Mediterranean Sea (volume name: atmospheric science, marine science and hydrology science)

Mediterranean Sea

extends between Asia, Africa and Europe, and is the largest land sea in the world. The western end is connected with the Atlantic Ocean through the narrow and shallow Strait of Gibraltar (the narrowest point is only 13 kilometers and the depth of the sill is 32 meters); The northeast is connected with the Black Sea by Aegean Sea through Da Daniil Strait, MarMalakhei Strait and Bosphorus Strait. In the southeast, you can reach the Indian Ocean from the Red Sea through the Suez Canal. The main seas in the sea area are tyrrhenian sea, Adriatic Sea, Ionian Sea and Aegean Sea. The Aegean Sea is dotted with islands. The Mediterranean extends about 4 kilometers from east to west. The widest point from north to south is about 18 kilometers. It covers an area of 2.516 million square kilometers, with an average depth of 1,494 meters and a maximum depth of 5,53 meters.

Geological landform The coastline of the northern shore of the Mediterranean Sea is tortuous and has many bays. The south coast is straight, with many lowlands and occasional bays. The continental shelf is mostly narrow, with an average width of less than 4 kilometers. Some areas are also wide, such as the Rhone estuary delta, Gabes Bay, Nile estuary delta and Ebro estuary delta, where the shelf width is 65-27 kilometers; The shallow water area in the northern Adriatic Sea can reach 48 kilometers. The continental slope is steep, and the deep-sea basin is often divided by the ridge.

An underwater ridge (with a depth of 366m) between Sicily and the northern end of Tunisia divides the Mediterranean Sea into two parts: the east and the west. In the west, there are three basins in turn: to the east of the Gibraltar Strait is the arvo Basin; To the east, Sardinia and Corsica are the Algerian basin; Further east is the tyrrhenian sea Basin. There are two main basins in the east: the Ionian basin south of Italy and Greece; East of Crete is the Levant Basin.

the distribution of seabed sediments is: shallow sea area of continental shelf, mainly calcareous mud and sand in the east, and many terrigenous materials or shell sand in the west; The deep waters of the continental shelf are mainly mud with a small amount of sand. In the deep water area, the eastern part is mainly black sapropelic mud produced by insufficient oxidation of organic matter, and a small amount of diatoms are scattered; The western part is mainly gray calcareous mud, and the Adriatic area is dominated by gray and blue soft mud.

the distribution of volcanic materials is closely related to the location of volcanoes, and they are mixed with non-volcanic materials. In addition, there are loess deposits formed by weathered materials, coarse-grained deposits, and fine-grained deposits caused by turbid flow.

According to submarine expansion and plate theory, the Mediterranean Sea is the remaining waters of the Tethys Sea. The Tethys Sea (also known as the ancient Mediterranean Sea) can expand eastward in a fan shape when it is at its maximum, and connect with the ancient Pacific Ocean. Since Mesozoic, the Eurasian plate in the north of Tethys Sea has been moving towards Africa, Arabia, India and other plates in the south, which gradually narrowed the sea area. The present Mediterranean is caused by the complex relative movement and collision between the African plate and the Eurasian plate from Mesozoic to Cenozoic Miocene (the upper land of the two plates did not contact directly, but in the northern part of the eastern Mediterranean, the lower part of the African plate has dived under the Eurasian plate, making the Aegean Sea and southern Italy full of volcanoes and earthquakes).

Climate The Mediterranean summer is controlled by subtropical high, which is hot and dry. In winter, it is warm and humid in the westerly belt with frequent cyclone activities. The climate is changeable in spring and short in autumn. The prevalence of strong winds is a prominent climate feature in the Mediterranean. Except for the south coast of Tunisia, the airflow flows in the gap between mountains. Through the gap between the Alps and the Pyrenees, there is a dry, cold and northwest Mistral wind. Through the gap in Trieste is a strong, northeast Bra wind; There are cold and strong Leifantaila winds in the east and Vendaval winds in the west passing through Gibraltar. A warm, dusty, southeast wind blows from the Sahara desert in Africa; The easterly winds in the eastern Mediterranean are stable, while the northwest and north winds are dry; Sirocco from Africa is hot and dry.

the total annual precipitation varies greatly from region to region. Along the coast of North Africa, from Gabes in Tunisia to the east, the annual precipitation rarely exceeds 25 mm; In Dalmatian, Yugoslavia, it can be as high as 25mm.

