Salinity refers to the total content of dissolved materials in water bodies. It means the amount of salt dissolved in seawater. It is calculated as the amount of solid material (in gm) dissolved in 1,000 gm (1 kg) of seawater and it is generally denoted as parts per thousand(‰) or ppt. Salinity is a very important component that affects the physical and chemical properties of seawater. It determines the temperature of oceans and their surroundings, pressure, density, freezing point, insolation, evaporation, humidity, and oceanic currents flow. It influences seawater movements and the habitat of fish and other marine life. Ocean Salinity is affected by several factors. These are evaporation, rainfall, river water influx, ocean currents, atmospheric pressure, wind direction, and global warming. The average salinity of oceans and seas is 35‰. Though it is not equally distributed, it varies spatially and temporarily with oceans and seas. This variation is both horizontal and vertical; horizontal distribution means the variation of salinity at the surface level of the ocean and vertical distribution refers to salinity changes with the depth of the ocean.
Factors Affecting the Salinity of the Seawater:
There are many affecting factors such as the weather including evaporation, precipitation; rivers, winds, ocean currents, sea waves, and global warming.
- Evaporation: The rate of evaporation and salinity are positively related. It is observed that with higher the evaporation rate, the higher the salinity. With high temperature and low humidity, more evaporation takes place and salt concentration increases and total salinity increases too. It is found that salinity is higher in the tropics than at the equator.
- Precipitation: It is inversely proportional to salinity. Higher precipitation results in lower salinity and vice versa. Hence, the regions of high precipitation record lesser salinity than the regions of low precipitation. The melting water of ice from polar areas in the temperate regions increases the volume of ocean water and results in lower salinity. Basically, high precipitation raises the amount of ocean water and reduces the salt-to-water ratio.
- The Influx of River Water: Rivers reduce the salinity by diluting the salt concentration of the sea and ocean. If a number of rivers are flowing into a specific location of the ocean, then there is a high chance for low salinity. As big rivers contain a large amount of fresh water, which is enough to decline the salinity of the ocean. For example, lower levels of salinity are found near the mouths of the Ganga-Brahmaputra, the Amazon, the Congo, etc. and the effect of river water is more when it discharges in the enclosed sea. Such as in the Gulf of Bothnia situated in the Baltic Sea, salinity reduces by five parts per thousand as river water adds in it.
- Ocean Currents: They affect the spatial distribution of ocean salinity. Oceanic currents affect the salinity by increasing and decreasing it. Oceanic currents have less impact on the enclosed sea’s salinity however the impact on the salinity of marginal seas with open sea ducts is significant.
- Atmospheric Pressure and Wind Direction: Anticyclonic conditions generally raise the salinity of the oceanic surface water when it has stable air and a higher temperature. At the high-pressure zone of subtropics, higher salinity can be observed. Winds and currents provide assistance in the redistribution of oceanic salinity.
- Global Warming: Global warming threatens the overall stability of ecological balance, not only salinity. It affects many factors such as temperature, ice melting, and many more, which directly and indirectly impact the salinity of seawater.
Distribution of Ocean Salinity:
Ocean salinity distribution can be studied in two ways. These are Horizontal distribution and Vertical distribution of salinity.
A. Horizontal Distribution of Ocean Salinity:
This distribution is further categorized into 2 ways i.e. Latitudinal distribution and Regional distribution of salinity.
Latitudinal Distribution:
The basic principle of salinity distribution is that it decreases from the equator to the poles. Though the highest salinity is never found on the equator due to high rainfall, it is found maximum in the tropical regions. These regions have high temperatures and humidity and less precipitation as compared to the equator. The Equator has 34‰ salinity whereas the highest salinity is found between 20 degrees to 40 degrees north. Salinity decreases towards polar zones due to the influx of melting ice water. Thus on the basis of the latitudinal distribution, salinity is divided into 4 zones: equatorial zones have comparatively low salinity, tropical zones have maximum salinity, temperate zones have low salinity and sub-polar and polar zone have minimum salinity.
Regional Distribution of Salinity:
Oceans are divided into three categories on the basis of salinity variation. These are:
- Seas having higher salinity: Theses are the Red Sea, Mediterranean sea, and other enclosed seas.
- Seas having normal salinity: Theses are Caribbean sea, Gulf of California, Gulf of Mexico, etc.
- Seas having below normal salinity: The Arctic Ocean, Antarctica ocean, Bering sea, Andaman sea, Baltic sea, etc.
B. Vertical Distribution of Salinity:
At the upper levels of oceanic water, maximum salinity is observed whereas with an increase in depth it reduces. A thermocline zone is a transition zone between the upper layer of maximum salinity and the lower layer of minimum salinity. Though this pattern should not be taken as a general principle because the salinity of the ocean is very complicated particularly when it is vertical distribution.
Up to 200 fathoms, the salinity increases with an increase in depth, and beyond it salinity decreases with an increase in depth. The zone which shows a drastic change in salinity is called as Halocline zone. In this zone, salinity decreases with an increase in depth at lower latitudes, and salinity increases with an increase in depth at higher latitudes.
At the equator, salinity is low due to higher precipitation and water transfer by equatorial currents. Below the surface water, the highest salinity is observed due to the addition of water by rainfall and salinity becomes low again at the bottom. At depth of 300 meters to around 1000 meters salinity decreases at the equatorial latitudes though it becomes almost constant after the 1000m depth zone.
At the depth zone of between 300 to 1000, the vertical distribution of temperature varies, the density of seawater also varies and due to this, the salinity of oceanic water also varies. In this zone, seawater density changes rapidly and this zone is called Pycnocline. In this zone, density increases with an increase in depth at lower latitudes but has a higher salt density at higher latitudes.