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

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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.

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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.

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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 an eddy and bring vertical movements to the coast. Those movements could have a direct influence upon the distribution of nutritive salts and the phytoplanctonic biomass. These eddies disturb the circulation of Levantine Intermediate Water (Millot, 1987-b) and take the Atlantic origin waters to the Algerian coasts out to sea. Their influence can be noticed in the North but not in the East, since their progression is blocked by the development of the continental plateau along Tunisian coasts.

Figure 2. Circulation of Atlantic origin water. In Millot, 1987-a.

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Figure 3. Horizontal distribution of the minimum of salinity and flow of the Atlantic current. In Brandhorst (1977).

The way the Atlantic current flows along Tunisian coasts has been outlined by Brandhorst (1977). Based upon the horizontal distribution of the minimum of salinity, this author has been able to establish that the Atlantic current vein, interesting for its 200 first metres, surrounds the Cap Bon and is oriented to the South. In the North of Lampedusa, this current is divided into two branches: the first one going to the South-East leaving the Island to the west, and the second one oriented to the South and feeding the circulation along the Gulf of Gabes (figure 3).

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2. Circulation of Levantine Intermediate Water

As for the general circulation of LIW, its study is easier because its hydrological characteristics make it less complex to identify. It is simple to reveal the presence of LIW, since its influence is mostly characterised by temperature and salinity maximum rates ranging from 14,10ºC and 38,75 in the Siculo-Tunisian Channel to 13,25ºC and 38,50 in the Straits of Gibraltar (Guibout, 1987); outside the main veine, the intermediate maximum rates can be slighltly weaker. The density of LIW is intermediate between that of the superficial water with an Atlantic origin and that of the deep water formed in winter in the North of the West basin. LIW does not experiment very noticeable seasonal variations in the Straits of Sicily, but it presents a strong seasonal variability along the Spanish coasts (Font et al.1988) and in the Straits of Girbaltar. As a result of its determinant intervention in fundamental processes such as the formation of deep water, it experiments all along its trajectory the effects of mixes and it can be affected by different phenomena on a smaller or larger scale.

On a first approach, the circulation of LIW results from a balance between the pressure gradient and the Coriolis force along with an entrance by the Channel of Sicily and an exit in the Straits of Gibraltar in extreme conditions. With such hypothesis, the role played by bathymetry at the continental slope should be determinant.

Basing upon these hypothesis, which are consistent with the current data analysis, Millot 1987-a (figure 4) has proposed a relatively simple and easy-to-verify circulation scheme. According to this scheme, a main veine along the coasts of Italy, France and Spain along with the bags of LIW found in the Algerien basin are carried there from the coasts of Sardinia by middle-scale tourbillons (Millot 1987-b). The role of the Thyrrenian Sea can be stressed, since it is there where LIW makes virtually the complete turn and where, partly due to a not very intense circulation, the double-diffusion phenomenon at the level of LIW causes a clear stratification of the deep layers. Note that the sheme proposed by Millot is very different from that elaborated by Wüst (1961) (figure 5).

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Figure 4. Circulation of Levantine Intemediate Water. In Millot 1987-a and Benzhora and Millot, 1993-a.

Figure 5. Circulation of Levantine Intermediate Water. In Wüst (1961).

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In any case, the evolution of LIW beyond the Siculo-Tunisian circle has a discordant point. In fact, there are three possible exit routes for LIW (Lacombe and Tchernia, 1972): - A branch recirculating around the Thyrrenian Sea and which would re-exit by the Sardinian Channel in the South of the island. - A branch entering directly into the Sardinian Channel with no recirculation. - A branch evacuating towards the Ligure Sea through the Corsica Channel.

All the present problems are centred in the definition of the proportions regarding these three branches (in case they exist).

STRAIT HIDROLOGY

The Strait of Gibraltar has an average length of 20 km and a maximum depth of about 1200 m. It represents a natural border that separates both the Atlantic Ocean and the Mediterranean. Due to its location, it is a privileged place for fishing migratory species, specially Tuna species that migrate from the Atlantic through the Strait for spawning purposes.

1. Salinity

The Strait of Gibraltar is characterized by two water masses: a superficial one from the Atlantic, slightly saline (s= 36.6‰) and another from the Mediterranean with higher salinity (38‰). On the west side of the Strait, salinity reaches 36‰ for 140- 200 m deep. For depths greater than 240 m, salinity can range from 38.21‰ to 38.3‰. As regards the western part of the Strait, the observed salinity values range from 36.3‰ in the south to 37‰ in the north. As regards deeper locations, the observed salinity values are those characteristic for the Mediterranean waters: 38.4‰. These values are highly affected by tides (Bouayad, 1977).

2. Temperature

The Atlantic current is an inflowing water mass of variable depth, running easternwards, and hoter than the Mediterranean water mass. As regards the Spanish coast, this water mass is slightly hoter for the whole year.

Though the temperature becomes lower with depth, the gradient corresponding to this descend varies between locations, greatly affected by the height of the tides. Seasonal sea surface temperature, highly variable, depends on the interchanged volume of water among both oceans. Sea surface temperature is also dependent on the tidal coefficient as well as tidal height. (Erimesco, 1965)

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3. Currents

Apart from the tidal currents, the Strait of Gibraltar is affected by currents due to water interchange among the Atlantic and the Mediterranean Sea (Figure 6).

Water from the Mediterranean Sea, denser and with higher salinity, inflows to the Atlantic Ocean in the south part of the Strait following an in-depth way. On the other hand, in the north part of the Strait, inflowing water masses from the Atlantic follows a surperficial path (Figure 7), which is in part due to the topography of the Strait: deeper in the Moroccan coast as compared to the Spanish coast (Bouayad, 1977).

Tidal currents superimpose those currents resulting in water mixing processes that follows the currents direction (Erimesco, 1965).

4. Tide

Two independent tides, one from the Atlantic and another from the Mediterranean, drive water interchange between both seas and give rise to seemingly irregular currents in the Strait of Gibraltar. Up to 200 m in depth, the direction of the current at Spartel changes depending on the tide: westernwards during low tide and easternwards during high tide

Tidal currents give rise to the so called “raz de courants” phenomenum; suddenly waving processes in previously steady waters. These phemomena, due to convergence of antagonist tidal currents, are characterized by short duration.

