In the preceding discussion it was shown that the rapidity of evaporation is the most important factor affecting the salinity of the sea water. It is, therefore, natural that generally there is a downward decrease in the salinity.
In fact the highest values of salinity, according to Defant, in the individual oceans are observed at the surface or in the uppermost layers. With a view to clearly demonstrate the vertical distribution of salinity Defant divided the oceans into two broad zones or spheres: troposphere and stratosphere.
The word “troposphere’ is used for the surface layers of the oceans where the temperatures in the low and middle latitudes are relatively higher and where there are strong surface currents. The word ‘Stratosphere’, on the other hand, is reserved for cold water masses found in the oceanic deeps.
The salinity distribution in the sub-layers of the oceans is rather more complex. Even in the tropospheric part of the oceans there is no uniformity in the distribution of salinity, because it is closely connected with the interrelationship between evaporation and precipitation.
On the contrary, in the oceanic stratosphere the distribution of salinity is characterised by uniformity. Here it may be pointed out that because of differences in the physical and chemical properties of various water masses the distribution of temperature as well as salinity at different levels becomes all the more complex.
In all the oceans between 40° North and 50° South latitudes the upper layers of varying thickness are characterised by a rapid decrease in salinity downwards. At a depth of 800 m the salinity ranges from 34.3%o to 34.9%o.
However, below this level salinity registers an increase at a depth of 1600 to 2000 m, where it is found between 34.8%o and 34.9%o. Beyond this depth, once again salinity goes on decreasing gradually. This gradual decrease in the percentage of salinity in the oceanic water continues until the bottom is reached.
Another peculiarity in the vertical distribution of salinity is the fact that the uncommon increase in salinity between 800 and 1000 m levels, which may be termed as salinity inversion, slowly disappears in higher latitudes in both the hemispheres.
In fact, this increase in salinity ends between the polar fronts and the poles. In the North Atlantic the intermediate salinity minimum abruptly disappears at a depth of 800 m in mid- latitudes, and there is a continuous decrease in salinity from the surface down to the bottom of the oceans. Thus, the lack of uniformity in the vertical salinity distribution in the North as well as South Atlantic is clear.
In the Atlantic Ocean the vertical distribution of salinity in the tropical and sub-tropical belts has been investigated very closely. Almost all stations in the above two belts were found to have the same degree of salinity. This is called a homo-hyaline top layer.
The thickness of this layer differs from that of the thermal top layer. Of course, its thickness is somewhat smaller. However, in many cases just below this quasi-thermal top layer, there is a well-developed salinity maximum in the upper part of the thermo-cline.
This salinity maximum is the most important feature of the vertical salinity distribution in the upper tropospheric layers of the oceans. Below the 50 m level salinity increases from 36.1%o to 37%o.
Again from this level the salinity decreases downwards. In this ocean the maximum salinity layer is found just above the thermo-cline. The maximum salinity layer is ever present in all the tropical and subtropical regions. At 25° South and 30o North latitudes a thin layer of maximum salinity is found just above the thermo-cline or sometimes even within it in both the hemispheres.
From the preceding discussion it is evident that the layer with the maximum degree of salinity is formed from the lowermost parts of the subtropical highly saline water by currents flowing towards the equator.
In this way this layer with maximum salinity manages to come under the lower-salinity surface layers of the equatorial region. This forms a part of the upper circulation in the oceanic troposphere.
Between the subtropical belts of highly saline water there are two belts with lower salinity located just above or inside the thermo-cline. The first belt is found between 10°N and 15°N latitudes, and extends from 45°W longitude to the coast of Africa.
The second belt which is rather narrow is found in the middle part of the Atlantic Ocean. Its latitudinal and longitudinal extent is from 2o to 3°S latitude and 30° to 10°W longitude. It is found in the central part of the South Atlantic Ocean.
These belts form the southern and northern limits respectively of the subtropical water masses extending towards the equator. The maximum salinity is found again between these two belts from 7°N latitude to the equator.
Undoubtedly the tropical and subtropical surface layers of maximum salinity are closely related with the tropospheric circulation in these regions.
The thickness of this layer is about 50 m and the degree of salinity is 35.5%o. This subsurface layer extends up to the equator below the surface layer of low salinity. The spreading of this layer is due to advection and turbulence.
The accumulation of warm and saline water in the subtropical zone moves towards the equator as well as the poles as subsurface flow in deeper water. In the southern hemisphere the flow of the saline water takes place at a depth varying from 100 to 150 m, but the salinity goes on decreasing because of mixing.
On the contrary, this subsurface layer of higher salinity is connected with the Gulf Stream. The flow of this layer of higher salinity is directed towards the coastal areas of Norway or even beyond to the polar areas. In the same way, the more saline water of the subtropical areas in the Indian and the Pacific Oceans moves towards higher latitudes.
The salinity of the oceanic water is lowered at a depth of 800-1000 m. In the southern hemisphere the mass of low salinity water crossing the equator moves into the Atlantic at 20°N latitude.
This movement takes place at a depth of 800 m. But the Arctic Ocean is characterised by the absence of intermediate cold water mass of this type. However, in the western part of the North Atlantic such water mass is found extending from the Polar Regions to Newfoundland.
In the Indian Ocean the intermediate low salinity water mass is found below the highly saline water mass of the subtropical zone. Its depth below the surface lies between 1000-1200 m.
In the Pacific Ocean, on the other hand, the polar water masses with low salinity move towards the equatorial region from north as well as south. This water mass forms in the Antarctic Ocean between 50° – 60°S latitudes, while the Arctic water mass of this nature forms in the Sea of Okhotsk.
In the western and central parts of the Pacific Ocean this water mass is found up to 10°N latitude. But there is no such water mass in the eastern Pacific. Remember that in all the oceans, the vertical thickness of this polar water mass is approximately 600 m.
In the Atlantic Ocean at a depth of more than 1500 m salinity gradually increases upto 20°- 40°N latitude, but beyond this limit salinity starts decreasing in the upper layer of this water mass.
The North Atlantic water that enters in the Mediterranean Sea continues eastward to Cyprus where, during the winter, it subsides to form the Mediterranean Intermediate Water. The temperature and salinity of this newly formed water mass is 15°C and 39.1%o respectively.
This water flows west along the coast of North Africa at a depth ranging from 200 to 600 m and enters into the North Atlantic as a subsurface flow through the Strait of Gibraltar. As it passes through the strait, its temperature and salinity drop to 13°C and 37.3%o respectively.
Because of its high salinity it spreads in all directions at a greater depth. It is because of this water mass that at a depth ranging from 1300 to 2500 m the salinity registers an increase.
In the same way, in the Indian Ocean at a depth of 1500-2000 m the salinity increases due to mixing with the more saline water of the Red Sea. In the western and middle part of the Indian Ocean salinity increase is encountered in the deeper parts.
In the Pacific Ocean investigations made below the depth of 1500 m show that there is a homogeneity in the vertical and horizontal distribution of salinity. The average salinity at this level ranges from 34.65 to 34.68%o.
The marginal seas like the Mediterranean and the Red Sea are missing in the tropical and the subtropical regions of this ocean, so the supply of more saline water is not possible on its bottom.
In the extreme north of the Atlantic Ocean the deeper layers near the ocean bottom have salinity varying from 34.67 to 34.92%o. Due to the paucity of salinity data, the degree of salinity in the deeper layers of oceanic water of the Pacific and Indian Oceans are not exactly known.