Hydrological characteristics: The surface seawater in the east and west basins presents cyclone circulation. After the surface water of the Atlantic Ocean flows in through the surface of the Gibraltar Strait, the mainstream flows eastward from the coast of North Africa and divides into two branches off the coast of Tunisia: one continues eastward and enters the eastern Mediterranean, forming a large cyclone circulation (see figure). Part of the water flows into the western Mediterranean below the surface of the Sicilian Strait. The other branch bends to the northwest of Sicily off the coast of Tunisia, moves to the northwest along the central part of tyrrhenian sea, and finally returns to the Strait of Gibraltar, forming another large cyclone circulation in the western Mediterranean. Some of these waters flow into the Atlantic Ocean below the surface of the Strait of Gibraltar. Due to the winding coastline and numerous islands in the north, a series of small eddies and local currents have been produced.

Because the evaporation is much greater than the sum of precipitation and runoff, the water level in the Mediterranean Sea drops and the salinity increases, which causes the seawater in the Atlantic Ocean and the Black Sea to flow in from the surface, while the deep water in the Mediterranean Sea flows out from the depths. The surface water flowing in from the Atlantic Ocean is located at a depth of 125m, with an average flow rate of about 1cm/s, an average flow rate of about 1.75x16m3/s and a salinity slightly higher than 36.. The average flow rate of the deep Mediterranean water flowing into the Atlantic Ocean is about 1.68×16 m3/s, the average flow rate is about 1 cm/s, and the salinity is greater than 37..

The seawater flowing from Malakhei, Mar., through the Da Daniil Strait is located in the shallow surface layer of 3m, with an average flow rate of about 1.26× 14m3/s, an average flow rate of about 75cm/s and a salinity of 3.. Below this layer, the average flow of Mediterranean water flowing into the Malakhei of Mar is .61×14 m3/s, the average flow rate is about 35 cm/s, and the salinity is greater than 38..

water masses can be divided into surface water, middle water and deep water. The thickness of surface water is 1 ~ 2 meters. There is a lowest water temperature layer in the lower bound of the surface water in the western Mediterranean. In the eastern Mediterranean, however, this low temperature layer disappears, showing a water temperature decreasing layer. The depth of the lowest or decreasing layer of water temperature means the depth of vertical convection in winter, where the temperature and salinity are equal to those of surface water in winter. Middle water, located between 6 meters deep in surface water. The main feature is that there is a highest salinity value in the layer of 3 ~ 4 meters. In the western Mediterranean, there is also the highest water temperature layer at almost the same depth, while in the eastern Mediterranean, there is a second water temperature decreasing layer. Deep water, located in the middle water to the bottom of the sea. It can also be divided into two parts, that is, the transition layer between middle water and deep water at 6 ~ 15 meters. The water temperature and salinity decrease with the increase of depth. The other layer is located at a depth of 15 meters, the temperature increases slightly with the increase of depth, and the salinity is relatively uniform, ranging from 38.4 to 38.6.

the average surface water temperature is higher in the east than in the west, and the highest water temperature appears on the Libyan coast in khalij surt, which is 31°C in August. Followed by Isken Delun Bay, which is 3°C in August. The lowest water temperature appears in the far north of Adriatic Sea, which is 5.2°C in February. In the dead of winter, the Trieste Bay occasionally freezes. Annual range of surface water temperature is greater than 9 C. The water temperature of middle water and deep water has little change, usually between 12.7 and 14.5 C.

the average salinity of the surface layer is about 38.. Adriatic Sea is only 35., and Aegean Sea is as high as 39.. The salinity of middle water and deep water changes little, usually between 38.4 and 39..

The dissolved oxygen content of surface water is very high, while the content of middle water decreases from east to west, and the content of transition layer between middle water and deep water is the lowest, and the content of deep water increases slightly.

the tide is a regular or irregular semidiurnal tide. The tidal range in most areas is not big, all below .7 meters. The maximum tidal range appears on the east coast of Tunisia, reaching 1.7 meters. The phenomenon of wind increasing water is obvious. Near Cape Coase in Corsica, sometimes the water level can rise by 3.5 meters. Strong tidal current can be observed in the waters of the strait.

organisms and resources: due to the shortage of phosphate and nitrate, the content of organic matter in seabed sediments is very small, which limits the growth of marine organisms. The highest value of Mediterranean primary productivity appears in the coastal waters of Egypt at the mouth of the Nile (March-May). The extremely low areas are mainly in the coastal areas of Naples and Dodd-kanis Islands, as well as the coastal areas where Gabes Bay, Egypt and Libya meet.

Algae are abundant, with 157 species along the coast of Greece alone. There are as many as 53 species of fish, but the number is small, and there is no fishing ground with a large number of fish. Other economic creatures include dolphins, lobsters, oysters and crabs, as well as cuttlefish, jellyfish, corals, sponges and turtles. In addition, the Mediterranean sea water has a high salt content, strong sunshine in summer and little precipitation, so the coast is a good place to dry salt.

yang Tianhong