THE MAIN CURRENT CIRCUITS

The surface of the Mediterranean is marked by liquid roads which are not just there for the use of sailors and oceanographers, but for the many organisms which move, actively or otherwise, following currents which travel the high seas, or move along the coastline or insinuate themselves into one of the basins.

Whatever their destination and their itinerary, these surface currents mainly follow cyclonic-type routes  they travel clockwise.

The main current circuits are in the Balearic Sea, the Ligurian Sea, the lonian Sea and the Adriatic Sea.

There seem to be many of them and they appear a long way from each other, but they all originate in the same mass of Atlantic water, whose destiny can be followed almost kilometer by kilometer after it enters the Mediterranean.

Meanwhile, the Algerian Current loses little or nothing of its power and continues its journey eastwards, although it is once more forced to ranch before entering the Sicilian Channel.

The shallow seabed of the Sherky Bank diverts around one third of the volume of water in the current towards the Sardinian Channel and into the Tyrrhenian, where it forms a cyclonic current which varies widely over the course of the year.
 
Part of it hits the Ligurian coastline and joins up with the Ligurian-Provençal-Catalan current - ln the Western Mediterranean these main currents ,are associated with other, weaker, ones, mostly in the southern zones, which form cyclonic vortices about 50-250 kilometers in diameter.

These vortices are present all the year round,although their characteristics do change from season to season.

Some of these marginal currents, like the ones originating along the Algerian coastline, can affect water up to several hundred meters deep and in many cases they tend to move out towards the open sea where they gradually lose their identity.

The remains of the Algerian current which passes through the Sicilian Channel enters  the Eastern Mediterranean running centrally between Africa and Sicily, its flow rate differing according to the seasons.

Because the new current is wedged between a homogeneous mass of cold waters driven upwards by the winds which blow on the south-western toast of Sicily and the huge vortices of the anticyclonic currents which dominate the coastlines of Tunisia and Libya, occupying virtually the whole of the Bay of Syrte.

At 20 degrees of longitude it forms the African current, which runs along the coastline of the Levant Sea, where it becomes the Asia Minor current and affects the Turkish coast as far as the island of Rhodes, where it borders a cyclonic current extending to Cyprus.

There are other cyclonic currents in the Aegean, the Adriatic and the lonian Seas.

In addition to these currents, which are to varying degrees ail coastal, the Central Mediterranean current, which flows above the Mediterranean Rdige, is of considerable importance in the Eastern Mediterranean.

This current, blocked in the south bythe powerful vortex of the Marsa Matruk anti-cyclonic current on the Egyptian coast, travels mainly towards the Sea of Crete and Cyprus, strengthening the Asia Minor current, while a secondary branch first heads east before turning south.

The currents which form in the Mediterranean as a result of the influx of the Atlantic and of evaporation do not only affect the surface. There is an intermediate layer of water which lies in a band between a depth of 200 to 600 meters. lt is in continuai movement, following the route of the surface currents, but in the opposite direction.

This "counter" current is manifest right from its origin in the Sea of Levant at the opposite extreme of the Mediterranean from Gibraltar, where the highest levels of salinity are recorded as a result of the graduai transformation of the chemistry of the Atlantic water as it crosses the Mediterranean.

The Mediterranean climate increases its sait content, terminating in the east at 39.1 grams per thousand, equal to 39.1 grams of sait in every 1000 grams of water.

In view of the fact that the average salinity of the Atlantic when it enters the Mediterranean is a little above 36 grams per thousand, this increase is a marked one and, given the volume of water involved denotes tons more sait.

Water with these characteristics tends to be unstable and sensitive to changes in the winter temperature, when the sea's surface temperature falls below 17 degrees under the influence of north winds.

Thanks to the mechanisms described at the beginning of the chapter, this causes an increase in the density of the surface water and the formation of a layer 100-150 meters thick comprising the warm salty water from the summer and the Atlantic water which has stayed below the summer thermocline.

The lower limit of this mixture of waters of differing origins borders on a colder and more homogeneous intermediate layer, formed during the previous winter and capable of spreading more or less horizontally over the entire eastern basin, where it forms a layer at between 200 and 700 meters, feeding a flow heading towards the Western Mediterraneanat an estimated 0.7 million cubic meters per second, which is equal to 2/3 of the water which flows out at Gibraltar.

The flow towards the Atlantic seem therefore to be equal to their area of provenance, as the eastern basins occupy around 2/3 of the entire Mediterranean basin.

After looking at the origin and the fate of the intermediate current, it might be interesting to study the stages of its journey westwards.

Driven by the new winter water, the water of the Intermediate Levantine current goes deeper, widening out into broader sections. Scientists who have followed the route of the current over long periods have discovered the existence of a main branch which crosses the entire Mediterranean and two secondary branches: a weaker, southern current, which hits the Gulf of Syrte and a larger, northern one in the lonian Sea, which travels along the eastern coastline of the Adriatic. past the Strait of Otranto, at a depth of around 300 meters.

ln the Adriatic, the current mixes with the cold winter waters from the north of the basin and then exits towards the Ionian.

On the threshold of the Sicilian Channel, the cross-roads of the main currents and the demarcation line between the Western and Eastern Mediterranean, the intermediate current's main flow is held close to the bottom by the Atlantic water above it and is channelled between two long narrow passages  which run  alongside each other but at different depths 430 and 365 meters respectively.

