Salinity Distribution
The salinity of surface seawater is controlled primarily by the balance between evaporation and precipitation. As a result the highest salinities are found in the so-called sub-tropical central gyre regions centered at about 20° to 3° North and South, where evaporation is extensive but rainfall is minimal. The highest surface salinities, other than evaporite basins, are found in the Red Sea.
What is Salinity?
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Why is salinity important?
- Salinity, along with temperature, determines the density of seawater, and hence its vertical flow patterns in thermohaline circulation.
- Salinity records the physical processes affecting a water mass when it was last at the surface.
- precipitation/evaporation – salts excluded from vapor
- freezing/thawing – salts excluded from ice
- Salinity can be used as a conservative (unchanging) tracer for determining the origin and mixing of water types.
Factors determining ocean water salinity
- Evaporation: The salinity of water in the surface layer of oceans depend mainly on evaporation. Where the evaporation is greater, the salinity is higher, for example, Mediterranean sea.
- Freshwater flow influx: Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
- Where the freshwater flow into the oceans is greater, the salinity is lower.
- For instance, at the mouths of rivers such as Amazon, Congo, Ganga etc., the ocean surface salinity is found to be lower than the average surface salinity.
- Temperature and density: Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of an area.
- In general, regions with high temperatures are also, regions with high salinity.
- Ocean Currents: They play an important role in the spatial distribution of dissolved salts in ocean waters.
- The warm currents near the equatorial region push away the salts from the eastern margins of the oceans and accumulate them near the western margins.
- Similarly, ocean currents in the temperate regions increase the salinity of ocean waters near the eastern margins. For instance, Gulf Stream in the North Atlantic Ocean increases the salinity of ocean waters along the western margins of the Atlantic Ocean.
- Precipitation: Precipitation and salinity share an inverse relationship.
- In general, regions with higher levels of precipitation have lower levels of salinity. This is the reason why though the equatorial region is as hot as the sub-tropics, it records lower salinity than the sub-tropics since the former receives heavy precipitation in a day.
- Atmospheric pressure and Wind direction:Â anti-cyclonic conditions with stable air and high temperature increase salinity of the surface water of oceans
- winds help is redistribution of salinity, as they drive away saline waters to less saline areas resulting into decrease of salinity in the former and increase in the latter
Distribution of Salinity
Surface seawater salinities largely reflect the local balance between evaporation and precipitation.
- Low salinities occur near the equator due to rain from rising atmospheric circulation.
- High salinities are typical of the hot dry gyres flanking the equator (20-30 degrees latitude) where atmospheric circulation cells descend.
- Salinity can also be affected by sea ice formation/melting (e.g. around Antarctica)
- The surface N. Atlantic is saltier than the surface N. Pacific, making surface water denser in the N. Atlantic at the same temperature and leading to down-welling of water in this region this difference is because on average N. Atlantic is warmer (10.0 C) than N. Pacific (6.7 C).
- This is mostly because of the greater local heating effect of the Gulf Stream, as compared to the Kuroshio Current. Warmer water evaporates more rapidly, creating a higher residual salt content
- The influence of surface fluctuations in salinity due to changes in evaporation and precipitation is generally small below 1000 m, where salinities are mostly between about 34.5 and 35.0 at all latitudes.
- Zones where salinity decreases with depth are typically found occur at low latitudes and mid latitudes, between the mixed surface layer and the deep ocean. These zones are known as haloclines.
The spatial distribution of salinity across oceans, is studied in two ways:
- Horizontal Distribution of Salinity
- Vertical Distribution of Salinity
Horizontal Distribution of Salinity
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- On an average, salinity decreases from equator towards the poles. However, it is important to note that the highest salinity is seldom recorded near the equator though this zone records high temperature and evaporation but high rainfall reduces the relative proportion of salt. Thus, the equator accounts for only 35‰ salinity
- The highest salinity is observed between 20° N and 40° N (36‰) because this zone is characterized by high temperature, high evaporation but relatively low rainfall
- The average salinity of 35‰ is recorded between 100 -300 latitudes in the southern hemisphere
- The zone between 40 deg -60 deg latitudes in both the hemispheres records low salinity where it is 31‰ and 33‰ in the northern and the southern hemispheres respectively.
- Salinity further decreases in the polar zones because of influx of Glacial melt-water. On an average, the northern and the southern hemispheres record average salinity of 35‰ and 34‰ respecÂtively
Vertical Distribution of Salinity
- Salinity at the surface of the sea is decreased by the input of fresh waters or increased by the loss of water to ice or evaporation. Thus both the trends of increase and decrease of salinity with increasing depths have been observed
- Salinity increases with increasing depth from 300 meters to 1000 meters in high latitudes i.e. there is positive relationship between the amount of salinity and depth because of denser water below but salinity becomes more or less constant beyond 1000 m depth
- Salinity decreases between the depth zone of 300 meters to 1000 meters in the low latitudes but it becomes more or less constant beyond 1000 m depth
- It appears from the above mentioned trends of vertical distribution of salinity that there is rapid rate of change of salinity (both increase and decrease) in the depth zone of 300m-1000m. This zone of steep gradient of salinity is called halocline
- Maximum salinity is found in the upper layer of the oceanic water. Salinity decreases with increasing depth. Thus, the upper zone of maximum salinity and the lower zone of minimum salinity is separated by a transition zone which is called as halocline, on an average above which high salinity is found in the low latitudes while low salinity is found in the high latitudes
Regional distribution of water salinity across the oceans
- Indian Ocean: The average salinity of the Indian Ocean is 35 parts per thousand . The low salinity is observed in the Bay of Bengal due to the influx of river water by the river Ganga. On the other hand, the Arabian Sea shows higher salinity due to high evaporation and a low influx of freshwater.
- Pacific Ocean: The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
- Atlantic Ocean: The salinity in the Atlantic ocean varies between 20 to 37 parts per thousand according to the location.
- For example, Near the equator, there is heavy rainfall, high relative humidity, cloudiness and calm air of the doldrums.
- Whereas, The polar areas experience very little evaporation and receive large amounts of freshwater from the melting of ice. This leads to low levels of salinity, ranging between 20 and 32 parts per thousand
- North Sea: In spite of its location in higher latitudes, it records higher salinity due to more saline water brought by the North Atlantic Drift.
- The Mediterranean Sea: The Mediterranean Sea records higher salinity due to high evaporation. Surface waters average about 38 parts per thousand in this sea
- The Baltic Sea: Baltic Sea records low salinity due to influx of river waters in large quantity, averaging around 35 parts per thousand
- The Black Sea: Salinity in the Black Sea is very low due to the enormous freshwater influx by rivers, averaging around 13–23 parts per thousand
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