Rainwater, ocean water, or any other types of water available in nature contains some mineral salts dissolved in them. Salinity is used to determine the total quantity of salts dissolved in seawater. 

• Salinity of water is the total number of salts (measured in grams) dissolved in 1000 kilograms of seawater. Therefore, it is an essential attribute of seawater
• Salinity is expressed in PPT i.e. Parts every thousand
• The highest limit of brackish water is the water with a salinity of 24.7% 

Factors that determine Ocean Salinity 

• Precipitation and evaporation are the two factors responsible for the salinity of ocean waters present on the surface of an ocean
• Procedures like thawing and freezing affect the salinity at the surface in polar areas. The influx of fresh water from rivers determine the surface salinity in the coastal areas
• By transferring water to other regions, wind also plays a role in the salinity of water of a particular area
• Oceanic currents also create differences in the salinity of ocean waters
• Water’s density, temperature, and salinity are interlinked to each other. Therefore, any modification in density or temperature affects water salinity in a given region

Horizontal Distribution of Salinity 

• A regular open ocean has a salinity ranging from 33 PPT to 37 PPT. The Red Sea, which is surrounded by land on all sides (landlocked) has a salinity of 41 PPT. The salinity varies between 0 and 35 PPT in the Arctic and estuaries according to the season
• Evaporation is maximum in the regions that are hot and dry. Therefore, the salinity of ocean waters in such areas can reach up to 71 PPT
• The Pacific Ocean exhibits a great variation in salinity because of its huge area extent and shape
• Northern Hemisphere’s western areas have a low salinity between 35 PPT to 31 PPT. It is due to the entry of melted fresh water from the Arctic region. Similarly, the salinity reduces to 33 PPT after 15° – 20° south
• The Atlantic Ocean has an average salinity of 36 PPT. The salinity is the highest between 15-degree to 20-degree latitudes. The highest salinity of 37 PPT has been noted between 20-degree North and 30-degree North and between 20-degree West to 60-degree West. It finally gets reduced as we move to the North
• As the Baltic Sea receives large quantities of fresh water from rivers, it has a low salinity. The Mediterranean Sea has a higher salinity because of the high rate of evaporation. 
• Black Sea receives enormous quantities of fresh water through the rivers. Therefore, its salinity is very low
• The Indian Ocean salinity is 35 PPT on average
• River water influx is the major reason due to which Bay of Bengal exhibits a low trend when it comes to salinity. The lower influx of river water and high evaporation are the reasons behind the high salinity of the Arabian Sea

Vertical Distribution of Salinity 

Salinity varies with depth. However, the variation also depends on the sea location. 

• Evaporation or water loss because of ice are the main reasons behind higher salinity at the surface. Influx of river water i.e. fresh water reduces the salinity
• In deeper regions, the ocean salinity remains constant as there is no chance of water evaporation there. Also, the ocean salinity of these areas remain constant as salt addition has no effects here
• There is a noteworthy difference between the salinity of deep and surface zones
• Seawater with higher salinity shrinks below the water with lower salinity. It leads to stratification i.e. creation of layers of saline water
• Usually, salinity enhances with depth and a vertical zone called halocline is created. This zone is a column of water that shows rapid changes in salinity based on depth. The density of seawater is higher due to the high salinity of water

Conclusion

The overall concentration of dissolved salts in a body of water is referred to as its salinity.When determining salinity, we look at how much salt dissolves in a litre of seawater.Parts per thousand is the most common way to express it (ppt). The upper limit for ‘brackish water’ fixation has been 24.7 percent salinity. Natural waterways and biological systems depend heavily on it for several chemical properties.