Definition: Salinity

Salinity is the total amount of solid materials, in grams, dissolved in one kilogram of sea
water. Salinity of the ocean water simply means the presence of dissolved salts in the
seawater.
Salinity is defined as the ratio between the weight of the dissolved solid materials & the
weight of the sample seawater. One of the fundamental differences between pure water and
seawater is that the latter contains salt in dissolved form.
According to P.R. Pinet, 2000, “salinity is the total mass expressed in grams off all the
substances dissolved in one kilogram of seawater, when all the carbonate has been converted
to oxide, all the Bromine and Iodin have been replaced by Chlorine , and all organic
compounds have been oxidized at a temperature of 480 degree C.”

Some of the basic features of salinity are:

Seawater has unique properties:
 it is saline, its freezing point is slightly lower than fresh water,
 its density is slightly higher,
 its electrical conductivity is much higher, and it is slightly basic.
 The viscosity (i.e., internal resistance to flow) of seawater, for example, is higher than
that of fresh water because of its higher salinity.
 The density of seawater also is higher for the same reason.
 Seawater's freezing point is lower than that of pure water, and its boiling point is
higher
 Presence of dissolved solids in water.
 Materials must have the properties of solid state.
 Materials must have the property of solubility in water.
 Salinity is not only confined to seawater, rather it applies to all water on the earths
surface.
Salinity is an important factor in determining many aspects of the chemistry of natural
waters and of biological processes within it. A contour line of constant salinity is called
an isohaline, or sometimes Isohale.

Salinity unit:
It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; Salinity of
seawater is usually expressed as the grams of salt per kilogram (1000 g) of seawater.
On average, about 35 g of salt is present in each 1 kg of seawater, so we say that the average
salinity of the ocean salinity is 35 parts per thousand (ppt).
There are a number of methods available for measuring the salinity of water. The most
precise measurements utilize direct chemical analysis of the seawater in a lab setting, but
there are a number of ways to get immediate salinity measurements in the field. For a quick
estimate of salinity, a hand-held refractometer can be used (right).
This instrument measures the degree of bending, or refraction, of light rays as they pass
through a fluid. The greater the amount of dissolved salts in the sample, the greater the degree
of light refraction. The observer traps a drop of water on the blue screen, and looks through
the eyepiece. The dividing line between the blue and white sections of the scale (inset) can be
used to read the salinity.
Salinity of 24.7 (24.7 o/oo) has been considered as the upper limit to demarcate ‘brackish
water’.

Role of Ocean Salinity

 Salinity determines compressibility, thermal expansion, temperature, density,

absorption of insolation, evaporation and humidity.
 It also influences the composition and movement of the sea: water and the distribution
of fish and other marine resources.
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.

Salinity Pattern:
Six ions out of many different dissolve substances comprise about 99.4% of all the dissolved
ions of seawater.

Ions g/kg in seawater % of ions by weight

Chloride 19.35 55.07%
Sodium 10.76 30.6%
Sulphate 2.71 7.72%
Magnesium 1.29 3.68%
Calcium 0.41 1.17%
Potassium 0.39 1.1%
99.36%

Composition of Salt in Sea Water

Sea water contains a complex solution of several mineral substances in dilute form because it
is active solvent. The total amount of salt is gradually increasing because it is brought from
the land every year.
Sr. No Salts Content(in%0) Amount(in%)

1 Sodium chloride (Nacl) 27.213 77.8

2 Magnesium chloride (MgCl) 3.807 10.9

3 Magnesium Sulphate 1.658 4.7

(MgSO4)
4 Calcium Sulphate (CaSO4) 1.260 3.6

5 Potassium Sulphate (K2SO4) 0.863 0.3

6 Calcium Carbonate (CaCO3) 0.123 0.3

7 Magnesium Bromine (MgBr2) 0.076 0.2

Salinity levels

The relative Proportions of ions in seawater.

