim current that is intensified by the inflow of three major rivers (the Axios, the Aliakmon, and the Loudias) into the gulf. The current is further complicated by the effects of two northerly winds, the meltemi and the local vardari, a strong northwesterly valley wind that blows irregularly from the Axios (ancient Vardar) river valley (Olson et al. 2007: 1904),2 As well as a sea breeze that forces strong diurnal surface currents within the gulf (Hyder et al. 2002). Contexts such as the Thermaic Gulf amply demonstrate multiscalar atmospheric, hydrospheric, and terrestrial forces interacting to create local conditions that are highly malleable and distinctly seasonal (Kourafalou and Barbopoulos 2003).
4.7 Distribution, size, and intensity plots of eddies in the Aegean. Olson et al. 2007: 1914, fig. 15. Courtesy American Meteorological Society.
World History