The wider and deeper of the two, closer to the Sicilian coast, faces North and it through here that the eastern water flows into the Tyrrhenian is the beneficiary of the greater part of the Intermediate Levantine current, whose denser and colder waters rise up the basin in a long, anti-clockwise, movement.

Locked between the Italian peninsula and the block of the larger Mediterranean islands, the waters from the east lose some of their original characteristics before exiting along the southern coast of Sardinia and joining up with the secondary branch heading north.

The intermediate current, transformed but still homogeneous and recognisable, once more heads north-west, driving along the Spanish coastline and forming a single liquid Iode which, compressed between the surface waters and the deep waters, is forced to head upwards towards the Gibraltar threshold and out into the Atlantic, closing the cycle started such a long time earlier.

Below the Intermediate Levant current, protected by a barrier hundreds of meters deep, the waters of the Mediterranean seem to tire of the continual coming and going to and from the ocean and look for a niche in which they can rest in a situation of relative calm.

The thresholds which divide the different basins finally come into their own, functioning as barriers and no longer as obstacles to be overcome, isolating the masses of cold,
homogeneous water which hardly ever mix with each other and which are once again separated by the two great blocks of seabed which rise up between Sicily and Africa.

Yet, despite this isolation, in spite of the fact that it is identifiable and has a physiognomy
of its own, the liquid kingdom which occupies the bathymetrical 1000-meter band cannot be cut off from the rest of the Mediterranean's waters and is subject to the same laws.

Once more, in fact, it is the reduction in water temperature and its increase in density that create these particular deep waters originating in the Western Mediterranean, prevalently in the Ligurian-Provençal basin, in the center of the Gulf of Lions.

The formation of the deep waters is not, however, as simple as that.
lt requires the joint action of strong and prolonged winds and of severe winters to
produce water which is sufficiently cold and dense to fall down to the bottom of the Mediterranean without the intervention of other forces.

And it is the unique meteorological conditions found in winter in the Gulf of Lions,
which did not lightly gain its infamous reputation amongst sailors as one of the most feared of all patches of sea, which create these idealconditions.

The cold surface waters coming from the coast are massed in a small area and other
waters are attracted from the depths below.
This accumulation process continues until the masses of water involved become like a gigantic, liquid skyscraper of dubious stability.

It only needs a few days of the Mistral sometime between February and March to destroy its apparent stability, lowering the temperature the little that is necessary
to transform the accumulated water into an avalanche plummeting down towards the bottom of the sea.

In fact, the definitive transformation is simply a consequence of vertical up and down movements produced by the wind as it acts on the volume of water produced by the forces of nature.

Some scientists have compared the movements which take place in this gigantic sphere of cold water to those which occur
in a pan of boiling water.

Vortices, side-slips, descents and upward surges all take place simultaneously, involving a volume of water estimated by some scientists at 5000 kilometers -a drop compared to the entire Mediterranean, but still 1000.000.000.000 litres of water multiplied by 5.000.

The movements which take place in this mass demonstrate the size of the forces involved, which are capable of acting at depths of more than 2,000 meters on the oldest and outermost strata of the deep water, which is partially, although very slowly, replaced. It has been calculated that the deep waters in this sector of the Mediterranean has changed over a period of 100-150 years.

The prolonged time of residence and the extraordinary genesis of these waters do affect the characteristics they reveal in chemical-physical analysis. Water samples taken at the center of the Western Mediterranean show such remarkably consistent temperature and salinity values (12.7°C and 38.40 per thousand respectively) that in 1970 a proposal was put forward to use them as the standard for oceanographic analysis. It was certainly an important discovery, but the Mediterranean was and is a changing sea, as the most recent surveys show. Analyses carried out in 1972 and 1973 in virtually the same areas led to the discovery of anomalous layers of water - slightly warmer (+ 0.03°C) and saltier (38.42 per thousand).

These higher values disappeared in the years which followed, only to reappear at the beginning of the I980s in the Gulf of Genoa and off the coast of Sardinia.

The presence of anomalous masses of water like these is seen now as proof of the proogressive warming of the Mediterranean, a much moretangible sign of which is the movement northwards of the areas of distribution of many species which were until now found only in much more southern regions. These in their turn have for some time now been undergoing a partial tropicalization as far as fauna is concerned, with the Lessepsian migrations which have led more than a hundred species from the Red Sea (mainly fishes and molluscs) to cross the Suez Canal and colonise the Eastern Mediterranean in particular.

This area, which has not been sceintifically studied to the extent of the western part, has a similar reserve of deep water which accumulates and circulates in the abysses of the lonian - the deepest part of the Mediterranean.

Below the 1000-meter mark there is a homogeneous layer of cold water created by phenomena which occur on a distant
part of the surface. The winter cooling and the Bora, an impetuous, icy, wind which blows from the east of Europe, cool the temperature of the waters in the relatively shallow Northern Adriatic. The special characteristics of the waters in this part of the Mediterranean, including the tides which play such a large part in the "high water" phenomenon which affects Venice, make it the most Atlantic of the Mediterranean Seas. This extremely cold and dense water gives rise to a current which heads south along the coast of the Italian peninsula until it reaches the relatively warm, salty water from the east, with which it mixes, before its density causes it to fall, past the Strait of Otranto, towards the abysses of the lonian.

Even in this case it is not totally segregated. Convection and internai currents moving a few hundreds of litres per second ensure that these waters are slowly replaced, a process which is completed every 120 years.