The Mediterranean Sea

The Romans called the Mediterranean Sea Mare Internum ("Internal Sea") and Mare Nostrum ("Our Sea"). In the seventh century AD Isidore of Seville was the first to use the name Mare Mediterraneum ("Sea in the Middle of the Land"). The names stress its major characteristic: it is connected to other seas only through narrow straits. The Atlantic Ocean can be reached through the Strait of Gibraltar, which is a little under 13 kilometres wide at its narrowest point, and 320 metres deep. The Black Sea is reached through the Dardanelles (depth 70 metres), the Sea of Marmara, and the Strait of the Bosporus. The Suez Canal is a modern, man-made break-through to the Red Sea. Large islands are Corsica, Sardinia, Sicily, Crete, and Cyprus. The Adriatic Sea, Mare Hadriaticum, was in antiquity the sea composed of the area between Italy and Dalmatia, but also the sea to the south, bordered by Sicily, Malta and Crete.

A submarine ridge (at a depth of c. 360 metres) between Sicily and the African coast divides the sea in a western and eastern part. The western part is subdivided into three large submarine basins. The Alborán basin is to the east of Gibraltar, between Spain and Morocco. To the east, and to the west of Corsica and Sardinia, is the Algerian basin. The Tyrrhenian basin is between these islands and Italy. The eastern part is subdivided into two major basins. The Ionian basin is to the south of Italy and Greece. The Levantine basin is to the south of Turkey. Between Greece and Turkey is the Aegean Sea.

Important mountain ranges surrounding the western half of the sea (on which we will focus) are the Sistema Ibérico, the Pyrenees, the Massif Central, the Alps, the Apennines, mountain ridges running parallel to the coast of Dalmatia, and the Atlas Mountains in the Maghreb. Many rivers reach this part of the sea, between the mountains: the Ebro in Spain, the Rhône in the south of France, the Po in the north of Italy, and the Tiber, to name but a few.

The western half of the Mediterranean Sea seen from a Space Shuttle.

The sea has given its name to "mediterranean seas". Mediterranean seas are characterized by very restricted water exchange with the oceans. Two types can be distinguished. If precipitation and river input exceed evaporation, such a sea is called a humid mediterranean sea. An example is the Black Sea. Because evaporation does not compensate the supply of water, another outlet is needed: the excess water flows out through the Bosporus. In other seas, such as the Mediterranean Sea, evaporation exceeds precipitation and river input. Such a sea is called an arid mediterranean sea. The Mediterranean Sea receives from rivers such as the Rhône, Po, and Ebro, and from precipitation only about one-third of the amount of water that it loses by evaporation. Therefore an extra supply of water is needed. It comes from the Black Sea and especially from the Atlantic Ocean, through the Strait of Gibraltar. For shipping an important result is, that the currents in the Mediterranean sea are not wind-driven, but created by the Atlantic water inflow. The general water budget is shown below (table from the Encyclopaedia Britannica):

Gains	Cu m/sec	Losses	Cu m/sec
Inflow from the Atlantic	1,750,000	Outflow to the Atlantic	1,680,000
Inflow from the Black Sea	12,600	Outflow to the Black Sea	6,100
Precipitation and rivers	38,900	Evaporation	115,400
Total	1,801,500	Total	1,801,500

The Strait of Gibraltar seen from a Space Shuttle.

The main circulation in the sea is a counterclockwise movement of the water. The water coming in through the Strait of Gibraltar flows eastward along the north coast of Africa, and then branches off. A counterclockwise current is created in the eastern part. In the western part the current continues along the north coast of Sicily, and then moves to the north, west and south, along the coasts of Italy, France and Spain, back to Gibraltar. The outflow to the Atlantic Ocean is a subsurface current below the inward current. The inward current flows with a speed of one to two knots (one knot = one nautical mile or 1852 metres per hour). It is more powerful in summer than in winter, because there is more evaporation during the summer. There are a few secondary currents moving in the same direction: there is one to the north of Algiers, and another one to the west of the Tiber. These special currents are strong and dangerous in narrow channels, such as the Strait of Messina.

Some natural features and modern names.

Seas that are almost completely closed have, like lakes, only a very small tidal range, i.e. a small difference in sea level between high and low water. In the Mediterranean Sea tides are only significant in the Gulf of Gabes (to the south-east of Tunisia) and the northern Adriatic. The general mediterranean tidal range is about 28 centimetres. In the Adriatic it is about 90 centimetres. The latter sea can almost be regarded as a channel, between the straight Italian coast and the coast of the Balkan peninsula, with many small islands, most of which run parallel to the coast. In the Adriatic Sea not only the tidal range is different: the surface currents are created primarily by the wind. They can reach a speed of three and a half knots.

In the summer most mediterranean winds come from the north. A number of special winds occurs. Some of these are:

• Levanter. A strong, cold eastern wind of the western sea and the southern coasts of France and Spain. It is mild, damp and rainy, and most common in spring and fall. It reaches its maximum intensity in the Strait of Gibraltar.
• Gibleh (also called Khamsin). A hot, dry, dusty southern / south-eastern wind in North Africa and Arabia in late winter and early spring.
• Sirocco (called in Spain the Leveche, in the Gulf of Lyon the Marin, and in Morocco, Algeria and Tunisia the Chili). It is a warm, humid southern / south-eastern wind over the sea and southern Europe. It is frequent in spring, early summer and fall. It starts in North Africa as a dry wind (the "dry Sirocco"), transporting dust from the Sahara and the Arabian desert. It picks up moisture as it crosses the sea (the "damp Sirocco"), and then brings rain and fog. It blows evenly and the maximum force is 6, so it does not become a gale.
• Mistral (or Maestrale). A cold and dry, strong northern / north-western wind in southern France along the lower Rhône Valley and the Alps-Pyrenees gap. It is strongest and most frequent in winter and spring. The velocity is intensified because it blows through narrow valleys. It begins very suddenly, and can reach gale force.
• Libeccio. A south-western wind blowing over Corsica and Sardinia.
• Tramontana. A dry, cold, strong northern wind off the west coast of Italy.
• Bora. A very strong, cold north-eastern wind, passing through the Trieste gap. It can blow throughout the year, but is commonest in winter. In the summer it usually lasts two days, in the winter up to fourteen. It begins very suddenly. The strength ranges from a light breeze to storm with gusts of wind. At the foot of mountains it reaches hurricane force.