The shores of the Black Sea make up an almost unbroken line like lakes have. Such seas are called closed, or internal - they are closed within land, surrounded by it with all sides. This vast sea is connected to the Mediterranean Sea and, through it, to the Atlantic Ocean by two narrow straits. The first strait, the Bosphorus, is a 30-kilometer water corridor separating Europe and Asia, it leads from the Black Sea to the small Sea of Marmara. The second strait - the Dardanelles, through which you can get out of the Sea of Marmara into the Mediterranean. The Dardanelles are a fairly wide and free waterway, but the Bosphorus is a narrow one - there is even a bridge across it - and a shallow (less than 40 meters in the smallest place) strait. Therefore, there is no free exchange of water, mixing, between the Black and Mediterranean seas. But nevertheless there is a current in the Bosphorus Strait - it is noticeable in the rubbish floating near the shore - there is a lot of it, because the huge ancient city of Istanbul is located on the Bosphorus shores (the former capital of the Byzantine Empire - Constantinople).
Where does this current come from? What force pushes water from the Black Sea? This can also be understood by examining the map - see how many rivers flow into the sea. There is no other inland sea in the world into which so much fresh water flows in - it is brought in by the largest rivers of Europe, the Danube, the Dnieper, the Bug, the Dniester, the Don, the Kuban, the Rioni, and thousands of small rivers and streams. Total - 350 cubic kilometers of river water per year. The earth's surface from which the rivers collect this water is 5 times the area of the Black Sea itself.
Black Sea History
Satellite photo of the Bosphorus region: the length of the strait is 30 km, the smallest depth is 32 m, the smallest width is 750 m. In this place, the coasts of Europe and Asia are connected by a bridge. More than 15 million people live around the Bosphorus, this is the city of Istanbul and its suburbs
The Danube is the second largest river in Europe after our Volga. It begins in streams in the alpine glaciers, and then the Danube and its tributaries flow through the territory of 17 European countries, there are several capitals on its banks, and it alone brings more than half of the water to the Black Sea. A particularly powerful stream of fresh water falls on the northwest "corner" of the sea.
Black Sea Salinity
A very large river flow is a key physical factor determining the properties of the waters of the Black Sea and its biological structure. What happens to the water in the sea, if it is almost closed and many rivers flow into it? The inflow of ocean water through the Bosphorus is small, and the rivers bring fresh water, which dilutes the sea water. As a result, there is 2 times less salt in the Black Sea water than in the ocean or in the neighboring Mediterranean Sea. 33–38 grams of salt are dissolved in a liter of normal oceanic water, and 16–18 in the Black Sea. To indicate salinity there is a special icon% o - ppm, in Latin - thousandths.
The main share in sea salt is table salt - sodium chloride, there are others, and their ratio is quite constant in different seas. Actually, all the salt in the salt shakers on our tables is made of sea salt. Just do not try to salt the food with sea salt - those present in it, in smaller quantities, other salts, such as sulfates, really do not like your stomach :).
It is easy to see sea salt - splash water from the sea into the recess of some stone heated by the sun on the beach - in a few minutes the water evaporates, and the salt ions combine to form sparkling crystals. The more salt dissolved in water, the harder it is. Therefore, when light water flowing from the Black Sea with low salinity encounters heavy ocean water in the Bosporus, it rises above it. Part of the heavy marble-sea water flows along the bottom of the Bosphorus into the Black Sea, increasing the salinity of its depths. This stream is called the Bosporus countercurrent, it is 2 times weaker than the main, surface current directed to the Sea of Marmara.
Below 200 meters the salinity in the Black Sea is already 20%, and at the bottom of the sea, at a depth of 2 kilometers, it can reach 30%. Two currents in the Bosphorus Strait opened in the XV century, the Italian merchant Marcilla - trade was his business, and his passion and vocation were the exploration of the seas. He lowered a lot from his boat - a load on a string, serving seamen to measure the depth, in the waters of the Bosporus and noticed that if the lot was shipped deeply, it was blown towards the Sea of Marmara, and if the load was suspended above the bottom, the string deviates to the Black Sea . Centuries later, hydrographic research in the Bosphorus was continued by Russian scientists under the leadership of Admiral Stepan Makarov.
The main Black Sea current (BSA) is counterclockwise around the entire perimeter of the sea, forming two noticeable rings (Knipovich glasses, named for one of the Russian hydrologists who described these currents).
At the heart of such a movement of waters and its directionality is the acceleration imparted to water by the rotation of the Earth — Coriolis force. However, on such relatively small water area, like the Black Sea, the direction and strength of the wind are no less important, and for this reason - the main Black Sea current is very changeable; sometimes it becomes poorly distinguishable against the background of smaller-scale currents (those that create wind above the sea), and sometimes the jet velocity of the main Black Sea current reaches 1 meter per second. This current extends only to the upper layer of the sea - no deeper than 100–150 meters. In the coastal area, whirlwinds of opposite PFB directivity are formed — anticyclonic gyres, especially expressed at the Caucasian and Anatolian coasts (they are shown on the map). Local longshore currents in the surface layer of water are usually determined by the wind, their direction can change during the day - these currents are easily seen from the coast.