In adition, the Strait bottom, due to its rough topography (Figure 8), induces currents deviation. This complicated system is on the basis for the observed irregularities in temperature and salinity (Bouayad, 1977).

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Figure 6.- Tidal currents (S.H.O.M in Rhazi, 1987)

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Figure 7.- Surface currents in the Strait of Gibraltar (speed in knots) (S.H.O.M in Rhazi, 1987)

Figure 8.- Strait of Gibraltar bottom topography (Erimeson, 1964)

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ALBORAN SEA

The Alboran Sea can be considered as a channel of about 170 km wide, where the complex phenomena produce its condition of waiting room between two different oceanographic basins as the Atlantic and the Mediterranean.

The difference of mean sea level, between western and Eastern the sides of the Straits, that is of about 14 cm, causes the entrance of an Atlantic water vein. In the gulf of Cadiz, next to the Strait, we can distinguish two types of masses of water:

- Atlantic Central Water (A.C.W), coming from the Atlantic subtropical convergence zone, located between the 100 and 1000 meters, with a salinity that decreases with the depth between 36,2 % and 35,4% and temperature between 16ºC and 9ºC; - Atlantic Surface Water (A.S.W), that directly covers the layer influenced by the seasonal changes. The salinity of A.S.W., that decrease slower than A.C.W. with the depth, is between 36,2% and 36.4%.

These two types of water are mixed partially in the Strait and constitute the water vein that enters the Mediterranean through Alboran. The temperature of this water can slightly be colder than Mediterranean ones of Alboran in summer, and hotter in winter.

The A.C.W., that is in the western side of the Gibraltar Strait at 100m (Gallagher et al., 1981) due to the upwelling of this type of water which takes place in the Moroccan Atlantic coast, goes into the Alboran Sea in mayor amounts when it comes from the south part. The type (A.C.W) only is found at the eastern part of the Straits in died tide in the lintel, due to the amplitude of the internal waves with the high tide produces a great mixture which makes disappear (A.C.W). (Gascard and Richez, 1985).

The Atlantic water vein that enters Alboran following the axis of the Strait, (the axis can be located in the 15ºC isotherm to 100m of depth), displays instabilities in form of meanders and eddies that are of great importance in the environmental characteristics since they generate processes of convergence and divergence of interest in the characterisation of the different masses of water (Cortes, Gil, García, 1984).

The Atlantic water vein creates a permanent termohaline front of about 30 km wide, which constitutes the North border of a permanent anticyclonic turn that occupies western half of the Alboran Sea. This cycle accumulates light waters in funnel form and although it has permanent character, his position and size is variable (Cheney and Doblar, 1982). Explanations of all type that justify the appearance of the turn, related to vertex (Nof, 19785, experiments of Laboratory (Miller, 1979) and models of gravity reduced of a system of two layers have occurred (Preller and Hulburt, 1982).

In the surface layer of variable thickness of the Atlantic water, a seasonal termocline that sometimes is united to the due one to the mixture layer between the Atlantic mass and the Mediterranean one, begins to form from May until October producing a thermal gradient that can reach 50m. The maximum temperatures can range the 24ºC and the warmest waters

Aim 1/ 12 Project FAO-COPEMED / Grandes Pelágicos’2000 of Alboran are in the Eastern zone and the centre of the western anticyclonic turn. The thickness of the isothermal layer over the termocline is greater in autumn than in spring summer. A minimum of superficial salinity is observed in the zones where there is a accumulation of water of Atlantic origin, because the superficial evaporation causes a increasing of salinity in surface, and the mixture with Mediterranean saltier waters does it by down. Following this minimum the movement of the Atlantic water through the sea of Alboran can be seen and in the minimum east and at the convergence zones it is to greater depth.

The predominant winds in the Alboran Sea are Eastern and the Western. The Eastern are winds of the east associated to high pressures to the north and the northwest. The Western are winds of the west, associated to storms located to the north and the northwest of the Peninsula. The western winds favours the Atlantic water entrance, doing that the Atlantic water vein follows parallel 36ºN (Grill, 1984), leaving an wide zone between it and the Spanish coast, where cyclonic eddies can be produced. When the regime is of the eastern winds, seems that all this situation is reversed and it is observed that the turn occupies all the Western basin of Alboran. In this case, there is no space for the formation of cyclonic eddies and diminish the upwelling possibilities (Gil, 1985a). In the coastal zones the changes of wind regime affect of a remarkable way the thermal characteristics of the superficial layers (Gil, 1985b), being able to appreciate in spring-summer variations of until 5ºC in the surface temperature. A great variation with the change of wind regime has not been detected in the medium and great scale circulation. Although it seems that the response times are different in the superficial layers that in deepest (P.J Bucca and T.H Kinder, 1983): the Atlantic layer, that can be considered conventionally over the 37,5%, undergoes a seasonal variation, mainly in the coastal zone, with a minimum thickness in winter and a mayor values in autumn. (Gil, 1988).

The Atlantic vein, that surrounds anticyclonic cycle, passes to the south of the island of Alboran taking direction NE, and feeds a second anticyclonic eddy in the Eastern zone on Alboran. In other cases the current once crossed the meridian of the island of Alboran continues in east direction, to turn towards the north to the 1º, turning again towards the east once exceeded parallel 36ºN. So, it originates a circulation of cyclonic type (Lanoix, 1974; White, 1977, 1978). Regarding the causes of appearing a type and another one of circulation, some explanations related to have occurred relating the variations of the Atlantic flow (Herburn and the Violette, 1987; Werner et al.1988), and others where the topography of the coast (Whitehead and Miller, 1979) and the direction of entrance in the proximities of the island of Alborán influence (Cano and Gil, 1984). The Modified Atlantic water (MAW) that arrives until the Cabo de Gata finds the one which it arrives in Mediterranean direction SW. This produces a convergence and MAW it is pushed towards, originating a termohaline front in the Almeria-Orán line (Tintoré et al., 1988). Part of this water is retained by the Eastern anticyclonic eddy and follows the direction E, throughout the Algerian coast. A branch of the Algerian current takes direction N and happens through the Balearic channels forming the Balearic front.

