Dive Atlas of the World. Jack Jackson

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      THE MEDITERRANEAN

      by Jack Jackson

      ALMOST LANDLOCKED, THE MEDITERRANEAN SEA LIES between Africa and Eurasia, exhibiting evidence of a complex geological history. Elongated from west to east, in the west the Mediterranean extends to the Strait of Gibraltar between Spain and Morocco, only 13km (8 miles) wide at its narrowest point and its only connection with the Atlantic Ocean. To the east it is bordered by the shores of Turkey, Syria, Lebanon and Israel. Nearly 4000km (2500 miles) wide, including the Sea of Marmara, it occupies an area of approximately 2,510,000 sq km (970,000 sq miles).

      To the northeast the Mediterranean Sea connects with the Black Sea through the Dardanelles, the Sea of Marmara, and the Bosphorus Strait. To the southeast the Suez Canal, an artificial waterway across the Isthmus of Suez, connects with the Red Sea.

      Diving in the Mediterranean has considerable variety. Seismic action, uneven deposition of river sediments, some of which were remelted, and uneven erosion together with the movements of the sea and the emergence and submergence of the land have resulted in a variety of coastlines. The northern shores of the eastern Mediterranean are complex and have variable fold mountains. The north coast of Africa bordering the eastern Mediterranean is low-lying and uniform except for the Cyrenaica highlands east of the Gulf of Sidra in Libya. Diving is popular everywhere with the local inhabitants, but there are some areas where only local nationals are allowed to dive and others that do not attract visiting divers. At Gibraltar and the marine reserves and Balearic Islands of Spain, the marine life is still good, but diving resorts on the Mediterranean coast of Morocco have not proved popular. The Mediterranean coast of France has good wrecks. Corsica and Sardinia have everything from wrecks to caverns. Algeria would have good diving but has never developed it; Tunisia has and its diving is quite good. The Tyrrhenian and Adriatic coasts and Sicily are popular and have plenty of wrecks. Malta, Gozo and Comino are particularly popular with UK clients and the government has scuttled six ships for divers. While not a problem for divers, Great White Sharks have been seen in Sardinia, Tunisia, Malta, Sicily and the mouth of the Adriatic. Greece and Turkey limit the areas where foreigners are allowed to dive to protect ancient shipwrecks, although Turkey has just opened some of these areas. Libya, Syria and Lebanon have never developed their diving. Cyprus has good diving, Israel limits access to its Mediterranean underwater archaeological sites, but Egypt has opened up the recent finds near Alexandria.

      Much of the Mediterranean coastline has rugged hills rising sharply from the water, but Egypt and Libya have plains lying next to the sea. The largest island is Sicily while in decreasing size other large islands include Sardinia, Cyprus, Corsica and Crete. There are myriads of smaller islands and those of the Aegean are so numerous that the name Archipelago was formerly applied to the Aegean Sea. These islands have frequent earthquakes.

      The large volume of warm water gives the surrounding land a subtropical climate known as a Mediterranean climate, even when it occurs elsewhere. Most Mediterranean countries have hot, dry summers and mild, wet winters. In summer, the temperature may reach 27° (80°F) while in winter it rarely drops below 4°C (39°F). Egypt and Libya have tropical climates. Off the coast of Libya, the Gulf of Sidra can reach 31°C (88°F) in August.

      WINDS AND RAINFALL

      Airflow into the Mediterranean Sea is mainly through breaks in the mountain ranges. The cold, dry, northwesterly mistral passes through the Alps-Pyrenees gap. The strong northeasterly bora passes through the Trieste gap. The cold easterly levanter and the westerly vendaval pass through the Strait of Gibraltar. Hot, dry southeasterly winds, known locally as the sirocco, ghibli or khamsin, frequently blow into the Mediterranean from the Arabian Peninsula and the Sahara desert when low-pressure zones cross the sea in late winter and early spring. These winds reduce the heat of the surface water by evaporative cooling. This is enough to increase its density so that it sinks and increases the salinity of incoming Atlantic surface water.

      The distribution and quantity of rainfall is variable and unpredictable. Maximum rainfall is found in mountainous coastal areas. The Dalmatian coast of Croatia can get 2540mm (100 in) per year, while the north African coast from Gabès in Tunisia to Egypt gets less than 250mm (10in). Overall, the loss of water by evaporation is three times the combination of rainfall and river drainage into the sea.

      TIDES

      Most regions of the Mediterranean have semidiurnal tides. The average tidal range is only about 300mm (12 in), but the change in water level caused by strong winds can be four times greater. However, the Gulf of Gabès off Tunisia has a range of nearly 2m (6½ft). The Atlantic affects tides in the Strait of Gibraltar, but its influence declines further east. A strong current flows into the Mediterranean from the Black Sea.

      The tides are significant in the Aegean Sea where the Euripus Phenomenon (violent currents, variable in speed and direction) is named after the tide of the strait lying between continental Greece and the island of Euboea (Évvoia). These currents are mainly influenced by the winds.

       DENSITY CURRENTS

      Density currents are currents that move by the force of gravity acting on small density differences caused by variations in salinity or temperature. The surface layer, which is disturbed by the waves, is there because it is less dense than the deeper waters due to being warmer or less saline. The oceans are composed of layers of water that have distinctive chemical and physical properties, which move more-or-less independently of each other and which do not lose their individuality by mixing even after they have flowed for hundreds of kilometres (miles) from their point of origin.

      The water from the Mediterranean Sea flowing through the Strait of Gibraltar into the Atlantic Ocean is an example of this type of density current or stratified flow. The Mediterranean is enclosed in a basin that is relatively small compared with the ocean basins and, because it is located in a relatively dry climate, evaporation exceeds the supply of fresh water from river drainage. The result is that the Mediterranean Sea contains water that is both warmer and more saline than normal deep-sea water. Overall the Mediterranean water is denser СКАЧАТЬ