The Caspian Sea is special in numberous ways. The change in salinity from north to south and the watertransparency are just two of those. In this section the salinity, waves, flow of ice, currents and watertransparency are being described supported by maps.
The Caspian climate is described in a separate section.
Because a quite big extent of the sea along the meridian the watertemperature is subject to sizeable latitudinal changes most distinctly expressed in the winter period, when the seatemperature changes from 0-0,5oC on an edge of ice in the north to 10-11oC in the south, a difference of nearly 10oC. This difference decreases to 1-2oC in the summer: In the north - 24-25oC, In the south 25-26oC. The temperature on the east coast is 1-2oC lower than on the west coast.
The most general characteristic of water temperature is the distribution of the annual average temparature during many years over the water area. Precisely these values reflect the influence of the researched reservoir on climate of the adjoining land, though it is more smoothed with regard to separate seasons. This smoothness is reflected in the form of annual average isotherms too. (Figure 4.1) Far from the coast the accumulation of water heating in degrees changes from 11oC in the areas adjoining the Caspian near steppes to 18oC on southeastern tip of the sea mirror. The greatest influence on the form of isotherms is the drain of the following rivers: Volga, Terek, Kura, and also prevailing winds. The influence of cold deep waters getting to the surface is great in eastern half of the water area.
Annual average differences of water temperature on the surface of up to 20oC are observed in the northern areas. This is also observed near the east coasts of the South Caspian, but that depends on intensive warming up in summer and cooling down of shoals in winter. It is characteristic for central parts of the South Caspian that water temperature changes least during the year, which corresponds to small seasonal climatic distinction. The annual difference of surfacetemperature decreases to 14-15oC near western and eastern coasts of the Middle Caspian in the upwelling areas.
The salinity of Caspian Sea changes from the north to the south within a range of 1,0 to 13,5 parts per thousand. This difference is especially strongly present in the North Caspian. It is less obvious in other areas distinguished by selfrelative homohalinity. It is apparent from Figure 4.2 that the difference of the value around the wellhead and the freshened Volga's part compared to the southeastern water area is about 12,5% (Figure 4.2).
The isohaline 12,5% bending around the peninsula forms the ledge as if it moves more salty water masses to the east. This phenomenon is explained by freshening influence of the rivers' drain on the western coast of the Middle Caspian, which is allocated by branches of Main Caspian flow of cyclonic and anticyclonic directions. Practically all the rest space of superficial waters of the sea after 12,5 parts per thousand isokhaline is defined by salinity of 13,0-13,5% . Almost complete absence of a hydrographic network in the south-east Caspian area has caused raised salinity (13,5%) of the water in that area.
The common increase of the salinity happening in the northern areas is observed in the whole area of the sea in winter months. Following this the main part of the water area is characterized by isohaline 13,0-12,5% and less change of values.
In other parts of the sea extremes do take place. Like in the Gara-Bogas-Gol Gulf, where the salinity of the water reaches 300 parts per thousand.
The northern part of the sea freezes in the winter. Ice usually starts being formed in November. The whole northern part of the sea is covered with ice in severe winters.
In soft winters ice is formed in shoals within the 2-3 meter isobaths. The occurrence of ice in the middle and southern parts starts in December - January. Ice has its origin at the east coast, at the west coast it frequently is taken to by the currents from northern part of the sea.
The shallow gulfs freeze at the east coast of the middle Caspian during severe winters. Stacks of ice form shores and coves at formed at the coast in this period. The drifting ice is found up to Absheron peninsula at the west coast in abnormal cold winters (Figure 4.5).
Characteristics of waves in various areas of Caspian Sea strongly differ from each other. The main characteristics of the waves in the Northern Caspian sea are: height - 3m, the period - 10 sec., length - 85m. The waviness in southeastern and eastern direction has the largest repeatability in the western part of the Northern Caspian Sea. In the northeastern area of the sea the repeatability is biggest in western and eastern directions.
The waviness in various areas of Middle and South Caspian is characterized by the following features. Because of northern windss the greatest heights of 5% provision waves are 2-3 m in the coastal zone of Makhachkala - Derbent region, and are 4-6 m in the sea part of this area. The average periods and lengths of waves are accordingly 4sec and 16m at the coast, 5-7sec and 20-25m far from the coast. The wave size increases from the north to the south too.
The greatest wave heights are found around Absheron peninsula. During severe northwestern storms (> 25m/sec) in the epicenter the waves reaches heights of 7,5 - 8,0m, and during extreme storms 9-10 m. The eastern winds with a speed of 5-9m/sec cause waves with heights up to 1 m. The winds with speeds of 10-15m/sec cause waves of up to 2m in height. The period of the waves is increased to 3,0 - 4,7 sec. With eastern winds of 16 - 20 m/sec the waves reach heights of 3-3,5m with a period of 6 sec.
The whole southern Caspian Sea has a weak standard waviness (0,5-1m) due to the southeastern winds. Strong waves develop in case of northern and western winds on the east coast near the Cheleken region. The height of waves does not exceed 1 m in case of moderate and strong northwest winds (from 5 up to 15 m/sec) in the eastern area of the South Caspian. Storms of 16-20 m/sec cause waves of 2-3m, and heavy storms (21-25m/sec) waves of 3 - 4 m.
In order of importance the causes of the circulation in Caspian waters are:
- a wind and its transversal unevenness;
- a relief of the bottom forming some isolation between North, Middle and South Caspian;
the shape of the coastal line;
- a distinction of water temperature in different parts of the sea, an increase of temperature from the north to the south;
- a drain of rivers flowing into the sea, especially of the Volga river.
The common circulation Caspian Sea waters may be explained as follows. (Figure 4.3) The drain of the Volga River forks in the North Caspian. The first branch goes to the southwest and further follows along the west coast. The other branch turns to the northwest and following the western, the eastern and the southern coasts of the North Caspian, forms anticyclonic (directed clockwise) circulation of waters.
Further, the waters from the North Caspian enter into the Middle Caspian, moving along the west coast, flowing around Absheron peninsula, then going onward to the south and further turning according to the shape of the coast in eastern direction.
The current is directed in opposite direction at the east coast, so from south to north. A branch separates from this stream near Cheleken peninsula, turns to the west and joins the stream which is going along the west coast of the South Caspian. As a result of this the cyclonic circulation of waters (directed counter-clockwise) is formed in the deep-water hollow of the South Caspian.
In its turn another branch also separates from the stream, which is going along the western coast of the Middle Caspian. After that it turns more to the north to the Absheron peninsula, where it turns to the east and joins the waters going along the east coast in northwestern direction.
The main part of this stream turns to the left (to the west) at the south of Mangishlak peninsula and joins the waters going along the west coast of Middle Caspian, thus forming also the circulation of waters there. In this way the basic features of the currents' scheme show that there is a large external cyclonic circulation of the waters covering all parts of the sea, besides cyclonic whirlpools of the Middle and South Caspian waters.
General speeds of the current in the Caspian Sea are 20-30 mm/sec. The greatest values of current speed, which is reaching up 100mm/sec, are observed in an area near Apsheron peninsula, between Jiloy Island and Neft Dashlari.
The transparency of Caspian waters increases from the north to the south (Figure 4.6).
The North Caspian Sea has by a small transparency (0,5-1,0?), which is explained by a big inflow of river waters, containing rich organic and inorganic suspensions, a highly biological productivity of waters and small depths.
The transparency is not the same in various areas of Middle and South Caspian and increases in deep water. The transparency is about 10-15m in the central part of the sea almost in all seasons of year, and the maximal value (21m) was observed in South Caspian.