The existing saline balance in the Mediterranean takes place when saltier waters and less amount than the one than it enters by surface leave by the lintel of the Strait. On the type of water that flows to the Atlantic there are several proposals. Stommel et al. (1973) suggests Aim 1/ 13 Project FAO-COPEMED / Grandes Pelágicos’2000 the deep exit of water (S=38,44% B = 12.9ºC) due to a process of absorption by Venturi effect. Bryden and Stommel (1982) observed a deep water flow in the direction of the Straits of Gibraltar following the African coast. Kinder and Parrilla (1987) have observed exit of this type of water.

The Mediterranean intermediate water that is mainly in the north part of Alboran Sea (Parrilla and Kinder, 1984), (with values of B= 13,2 S (38,45 - 38,50)), has its origin in the Eastern Mediterranean, between Rodas and Cyprus, originating itself in the months of February - March with B = 15ºC and S=39,1%, which produces a vertical termohaline circulation that is extended gradually towards the western river basin. At the Straits of Sicily it reaches values B = 14ºC and S=38.70%. This intermediate water, in pure or mixed form with the deep water, also comprises of the flow that leaves the sea from Alboran to the Atlantic through the Strait.

CATALAN-BALEARIC

The termohaline general circulation in the Catalan-Balearic basin is of cyclonic type (Font and Miralles, 1978). Following the coast in SW direction, it runs the Catalan current, whose origin is the Provenzal-Ligur current, that transports waters near Córcega towards the Gulf of Leon and Catalan sea (Prieur, 1981), and the current of the Rhone that transports waters of fluvial origin until the Cabo de Creus. The Catalan current is associate to an enormous termohaline front that originates between continental fresh waters in the platform and the densest exteriors to it (Font et al., 1986). A branch of the Catalan current diverges when arriving at the Gulf of Valencia and the rest follows in South direction by the channel of Ibiza. The Balearic current is associated as well to a front whose cause is the relatively fresh water entrance of Atlantic origin by the Balearic channels. Therefore, the cyclonic termohaline circulation in the Catalan Sea intimately is bound to these two fronts and its intensification produces an increase in the speed of the currents. The Catalan front is characterised by salinity differences and the Balearic one by temperature differences. Instabilities at medium scale like eddies, filaments, etc, associated these fronts have been observed (Tintoré, 1988 ; Wan et al., 1988).

Regarding the types of masses of water of the Catalan sea, it can be differentiated three zones (Salat and Font, 1985).

- Waters of variable salinity of continental origin in the shelf, occupying the maximum extension in spring due to the greater fluvial contribution; - Waters at the centre zone, typically Mediterranean density waters; - Relatively fresh waters of Atlantic origin in the part of the Balearic Islands.

At the shelf, due to the contribution of fluvial origin and to the formation of the seasonal termocline in spring-summer, it forms a layer submissive the drag due to the wind and the seasonal variations that does not follow the general the termohaline current strictly. In winter the SW current at the shelf, arriving the Catalan current is reinforced until the Gulf of Valencia (Font, 1986). The vertical distribution is characterised by a layer of variable thickness with continental influence, put under seasonal changes, and a second layer Aim 1/ 14 Project FAO-COPEMED / Grandes Pelágicos’2000 between 100 and 200 ms where it appears the intermediate water of winter, formed by coastal water cooling with B % 13ºC and S % 38,1%-38,3% (Salat and Cruzado, 1981), and between the 300 and 500 ms there is a intermediate water B=13.2 S% 38,50%, and below 500m we found water of bottoms with B% 12,80 and S % 38.45%.

Upwelling of superficial waters in the shelf have been observed attributed the abrupt widening of the shelf, to the height of Salou cape, where the general current finds a barrier in its movement towards SW (Font et al., 1986).

NORTH TUNISIAN COASTS

Circulation along north Tunisian coasts must be considered a part of the different hydrodynamic mechanisms affecting the region delimited by Tunisia, the Sardinia Channel and the Straits of Sicily. This region forms the junction of three major subsets in the Mediterranean: the Algéro-Provençal Basin in the west, the Tyrrhenian Sea in the north and the East Mediterranean in the east (Ionian Basin). The transfers of water masses from one of the three subsets towards the other two are controlled by this common region. The water with an Atlantic origin (MAW) flows from the Algerian current through the Sardinia Channel forming a more or less diffused current (Millot, 1987). Although it has been well known for a long time that some of this water goes to the East Mediterranean (Ovchinnikov, 1966) after having passed the north Tunisian coasts, the quantitative estimation of flux is still not very accurate. The same holds with the intermediate Levantine water (LIW), which comes from the East Basin and penetrates into this region leaving Sicily to the west.

The Sardinia Channel is the least studied region of the West Mediterranean. Garzoli et al. (1979, 1981) observed two eddies, the permanence of which is not verified and stated the hypothesis of the separation of LIW into two branches in the south of Sardinia, the first one being parallel to African coasts and the second one being parallel to the coast of Sarde. The most recent results based upon the analysis of the data from MEDIPROD 5 campaign (Ben Zohra et Millot, 1993) do suggest that a part of LIW flows towards the east along the Algerian slope. Today, the reduced number of observations carried along Tunisian and Algerian coasts, can confirm or invalidate these hypothesis. Furthermore, there is no seasonal truthful estimation available about the fluxes through straits. In spite of its paramount importance, the Siculo-Tunisian Straits has not received the attention it deserves. However, some research conducted by Lacombe et Tchernia (1960), Wüst (1961), Ovchinnikov (1966), Grancini et Michelato (1987), Manzella et al. (1988, 1990)..., has provided some general aspects regarding the hydrology and the circulation of water masses, particularly in Sicily, although they are not clearly specified.

Within the frame of a research and development project regarding fishing carried out by FOA and INSTOP, Brandhorst (1977) reports some qualitative features about the currentologie along Tunisian coasts. These are based upon the hydrologic observations gathered in three observation campaigns. Brandhorst suggests that the sinking of isopycnal surfaces in the Galite Channel indicates either a water cascading of the heaviest waters Aim 1/ 15 Project FAO-COPEMED / Grandes Pelágicos’2000 on the channel edge or a strong current towards the East on its Southern flank. The strong bottom currents capable of hindering the trawling operations have often been observed in this area. At the level of the Sardinia Channel, the decrease of surface isopycnals towards the South indicates an intensification of the Atlantic current as the Tunisian coast is approached. However, not far from the shore (10 miles) the distribution of st values reverts, indicating the presence of a local crosscurrent towards the East.

