[Solved] Account for variations in oceanic salinity and discuss its multi-dimensional effects ( UPSC GS-1 mains 2017)
Salinity is defined as the ratio between the weight of the dissolved materials and the weight of the sample sea water. The average salinity in the oceans and the seas is 35‰ however it has significant spatial variations as follows. Salinity refers to the amount of salt dissolved in 1000 gms of sea water. It is usually expressed as parts per thousand or ppt. The salinity for normal open ocean ranges between 33 o/oo and 37 o/oo. Oceanic salinity varies significantly due to the free movement of ocean water and its distribution has two aspects:
Horizontal Distribution
- On an average salinity decreases from equator towards poles. However highest salinity is recorded at tropics and not at the equator because of high rainfall at equator.
- Marginal areas of the oceans bordering the continents have lower salinity than their central parts because freshwater is added to the marginal areas through the rivers.
- There is no control of latitudes on the distribution of salinity in the inland seas as salinity of the partially enclosed seas in higher latitudes is controlled by influx of melt water. E.g. Baltic Sea records relatively lower salinity than the North Sea though both are in almost same latitude. The areas of highest salinity (about 37o/oo, in Atlantic Ocean) are found near the Tropics due to active evaporation owing to clear skies, high temperature and steady Trade Winds.
- From the tropical areas, salinity decreases both towards the equator and towards the poles. Salinity is relatively low near the equator (about 35 o/oo, in Atlantic Ocean) due to high rainfall, high relative humidity, cloudiness and calm air of the doldrums.
- In polar seas, salinity decreases (20-32 o/oo) due to very little evaporation and due to melting ice yielding fresh water.
Vertical Distribution
- Salinity increases with increasing depth in high latitudes.
- Salinity is low at the surface at the equator due to high rainfall and transfer of water through equatorial currents but higher salinity is noted below the water surface.
- However, on an average higher salinity is found in the upper layer of the oceanic water and it decreases with depth.
- Thus, the upper zone of maximum salinity and the lower zone of minimum salinity is separated by a transition zone which is called Halocline zone.
- The areas of highest salinity (about 37o/oo, in Atlantic Ocean) are found near the Tropics due to active evaporation owing to clear skies, high temperature and steady Trade Winds.
- From the tropical areas, salinity decreases both towards the equator and towards the poles. Salinity is relatively low near the equator (about 35 o/oo, in Atlantic Ocean) due to high rainfall, high relative humidity, cloudiness and calm air of the doldrums.
- In polar seas, salinity decreases (20-32 o/oo) due to very little evaporation and due to melting ice yielding fresh water.
- Salinity & Water Cycle: Water in liquid state dissolves rocks and sediments which creates a complex solution of mineral salts in ocean basins. Conversely, in other states such as vapor and ice, water and salt are incompatible and water vapor and ice are essentially salt free. By tracking ocean surface salinity we can directly monitor variations in the water cycle: land runoff, sea ice freezing and melting, and evaporation and precipitation over the oceans.
- Salinity, Ocean Circulation & Climate: Ocean circulation in deep waters is primarily driven by changes in seawater density, which is determined by salinity and temperature. In the North Atlantic near Greenland, cooled high-salinity surface waters can become dense enough to sink to great depths.
- Salinity & Climate Density: The ocean stores more heat in the uppermost three meters than the entire atmosphere. Thus density-controlled circulation is key to transporting heat in the ocean and maintaining Earth’s climate. Excess heat associated with the increase in global temperature during the last century is being absorbed and moved by the ocean.
NASA studies suggest that sea water is getting fresher in high latitudes while saltier in sub-tropical latitude. This will significantly impact not only ocean circulation but also the climate in which we live.
Effects of variation in ocean salinity
- The freezing and boiling points are greatly affected and are controlled by addition or subtraction of salts in seawater.
- Salinity and density of seawater are positively correlated.
- Evaporation is controlled by salinity of the oceans thus the tropical cyclones which are driven by evaporation are indirectly controlled by salinity
- Coral reefs which are important for marine biodiversity can thrive only in saline water.
- Spatial variation in seawater salinity is a potent factor in the origin of ocean currents.
- It affects marine organisms and plant community.
Oceanic salinity is depending on multiple factors , at the equator is mild because high amount rain fall( though evaporation is more), if go to poles it increases but once we cross the tropics it again reduce. Seas enclosed by continent are more saline than open seas, fresh water flow to oceans will reduce salinity. Precipitation reduces salinity while evaporation increases the salinity. Wind also influence, if wind flow is high water get spread over a larger area, which reduces salinity. Ocean currents also contributed for variation of salinity Effects of salinity :
- Salinity determines compressibility, thermal expansion, temperature, density, absorption of insolation, ocean currents, evaporation and humidity.
- 2.It determines the presence of marine resource including creatures like fish.
- Sea surface salinity can have dramatic effect on the water cycle and ocean circulation which in turn affects the climate of planet.
- on sea voyage , travelers also get effected.
- Ships, maritime vehicles, submarines, war ships , strategic installations near by sea and in islands are also effected.
- 6.local whether conditions also changed in coastal areas.
- Salinity is the measure of all the salts dissolved in water (of which sodium chloride alone constitutes more than 77%). It is usually measured in parts per thousand (ppt) and the average ocean salinity is 35 ppt.
Factors affecting the salinity:
- The rate of evaporation: the waters fringing the high pressure belts of the trade wind deserts between 20° and 30° N and S have high salinity because of high rate of evaporation whereas temperate oceans have lower salinity.
- The amount of fresh water added by precipitation, streams and icebergs: salinity is lower than average in equatorial waters because of heavy daily rainfall and in oceans into which huge rivers like Amazon drain and the Arctic and Antarctic because of fresh water addition.
- The degree of water mixing by currents: in wholly or partially enclosed seas such as Caspian Sea waters do not mix freely with the ocean waters, hence they have high salinity whereas in open oceans salinity is uniform due to free mixing by ocean currents.
Impacts of salinity variation:
- Density differences leads to formation and circulation of oceanic currents via thermohaline process which controls the climate of the planet.
- It controls (through thermohaline circulation) the mechanism by which the oceans store and transport heat.
- It affects the marine biodiversity by providing right conditions for their survival. For example – planktons (which are the base of marine food chain) float because of high water density due to salinity.
- Oceanic salinity plays an important role in the growth and survival of marine organisms, circulation of oceanic currents and distribution of temperature and rainfall across the globe Ocean salinity is the saltiness or amount of salt present in the water body of the ocean. Salts compounds present in the water are sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate which dissolve into ions. The average salinity of the water body of the oceans is 35 parts per thousand. But due to some reason variations in the salinity has been noticed.
Possible reasons
- Where the evaporation rate is higher, due to adverse weather conditions, global warming, and low humidity. Water salinity is low where the evaporation rate is not high.
- where the rainfall is high, salinity is low.
- Large river-fed oceans have low salinity.
- Temperature directly affects the water salinity.
- Multi-dimensional effects
- The variation in water salinity causes different water density and temperature variation.
- Thus salinity affects the circulation of ocean currents.
- It affects climate change.
- It directly affects the marine biodiversity and the survival of the species.
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