In the Mediterranean
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Project FAO-COPEMED / Grandes Pelágicos’2000 SUBSCRIPT OBJETIVE 1 OBJECTIVE 1 : HIDROGRAPHICAL DESCRIPTION ......................................................2 GENERAL CIRCULATION IN THE MEDITERRANEAN................................................2 1. Circulation of Atlantic water ......................................................................................3 2. Circulation of Levantine Intermediate Water.............................................................6 STRAIT HIDROLOGY .....................................................................................................8 1. Salinity........................................................................................................................8 2. Temperature................................................................................................................8 3. Currents ......................................................................................................................9 4. Tide.............................................................................................................................9 ALBORAN SEA ..............................................................................................................12 CATALAN-BALEARIC..................................................................................................14 NORTH TUNISIAN COASTS ........................................................................................15 1. The Gulf of Tunis .....................................................................................................16 2. The Gulf of Hammamet............................................................................................17 3. The Gulf of Gabes ....................................................................................................18 HYDROGRAPHIC AND GENERAL CIRCULATION IN THE VICINITY OF MALTA............................................................................................................................19 1. General phenomenology and morphological characteristics....................................19 2. General Circulation...................................................................................................20 3. Synoptic scale phenomena........................................................................................22 4. Thermal Signature of Sea Surface Temperature.......................................................23 5. Phenomenology of Upper Layer Currents................................................................24 HYDROGRAPHIC AND OCEANOGRAPHIC CONDITION ALONG THE LIBYAN WATERS..........................................................................................................................25 1. Water Circulation .....................................................................................................25 2. The Surface Water Tempertature .............................................................................26 3. Salinity......................................................................................................................26 4. Meteorological Conditions .......................................................................................27 HIDROGRAPHYCAL BIBLIOGRAPHY ......................................................................28 Aim 1/ 1 Project FAO-COPEMED / Grandes Pelágicos’2000 OBJECTIVE 1 : HIDROGRAPHICAL DESCRIPTION GENERAL CIRCULATION IN THE MEDITERRANEAN The general circulation in an area like the Mediterranean Sea is in most cases the main factor controlling the circulation over the continental plateau and coastal areas. This general circulation is therefore the origin of the distribution of hydrological, biological, chemical, sedimentary and pollution characteristics of the different bodies of water. From now on, it becomes useful to grasp a summarised image of the hydrodynamic operation in the Mediterranean. The main force controlling general circulation of bodies of water in the Mediterranean results from its status of “concentration basin”. Due to different thermodynamic mechanisms (evaporation, transportation, formation of deep water in winter, etc.), the Mediterranean produces a relatively dense water which occupies 70% of the whole basin (MILLER and STANLEY, 1965) until it finally flows to the Atlantic. Opposite flows in the Straits of Gibraltar are due to the horizontal pressure gradients going to the Mediterranean on the surface layer and to the Ocean on the deep layer. The vertical spatial evolution of the density depicts a movement generally oriented to the East on surface layers, whereas on intermediate layers, the dominant movement aims to the West. However, the Coriolis force tends to generate cyclonic trajectories in the different sub-basins, beginning in the Straits of Gibraltar for surface waters, continuing in the north- east Levantine basin for intermediate waters and finally in the different sources of deep waters. However, there are significant exceptions to this scheme, particularly regarding surface waters: there are anticyclonic gyres to the West of the Alboran Sea, in many locations along the Algerian coast and in the Gulf of Syrte. Besides, there are several eddies in the central part of the different basins, especially in the Ionien Basin. However, the winds, which are very irregular in terms of intensity and spatial-temporal distribution, have an influence upon the general circulation, in particular in those regions where strong and irregular winds blow: for example the “Tramontana” has a very important role over the West side of the Gulf of Lion and along the North East coast of Spain. The salinity of the Atlantic waters grows as they progress to the East until they reach the Middle-West coasts. The hot and salted water which can be found on the surfance between the islands of Rhodes and Cyprus in the end of the summer, experiments an intense evaporation followed by a cooling in winter caused by dry and cold winds coming form high plateaus in Turkey. These atmospheric effects cause mixes in the first 150 to 200 metres, where a water body type is formed: Levantine water (t=15.7ºC, S=39.1o/oo, st=29.00), which flows in all the East basin at about 300 metres depth. This intermediate water goes slowly to the West. The moment it crosses the Sicily Channel, affected by the mixes of adjacent waters, these average characteristics turn to t=14.0ºC, S=38,75. This water feeds the West basin joining the cyclonic movement at an intermediate depth just below the Atlantic water and above the Deep Water. Aim 1/ 2 Project FAO-COPEMED / Grandes Pelágicos’2000 The global organization of this circulation corresponding to the characteristics described above, give the basin in the name of hydrodinamic laws, a North-South dissymetry, particularly in the West basin. In fact, circulation is very turbulent along the African continent and relatively stable along the European continent. On one side, the turbulences which would favour the dispersion of eventual pollution sources, would allow a relatively important development of the whole food chain. On the other side, a stable circulation parallel to the coast is not very favourishing, neither for the development of biological processes nor for the dispersion of polluting products. 1. Circulation of Atlantic water The major features of the Atlantic water path are globally well represented on the most common figures (figure 1, Ovchinnikov, 1966). The problem resides in the strong space and time variability which characterises this flow and in that estimations are often biased. This variability becomes evident particulary by two kinds of phenomena: the front areas and the medium-scale turbulent structures. In other words, the actual general circulation is not the image of current lines contracting in the straits and spanning in the succesive basins: currents encouraging superficial waters are strongly unstable and form meanders which generate eddies that can interact with them afterwards. Figure 1. Circulation of surface waters. In Ovchinnikov, 1966. Aim 1/ 3 Project FAO-COPEMED / Grandes Pelágicos’2000 The circulation of Atlantic waters between the Straits of Gibraltar and the Sicily Channel presents fundamentally different characteristics, which vary if one is along Moroccan, Algerian or Tunisian coasts. In the West, this circulation is relatively stable and closely related to the geography of the Straits of Gibraltar and the Alboran Sea. At the exit of this sea, the circulation is oriented from the Spanish coasts (2ºW) to the Algerian coasts (1ºW) virtually without stopping; this circulation takes the shape of a current vein which flows to the African coast and generally becomes stable from 1-2ºE (Millot, 1985). Then the meanders and the cyclonic eddies develop; the eddies derive to the East at a speed of several cm.s-1, but only the anticyclonic ones grow until they reach an approximate diameter of 100 km (figure 2). Then they separate from the coast and are sometimes identified by the thermographies for several months, when they can reach dimensions exceeding 200 km and go back to the coast to interact with the current vein (Taupier-Letage and Millot, 1987). These eddies should strongly condition the circulation along Tunisian coasts. No specific research has been conducted in the Sardinia Channel for the time being. However, it can be said that the circulation of waters, which is generally parallel to the Tunisian coast, can certainly become perpendicular in the presence of