1. The Gulf of Tunis Among the existing attempts to describe qualitatively the currentology of the Gulf of Tunis, we are going to quote the work of M. Belkhir and M. Hadj Ali Salem. These authors mainly wrote:

The soft winds blowing from the N-NE sector tend to push marine waters towards the S- SW coasts. When marine waters reach the middle of the axis that joins cap-Cartage and ras- Dourdas, they find a hotter and hyposaline water current coming from the opposite coastline sector S.W. This coastal flow is born near Oued Méliane and flows to the west coasts of the Gulf of Tunis up to cap Cartage, where it finds the marine waters which divide it into two branches. The first branch goes towards the N.W. and surpasses a water layer in opposite direction. This water layer is the translation of a deep crosscurrent coming from N.W.; this crosscurrent reinforces the second branch of the coastal flow, imposing its direction N.W.-S.E. towards the Sidi Errats - Korbous coasts, where it goes up with enough strength to push the hot thermal hyposaline waters, that find the marine water flux out to sea. Once out to sea, the thermal waters find a marine water flow with opposite direction induced by the winds N-NE., plunge in depth and thus explain the presence of a hotter and less saline water layer detected at 5-10 m depth in zone II (central zone of small Gulf of Tunis; immediate border to the coastal area).

In the region of the Gulf of Tunis, which was studied by P. Lubet et A.Azouz (1969), salinities remain relatively low all year; near 37 p.s.u. The Atlantic waters enter it in winter and spring; in summer and autumn the mixed waters (with salinities ranging from 37 to 37.5 p.s.u.) are predominating. The waters of the Gulf of Tunis are not submitted to the action of permanent currents. Only the dominant winds from the west coast can cause the formation of local rotating movements in the Gammarth bay. However, we can find a surface current NW-SE at 0.8 knots in the north of the Gulf and in the Siculo-Tunisian Straits. This current was evaluated by M.A. Guyot (1951) and drags the surface waters towards the East Basin of the Mediterranean.

Azouz concludes, very accurately, that the hydrologic study of the north coasts of Tunisia reveals complex phenomena deserving a deep study. Azouz states that the region –whose morphology is very rough, bristling with deep bottoms, reefs and pierced with multiple channels – is submitted to two opposite influences; that of the surface waters from the Atlantic, having their greatest extension in winter; and that of the Southern Basin, which seems to surmount towards the surface from spring to summer. It would still be necessary to precise the nature and the direction of the currents that play an important role in the distribution of water masses.

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2. The Gulf of Hammamet

The Gulf of Hammamet is close to the Siculo-Tunisian Straits, through which the exchanges of the two Mediterranean Basins take place. In the one hand, these exchanges are transferred by an Atlantic vein (Mediterranean Atlantic Water), relatively hyposaline flowing towards the Southeast between the surface and 200 m depth. On the other hand, they are transferred by a vein of Levantine intermediate water (LIW) circulating towards the North West between 200m and the bottom.

Taking into account its geographical situation and the depth of the water column, the Gulf of Hammamet is more specifically concerned with the Atlantic water (MAW) than with the LIW. The MAW flows along African coasts before contouring the Eskerkis bank, where it splits to form a first branch which flows along the north Sicilian coasts towards the Thyrrhenan Sea and a second branch which enters the Sicily Channel contouring the Cap Bon (Garzoli et Maillard, 1979; Morel, 1971). The recirculation of Atlantic water into the Gulf of Hammamet is relatively unknown. In fact, data obtained in situ with a correct space-time resolution are almost non-existent. Although they need to be validated a posteriori with in situ observations, the appearance of general circulation numerical models such as the one implemented in the Mediterranean by the group POEM (Physical Oceanography in the Eastern Mediterranean, 1992) or "EUROMODEL", provide several at-first-sight aspects regarding the local circulation in the Gulf of Hammamet. For the needs of our study, we have extracted from the files of the "EUROMODEL" (C. Herbaut, 1994), the current charts concerning our area of study, especially in August (figures n3-4) and February (figures n 5-6). This three-dimensional model, with primitive equations is forced by a realistic wind, by vertical air-sea fluxes and by horizontal fluxes in the straits.

Along the Gulf of Hammamet, the model returns correctly the flowing of the Atlantic water vein. Several miles away from the shore, the Atlantic current is mainly oriented towards the South, with values of 15 cm/s on surface and 10 cm/s at 40 m depth. In the actual Gulf of Hammamet, a cyclonic recirculation is identified at 40 m depth of the MAW with horizontal speeds of 1 or 2 cm/s. In summer (figures 3-4), the waters of the Gulf of Hammamet are thus regenerated by the Atlantic water vein. In winter, the apparent convergence of surface waters towards the coasts of the Gulf (figure 5) as well as the flowing towards the South East of bottom waters (figure 6), makes us assume the establishment of a vertical agitation during this season.

Brandhorst’s work (1977), shows that in summer (figure 1), the water column close to the site of Zelfal (within a 20 Km radius) presents a strong homogeneity in terms of temperature (T~ 22°C) and salinity (S~37.1 psu). Nevertheless, a levelling of isopycnals can be noticed (figure 2) (25.8< t< 26.2). In the case of an adventitious flowing being weakly diffused and conserving the potential density, this levelling can favour an elevation of the water (and also of particles) from the bottom (~ -50 m) to the surface and in direction out to sea.

It must be mentioned that contrary to the stability of the characteristics of LIW, the hydrological parameters of MAW present a seasonal variability (Manzella et al; 1988). This Aim 1/ 17 Project FAO-COPEMED / Grandes Pelágicos’2000 variability is marked by the evolution of the depth of the salinity minimum (characteristic of the heart of MAW). In winter, the heart of MAW is on surface whereas in summer it is situated on sub-surface (-50m) due to the strong evaporation.

