Salinity of Ocean Water, Factors Affecting, Distribution & Significance

Salinity of Ocean Water

The total content of dissolved salts in the sea or ocean is referred to as salinity. The amount of salt dissolved in 1,000 gm of seawater is used to calculate salinity. It is commonly expressed in ‘parts per thousand (ppt). The upper limit for fixing ‘brackish water’ has been set at 24.7% salinity. It plays an important role in determining several aspects of the chemistry of natural waters and biological processes.

The total salt content in 1000 grammes of seawater can be used to calculate the salinity of the ocean’s water. If the dissolved salt content exceeds 24.7 grammes per 1000 grammes of water.

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Factors Affecting Salinity of Ocean Water

Water salinity, temperature, and density are all linked. The Salinity of Ocean Water surface layer is influenced by Evaporation and  Precipitation. The freshwater flow from rivers influences surface salinity in coastal regions. Surface salinity in the Polar region is influenced by ice freezing and melting processes. The wind also influences salinity by transporting water to other areas. The variations in salinity are caused by ocean currents. The salinity of water in an area is affected by changes in density or temperature.

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Salinity of Ocean Water Significance

• The density of seawater, as well as its vertical flow patterns in the thermohaline circulation, are determined by salinity and temperature.
• The physical processes affecting a water mass when it was last at the surface are recorded by salinity.
• Freezing/thawing – salts excluded from vapour precipitation/evaporation – salts excluded from the ice
• For determining the origin and mixing of water types, salinity can be used as a conservative (unchanging) tracer.

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Salinity Distribution of Ocean Water

Surface seawater salinities are largely determined by the local balance of evaporation and precipitation. Rain from rising atmospheric circulation causes low salinities near the equator. High salinities are typical of the hot dry gyres (20-30 degrees latitude) that flank the equator and are where atmospheric circulation cells descend.

The surface North Atlantic is saltier than the surface North Pacific, making surface water denser in the North Atlantic at the same temperature and leading to the down-welling of water in this region. This difference is due to the fact that the North Atlantic is warmer (10.0 C) on average than the North Pacific (6.7 C). The influence of surface salinity fluctuations due to evaporation and precipitation changes is generally small below 1000 m, where salinities are mostly between 34.5 and 35.0 at all latitudes.

Salinity decreases with depth zones are typically found at low and mid-latitudes, between the mixed surface layer and the deep ocean. These are referred to as haloclines. Two approaches are used to investigate the spatial distribution of salinity across oceans ie Horizontal Distribution of Salinity & Vertical Distribution of Salinity.

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Horizontal Distribution of Salinity

Salinity decreases from the equator to the poles on average. It is important to note, however, that the highest salinity is rarely recorded near the equator, despite the fact that this zone experiences high temperatures and evaporation, but high rainfall reduces the relative proportion of salt. Thus, the equator accounts for only 35 per cent of salinity.

The highest salinity is found between 20° N and 40° N (36), which is characterised by high temperature, high evaporation, and relatively low rainfall. In the southern hemisphere, an average salinity of 35 is recorded between 100 and 300 latitudes. The zone between 40 degrees and 60 degrees latitude in both hemispheres has low salinity, which is 31 and 33 degrees in the northern and southern hemispheres, respectively.

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Vertical Distribution of Salinity

Salinity at the sea’s surface is reduced by the addition of freshwater or increased by the loss of water to ice or evaporation. Thus, both increasing and decreasing salinity trends with increasing depths have been observed. In high latitudes, salinity increases with increasing depth from 300 metres to 1000 metres, indicating a positive relationship between salinity and depth due to denser water below, but salinity becomes more or less constant beyond 1000 m depth.

Salinity decreases between 300 and 1000 metres in depth in the low latitudes, but it becomes more or less constant beyond 1000 metres in depth. According to the above-mentioned trends in vertical salinity distribution, there is a rapid rate of change of salinity (both increase and decrease) in the depth zone of 300m-1000m. This zone of steep salinity gradient is known as the halocline.

The upper layer of oceanic water has the highest salinity. With increasing depth, salinity decreases. Thus, the upper zone of maximum salinity and the lower zone of minimum salinity are separated by a transition zone known as the halocline, above which high salinity is found in low latitudes and low salinity is found in high latitudes on average.

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Salinity of Ocean Regional Distribution

• The Indian Ocean has an average salinity of 35 parts per thousand. The low salinity in the Bay of Bengal is caused by the influx of river water from the Ganga.
• The Arabian Sea, on the other hand, has a higher salinity due to high evaporation and a low freshwater influx.
• The Pacific Ocean’s salinity variation is primarily due to its shape and larger area extent.
• The salinity of the Atlantic Ocean varies from 20 to 37 parts per thousand depending on location.
• Near the equator, for example, there is heavy rainfall, high relative humidity, cloudiness, and the calm air of the doldrums.
• The polar regions, on the other hand, experience very little evaporation and receive large amounts of freshwater from ice melting. This results in low salinity levels ranging between 20 and 32 parts per thousand.
• Despite its higher latitude, the North Sea has higher salinity due to more saline water brought by the North Atlantic Drift.
• Due to high evaporation, the Mediterranean Sea has a higher salinity. This sea’s surface waters average around 38 parts per thousand.
• The Baltic Sea has a low salinity due to a large influx of river waters, averaging around 35 parts per thousand.
• The Black Sea’s salinity is very low due to the massive freshwater influx from rivers, averaging around 13-23 parts per thousand.

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Salinity of Ocean UPSC

Along with temperature, salinity has a direct impact on seawater density (salty water is denser than freshwater) and thus the circulation of ocean currents from the tropics to the poles. One method for delving deeper into the water cycle is to measure salinity. We discussed the salinity of ocean water in this article. This article investigates the factors that influence the salinity of oceanic waters and the surface salinity of the world’s oceans. Finally, we discussed the significance of the Salinity of the ocean which is useful for the UPSC exam.

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