3. The Gulf of Gabes A- Overview of physical characteristics

Hydrodynamics

In the north of the Italian island Lampedusa, the Atlantic current splits in two branches: the first one towards the south-east leaving the island to the west and the second towards the south, feeding the circulation along the Gulf of Gabes. The flux of MAW presents a well-marked seasonal variability.[Manzella et al., 1988]. The intensity of the second branch is consequently subject to fluctuations which affect directly the circulation along the Gulf of Gabes. In fact, in this region there is a local cyclonic permanent circulation [Ovchinnikov, 1966; Lacombe et Tchernia, 1972] which leaves a more or less important part of its waters to the sub-branch of the Atlantic current. However, it is difficult to quantify accurately the currents near this site taking into account the lack of proper in situ measures.

Hydrology

The Gulf of Gabes occupies a vast region of shallow water, which makes it susceptible to the effects of differential heat. In winter, the ambiant air, relatively cool, obtains enough heat from the water column, leaving it isothermal. This shallow region is in thermal contrast with the waters from deeper regions in the east of the plateau. In these deep regions, the heat that the surface layers lose to benefit the atmosphere is replaced by that available in deeper layers. On the contrary, this heat source is not available in the waters from the continental plateau -very shallow- and the water remains cold. Therefore, in order for this phenomenon to exist, the thickness of the shallow water must be weaker than that of the mixed layer. The localisation of fronts corresponds to a bathymetric countour equal to the depth of the mixed water (40 to 50 metres) and are easily detectable by satellite images. (figure 1).

This kind of phenomenon also takes place in summer, when the mixing in the shallow water region (near the coast) continues to be easier than in deeper regions. This has been observed in different Mediterranean regions, especially in the Adriatic (Bignami et al., 1990a-1990b), the Ligurian Sea and the Aegean Sea. It is the Sugimoto-Whitehead effect, dealing essentially with continental plateaux. The comprehension of meteo-oceanic mechanisms leading to the apparition and the persistence of this strongly energetic phenomenon constitutes an essential phase in order to apprehend the circulation of the waters of the Gulf of Gabes and their rate of renewal.

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HYDROGRAPHIC AND GENERAL CIRCULATION IN THE VICINITY OF MALTA

Hydrographic measurements in the vicinity of Malta are generally lacking. The first-ever physical oceanographic campaign in the Maltese coastal waters was carried out by the Physical Oceanographic Unit (University of Malta) in summer of 1992 during a two-week survey intended to study the circulation inside two embayments namely those of Mellieha and St. Paul’s. Bay. The data collected has enabled the identification of important physical processes characterising this coastal stretch over a range of spatial and temporal scales. This was followed by several other physical oceanographic surveys in 1993/4. The main area investigated included a considerable stretch along the NW coast of the Maltese Islands in which the phenomenology of the sea currents and water column structure was studied. No local hydrographic data however exists in the area to the south of Malta.

The contents of this part of the report are thus extracted from a paper entitled ‘A general description of the hydrodynamics in the Strait of Sicily and the Sicilo-Maltese Shelf Area’ presented by A. Drago at the kick-off meeting of WP6 of the Mediterranean Forecasting System Pilot Project (MFSPP) in Athens, December 1998.

1. General phenomenology and morphological characteristics

The Strait of Sicily is a large and dynamically active area connecting the eastern and western Mediterranean sub-basins. The flow in the region is mainly driven by the Mediterranean thermohaline circulation as well by local atmospheric fluxes. It consists of a two-layer flow with fresh modified Atlantic water (MAW) near the surface and a deeper salty Levantine outflow.

The influence of the strait geometry on the dynamics is very important. The fan-shaped configuration of the land boundaries has its narrowest constriction on the western extremity where the channel between Cape Bon (Tunisia) and Cape Lilibeo (Sicily) is only 1434 km wide. The highly irregular bottom topography in the form of a submarine ridge further limits this flow. This ridge is characterised by shallow banks along the Tunisian and Sicilian coast. A central narrow passageway cuts along the NW-SE axis of the strait, and forms an intermediate basin with an average depth of 500 m. Flat-bottomed deep trenches reaching depths of 1100 – 1200 m off Pantelleria, 100 m off Linosa and 1650 m in the Malta Graben are situated in the central part of this basin to the West of Malta. Owing to the nature of its different behaviour with respect to the main Mediterranean sub-basins, it is generally considered as a third sub-basin; the central Mediterranean Sub basin.

The Strait of Sicily connects in the east and west to the respective Mediterranean basins by a system of sills. The Sicilian channel on the western side constitutes the main exchange passageway for the Levantine Intermediate water (LIW) between the eastern and western Mediterranean sub-basins. It consists of a two-sill system. The first with a minimum depth of 365 m oriented to the north-northwest, the second with a minimum depth of 430 m oriented toward north (Frassetto, 1965). The major flux occurs through the narrower and deeper eastern passage close to the Sicilian shelf (Bethous, 1980). This channel has not Aim 1/ 19 Project FAO-COPEMED / Grandes Pelágicos’2000 received the same attention as the Strait of Gibraltar although it has been suggested that its role is as important. The entrance of the LIW from the east occurs mainly at the Medina sill to the southeast of Malta.

In the southeastern parts of the strait, the African continental shelf is very wide and covers more than a third of the areal extent of the Strait. In the Gulf of Gages, the bathymetry is shallower than 30 m for large stretches away from the coast. On the southern coast of Sicily the shelf is bounded by two wide (approx. 100 km) and shallow (100 m on average) banks on the western (Adventure Bank) and eastern extremities (Malta Channel area) while it narrows down considerably along its middle part. Along the eastern coast of Sicily and extending southward, there is a narrow Ionian shelf break which is very steep to the east of Malta (known as the Malta Escarpment). The shelf break fans out to the south and broadens to a gentle slope to the north of Libya.

The Maltese Archipelago, consisting of a group of small islands aligned in a NW-SE direction, is located on the southern most extremity of the Sicilian continental shelf. The topography of the shelf in this area is characterised by a plateau in the middle part, with an average depth of 150 m. The shelf is flanked by a submarine ridge which protudes as a submerged extension of Cape Passero and embraces the shelf area along the eastern and southern perimeter. The Maltese Islands represent the emerged part of this ridge while the Hurds bank to the northeast of Malta shallows to a depth of just over 50 m. To the southeast, a series of relatively shallow areas, and notably the Medina Bank, maintain an average depth of less than 300 m in the sea joining the Sicilian and Libyan continental shelves.

2. General Circulation

The region is known to contain a number of significant hydrodynamical processes and phenomena covering the full spectrum of temporal and spatial scales. The general circulation is dictated by the slow basin scale (vertical) thermohaline structure of the Mediterranean, and carries a significant seasonal and interannual variability. In the upper thermocline this circulation is characterized by an energetic and meandering stream known as the Atlantic Ionian Stream (Robinson et al., 1996). The circulation is further modified by strong mesoscale signals in the form of eddy, meander and filament patterns.

Using satellite-tracked drifters, Poulain (1998) reports on Langrangian measurements of surface currents between November 1994 and March 1997. The tortuous drifter trajectories are the result of both spatial and temporal variability and reveal the complexity of the sub- tidal surface patterns. These mesoscale processes are triggered by the synoptic scale atmospheric forcing. The heat and momentum fluxes at the air-sea interface represent the dominant factor in the mixing and pre-conditioning of the MAW on its way to the eastern Mediterranean.

From CTD and XCTD profiles taken during a cruise in November 1994, seven water masses were identified in the northern area of the Strait of Sicily and the northwest Ionian Sea (Robinson et al., 1996). Starting from the Levantine water at the bottom the successive Aim 1/ 20 Project FAO-COPEMED / Grandes Pelágicos’2000 overlying layers consist of Transitional, Fresh, Mixed, Modified Atlantic, Upper and Surface water masses. The horizontal distribution of these water masses amongst the different station can be valuable in identifying the type of processes in action.

Traditionally the three main water masses in the strait are the upper layer Modified Atlantic Water which enters through the Sicily channel as an extension of the north African Algerian coastal current, the deeper Levantine water moving towards west, and the 100 m transitional layer. The signature of the Modified Atlantic Water is seasonal and it is given by a salinity minimum (37.2) that is found at about 50 meters during summer and near the surface during winter (Wust, 1961; Manzella, 1988). The core of the Levantine Intermediate water is indicated by a maximum at a depth of about 300 meters with a temperature of 29.16 OC and a salinity of 38.75 at the channel. The Levantine Intermediate water has maximum salinity in the western and southwestern approaches of Malta. The renewal time of the total Levantine water in the strait is estimated to be 9 months, long enough to maintain a fairly constant salinity over the annual cycle. This also indicates that he characteristics of the Levantine Intermediate water incident into the strait from the eastern Mediterranean are also quite stable.

During winter, the Modified Atlantic water is more steeply sloped towards the African coast (Manzella et al., 1990; Grancini & Michelato, 1987). This is corroborated by analysis of the climatological buoyancy fields which actually indicate an intensification of the flow on the southern side of the channel during autumn and spring, whereas the flow is concentrated against the Sicilian side, reaching a salinity of 38.77. The transition layer is squeezed in only a few tens of meters thickness, sloping up from south to north as is typical in system with two layers flowing in opposite directions. The transition from winter to summer stratification occurs during the April – May period. By July, both temperature and salinity isolines are quite horizontal. During the summer months the transition layer is re- established to its normal uniforms thickness of about 100 m.

The MAW flows eastward mainly along the AIS. This swift topographically controlled current normally starts its path as a meander to the south of Adventure bank. It then proceeds southeastwards and loops back northward around Malta, forming the Maltese channel Crest. As it reaches the sharp shelf break to the east of Malta, it abruptly gains positive vorticity and tends to deflect with an increase looping northward meander forming the characteristic Ionian Shelf Break Vortex. South of this strong, the weaker flow of Modified Atlantic Water moves southeastward and mostly recirculates on the Tunisian and Libyan shelf, hence contributing very little to the mean inflow into the Ionian.

The northward flow along the Ionian shelf break is predominant during summer when the AIS is most intense and follows closely the Sicilian shelf break. The flow subsequently extends as a relatively strong velocity front into the northwestern Ionian where the summer circulation is mostly anticyclonic. The contrast in temperature of the MAW exiting the Strait of Sicily with the warmer Ionian Sea produces the Maltese front which constitutes a conspicuous thermal filament on sea surface temperature AVHRR maps. The general pattern of the circulation during winter is somewhat different. The AIS is less intense, and the MAW will tend to spread more along the interior of the strait. The exit of the MAW is shifted further south where the shelf break is more tenuous, and the most probable fate of Aim 1/ 21 Project FAO-COPEMED / Grandes Pelágicos’2000 the MAW is to progress along southeastward and southward branches. This situation is moreover favoured by an enhanced cyclonic component in the Ionian circulation especially during winter. Poulain (1998) reports that in winter, Langrangian drifters deployed upstream in the Strait of Sicily tended to avoid the Malta Channel route and proceeded south-southeastward to eventually reach the central Ionian.

The circulation pattern in spring and fall is more difficult to assess. On the basis of more updated climatologies (Brasseur & Brackart, 1998) and reliable ocean prediction systems with assimilation of detailed hydrographic data in the area, (Robinson et al., 1998 and Horton et al., 1997), it appears that the summer circulation pattern with a northward veering of the MAW over the Malta Escarpment is also common in both spring and fall. This is in contradiction with earlier studies such as by Tziperman & Malanotte-Rizzoli (1991) and by Ovchinnikov (1996) who conclude that on exiting the strait, the MAW will predominantly proceed to the north during summer and to the south and southeastward during the remainder of the year. The model by Zavatarelli & Mellor (1995) does not attribute very pronounced seasonal variability to the flow of the MAW into the Ionian Sea. The path of the stream is more complicated in winter than in summer, but generally follows a wide anticyclonic meander extending from Sicily up to almost the entrance of the Cretan Passage. These discrepancies in results reflect the difficulty of determining the annual cycle in the strait where various phenomena interfere on the general circulation and can actually switch between highly variable patterns of flow.

3. Synoptic scale phenomena In addition to the general circulation and its mesoscale variability, the Strait of Sicily and the Ionian shelf break region are also influenced by a number of significant synptic scale processes and phenomena. In particular, the phenomenology of the circulation in the Sicilo-Maltese shelf area is characterized by the frequent coastal upwelling events which bring to the surface cool water that is then swept along by mesoscale eddies. The upwelling zone extends over the whole southern coast of Sicily and is favoured by the northwesterly gusts that are numerous and can be particularly strong at any period of the year. The southeastward advection of these cold patches in the form of long plumes and filaments area very characteristic feature in the thermal IR images of the region. During the colder months, surface cooling due to the influence of polar air masses can result in the formation of large patches of relatively cooler water which accentuate the latitudinal thermal gradients in the northern parts of the strait, and enhance the contrast with the relatively warmer Ionian water at the Maltese Front. Closer to the land perimeter, the incidence of coastal currents is also very important. The Sicilian coastal current along the southern border of the island is wind driven and associated with local and remote storms. From thermal IR images, the vein of cold water along the Italian southern coast can often be followed down to Sicily. The penetration of cold Ionian water from the eastern coast of Sicily around Cape Passero over the shelf is favoured by the northerly winds and is often associated with the meandering frontal extension northward of the Maltese front.

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4. Thermal Signature of Sea Surface Temperature In the strait of Sicily, the processes described in the previous section can practically all be followed by correlated surface temperature signatures. The surface layer Modified Atlantic water itself, its characteristic lower sub-surface temperature, will lead to changes in sea surface temperature as it undergoes large scale movements with the associated vertical mixing. The winter cooling and summer heating of shallow areas can also be followed from a time series sea surface temperature maps. Like in the rest of the Mediterranean, the surface density is the strait is strongly dependent on the salinity, so that even relatively weak temperature gradients can be identified against the strong seasonal signal, provided that they persist in time and space.

In the area surrounding the Maltese Islands, various phenomena meld to produce a very variable and complex sea surface temperature field. The progression of the AIS and its eastward extension, the upwellings south of Sicily and the warming and cooling of the shallow continental shelf waters are amongst the main driving processes. Starting from December, the winter structures are characterized by a front to the south of Malta. This front is commonly broad and long, often taking the form of a V-shape pointing north westwards.

In April, the Sidral frontal zone is very much attenuated and this month is usually without significant sea surface temperature features. The end of May usually leads to the first summer features. The reduced surface cooling and the presence of stratification mark the onset of upwelling fronts. These fronts are very common in summer and show up as extensive plumes and tentacle-like structures which are advected by their interaction with the AIS. During major events of mistral winds the area of upwelled water can be extensive and actually engulf the whole shelf area. At the southern most tip of Sicily and over the Malta escarpment a hug filament of cooler water is usually established against the warmer Ionian water to the north and the warmer Sidra water to the south (Champagne-Philippe & Guevel, 1982). This Malta Front can protude southeastward to very long distances. On other occasions it is restricted to latitudes higher than 34oN under the clear influence of the northward veering of the outflowing AIS. When upwelling events are interrupted, a series of complex latitudinal fronts are established over the shelf in the Malta Channel. The cooler coastal current along the southern perimeter of Sicily often appears as a thin channel of cooler water. The autumn season presents the most complicated SST stuctures. The variability and the number of fronts is highest in this season. The thermal gradients are usually weaker than those in summer, but a mixture of thermals structures between those occurring in summer and winter can often be established. In October, a new frontal zone appears off the coast of Tunisia. It can be either continuous from southern Sardinia to the Strait of Sicily, or divided into two parts, one south of Sardinia, the other starting south of Cape Bon, and delineating the northern border of the eastern Tunisian continental shelf warmer water. In October and November, this frontal system is superimposed on the persisting summer structures of southern Sicily.

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5. Phenomenology of Upper Layer Currents

Current meter measurements made in the strait of Sicily are predominantly in the channel between Cape Bon and Marsala. The most comprehensive sea current data refers to the section between Sicily and Libya which was studied by AGIP in the framework of the Libyan Offshore to Sicily Gas Transportation System Project and conducted in 1981 – 1982 (Grancini, 1985).

On the basis of these measurements the current pattern across the vertical section between Sicily and Libya is reported to be rather complex, due to the presence of large continental shelves and deep trenches, to the variability of atmospheric disturbances and finally to the vertical density structure which couples with the wind stress producing barotropic and baroclinic responses of the current (Grancini & Michelato, 1987). On the Sicilian continental shelf the currents are characterized by a south eastward steady flow of the order of 25 cm s-1 throughout the year. Wind forcing increases the flow to 30 cm s-1 during winter. In the vicinity of Malta the stream funnels out to a more southward direction with a reduced average flow of 10 cm s-1. Further south, in the mid section corresponding to the deep trench separating the Sicilian and Libyan continental shelves, the steady flow of Modified Atlantic water in the upper layer is predominantly eastward, with an intensification to values of 10 cms-1 during winter and spring, especially in the vicinity of Medina bank. During autumn this steady eastward flow is spread further to the south.

Fluctuations of the upper layer currents are generally isotropic except on the Sicilian shelf where the coastal currents are characterized by a noticeable longshore variability. The variability of the dynamic processes over the Sicilian shelf area is evidenced by the high current variance which is reported by Granicini & Michelato (1987) to have values in winter that rare more than twice that of the surface flow in the central and southern area of the stait. The kinetic energy is mainly in the tidal frequency band and is found to be dominated by the semi-diurnal and diurnal signals. The semi diurnal currents do not exhibit significant seasonal changes in magnitude. Their consistency with depth reveals a dependence on the barotropic tide. On the other hand, the diurnal currents are baroclinic in nature and represent most of the current variability. They are reported by Granicni & Michelato (1987) to be mainly confined to the Sicilian shelf and to be particularly strong in the vicinity of Malta. The diurnal currents are stronger in summer. During winter they can completely disappear in some area. During summer, inertial currents occur thoughout the section between Sicily and Libya. Inertial events, each with a duration of about 10 days, occur in succession and give rise to inertial oscillations with periods between 20 h and 21 h. Their amplitude can reach 25 cms-1 and persist for several days on the Libyan continental shelf. On the Sicilo-Maltese continental shelf the diurnal currents and the inertial oscillations co-exist together and give rise to a distinctive diurnal-inertial spectral peak in summer.

The presence of intense diurnal subsurface flows in the NW coastal area of Malta, have recently been confirmed from measurements obtained during a physical oceanographic survey carried out in summer 19992. These diurnal baroclinic currents are believed to be the expression of a topographically trapped wave that takes the form of an internal Kelvin- Aim 1/ 24 Project FAO-COPEMED / Grandes Pelágicos’2000 like waveform in the deeper sea away from the shelf break and is accompanied by shelf wave modes propagating over the continental platform. An associated vertical oscillation of the order of 8m (Drago, 1997). The free surface displacement accompanying this internal tide is estimated to have a semi-amplitude of the order of 1.2 cm. From measurements of sea level in Mellieha Bay, Malta, covering a period of 42 months (1993 – 1996), Drago & Boxall (1998) report that the variability of the diurnal residual shows consistent peaks during the periods scannning mid-October to mid-January and Mid-April to mid-July respectively. These sea level signatures reveal the temporal variability of the underlying dynamical processes. The semidiurnal residue does not exhibit this variability and except for sub-seasonal variations, it remains practically constant throughout the year. In particular, the same diurnal can predominate over the diurnal residual during late winter and late summer.

A strong diurnal internal tide has also been detected at a station near the shelf break on Adventure Bank from time series of vertical temperature profiles (Artale et al., 1989). These sub-inertial oscillations of the thermocline are related to baroclinic flows with phase opposition between the upper and lower layers. They must be related to the same process of the trapping of energy at the diurnal frequency as that observed in the vicinity of Malta.

Low frequency current components in the strait are also remarkable. They are particularly energetic close to the Tunisian and Sicilian coasts as well as in the deep central areas of the strait. Their intensity is reduced over the shelves while they are hardly observed over the Libyan shelf. These low frequency currents can be very intense especially in winter, reaching up to 30 cms-1 and modulating the tidal oscillations with a mean periodicity of about 10-12 days. The relationship with metereological forcing at these time scales is rather complex. Grancini & Michelato (1987) attribute the origin of the low frequency currents along the full extent o the Sicilian southern coat to the forming by the local longshore wind. On the other hand, the low frequency variability within the Atlantic water flow is believed to be driven by larger scale non-local weather patterns. Local perturbations in the low frequency flow emanating from the Channel can result in the formation of mesoscale baroclinic eddies along the deeper central axis of the strait, and thus give rise to the low frequency clockwise rotating currents that are observed in these areas.

HYDROGRAPHIC AND OCEANOGRAPHIC CONDITION ALONG THE LIBYAN WATERS

1. Water Circulation According to the information available the hydrographic and Occeanographic condition along the Libyan waters are typical south Mediterranean one. The general circulation of water masses is lergely conditioned by the fact that this reagon constitutes an evaporation basin (Lacomb, et al 1960). In order to compensate for losses due to evaporation and to maintain a constant level, water has to come in from Aim 1/ 25 Project FAO-COPEMED / Grandes Pelágicos’2000 the Atlantic via the straits of Gibraltar. The water masses moves near the shores of north African coast and eastward along this coast.

Reports by Sogreah, 1977; Gerges et al (1974, 1977) and Gerges et al (1983), Zupanovic et al (1983) shows that the Atlantic waters (T= 15.0C° , S‰ = 36.3, σt = 27.0) occupies the upper layer of water, the thickness of which is about 25-30 meters in winter and 50 meters in spring, fig.( 1 ) shows the pattern of the surface water circulation in winter season which hypothetically covers the period from December to June in the central Medeterranean Zopanovic et al, 1983. The water passes over the ridge of the Tunisian strait and spreads along the coast of Gabes, moving eastward twards the Libyan coast where it then sub divided into two branches one return back and the other continue eastward reaching Misurata.

In a more detailed studies by Sogreah, 1977 during winter and summer season revealed that the majority of water passes between Malta and Panteleria and divided by Ilands of Linosa and Lampedusa into two parts one flow to North and one towards the Libyan coast (Fig. 2 and 3) where it meets another small current which has swept the coast from the channel between Panteleria and Tunisia and become stonger off Homs and Misurata, this promote mixing and causes deep up welling which has been confermed by nutrative salt measurement Sogrea, 1977.

2. The Surface Water Tempertature The surface water temperature along the Libyan water is fluctuated according to the annual cycle from around 29°C in August to about 12.5°C in January (Ramadan et al 1984). However the difference between surface layer and deep water layer is very small giving an annual mean differences of about 0.5°C higher in surface than in deeper waters. In spring as reported by Sogreah 1977 and revied by Zopanovic et al, 1983 that the surface temperature ranges between 16°C to 18°C. In theGulf of Gabes and extending as far east as the approches of Misurata Fig.( 4 ), then off Misurata and to the east the optimium water temperature range is between 16°-18°C, is found only at level of about 50 meters depth Fig.( 5 ), this water temperature goes deeper east word of Misurata to the Gulf of Sirte.

The bottom temperature ranges between 14 - 20 °C according to the distance from the shore as indicated by Sogreah, 1977 Fig. ( 6 ).

3. Salinity Broadly speaking, the whole of the Mediterranean is a mixture of three standard water types. Atlantic water with minimum salinity, intermediate water with maximum salinity and deep water with minimum temperature and high salinity in Spring the bottom salinity in Libyan waters of the Atlantic origin S‰ was S‰ < 37.5 ‰ while the intermediate water S‰ < 38.75, the deep water was S‰ < 38.75 Fig.( 7 ). In summer the bottom salinity of the Atlantic water was 37.5 ‰ the intermediate and deep water was 38.75 ‰ Fig.( 8 and 9 ) as reported by Sogréah, 1977.

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4. Meteorological Conditions Considering the meteorological situation over the Libyan coast, as reported by Contransimex (1977) and reviewed by Zopanovic, et al. 1983 that during spring season a very high speeds corresponding as small frequency winds from SSW, SSE and E directions. While in winter winds usually blows from the NW to NE Fig. ( 10 ). During summer the wind blows from S, SE to N and NW and refereed to as Gebli.

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