Latitudinal Variations of Salinity

• The most pronounced differences in salinity should be found in surface waters.
• Temperature is highest at the equator, and lowest near the poles, so we would expect higher
rates of evaporation, and therefore higher salinity, in equatorial regions
• but the salinity right along the equator seems to be a little lower than at slightly higher
latitudes.
• This is because equatorial regions also get a high volume of rain
on a regular basis, which dilutes the surface water along the equator.
• So the higher salinities are found at subtropical, warm latitudes
with high evaporation and less precipitation.
• At the poles there is little evaporation, which, coupled with ice
and snow melting, produces a relatively low surface salinity.
• high salinity in the Mediterranean Sea: this is located in a
warm region with high evaporation, and the sea is largely
isolated from mixing with the rest of the North Atlantic
water, leading to high salinity.
• Lower salinities, such as those around southeast Asia, are
the result of precipitation and high volumes of river input.
Salinity variations in the Pacific Ocean:
There is a wide range of salinity difference in the Pacific Ocean because of its shape and
larger areal extent.
 Salinity remains 34.85% near the equator.
 It increases 35% between 15-20 Degree latitude.
 Again decreases further Northward in the western part of the Pacific Ocean.
Salinity variations in the Atlantic Ocean:
 Average salinity is 35.67%
 Salinity recorded at 5N,15N,15S as34.98%,36% & 37% that respectively indicates
increasing trend of salinity.
 Salinity is higher along the Western margin than the Eastern margin.
 Comparatively low salinity is found in front of river mouths (St. Lawrence,
Amazon, Congo, Niger etc.)
Salinity variations in the Indian Ocean:
The spatial distribution of salinity in Indian Ocean is more variable & complex than the
Pacific Ocean & Atlantic Ocean.
 Average salinity is 35% found between 0-10 Degree North.
 Gradually decreases Northward in the Bay of Bengal.
 Higher salinity record is 36%, in the Arabian Sea

Annual mean global sea surface salinity

Figure shows the mean global differences between evaporation and
precipitation(evaporation-precipitation). Green colors represent areas where precipitation
exceeds evaporation, while brown regions are where evaporation is greater than
precipitations. Note the correlation between precipitation, evaporation, and surface salinity
Vertical Variation of Salinity
Most differences in salinity are due to variations in evaporation, precipitation, runoff, and ice
cover. All of these process occur at the ocean surface, not at depth.
 Salinity increases with increasing depth from 300m-1000m in high latitude.
 Salinity decreases between the depth zone of 300m-1000m in the low latitude.
 It appears from the above mentioned trends of vertical distribution of salinity both
increase and decrease in the depth zone of 300m-1000m.
  Salinity is low at the surface at the equator due to high rainfall and transfer of water.
 Maximum salinity is found in the upper layer of the oceanic water.
 Salinity in deeper water remains relatively uniform, as it is unaffected by these
surface processes.
 At the surface, the top 200 m is called the mixed layer.
 Below the mixed layer is an area of rapid salinity change over a small change in
depth. This zone of rapid change is called the halocline
 It represents a transition between the mixed layer and the deep ocean.
 Below the halocline, salinity may show little variation down to the seafloor.
 Low surface salinity at high latitudes, and higher surface salinity at low latitudes
Salinity profiles from two hypothetical sites in the open ocean, one from high latitude
and one from low latitude
Controlling factors of Salinity:
There is a wide range of variation in the spatial distribution of salinity within the oceans and
the seas. The factors and processes which affect spatial distribution of ocean salinity are
grouped in two categories:
 Factors that increase ocean salinity
 Evaporation
 Formation of ice
 Atmospheric high pressure
 Factors that decrease ocean salinity
 Ice melting
 Precipitation
 Influx of river water
Wind direction: Westerlies increase salinity along the western coast of land;
Circulation of ocean water: salinity varies for mixing; these also affect ocean salinity.
Significance of salinity
• Freezing and boiling points are greatly affected and controlled by addition of salts in
seawater.
• Freezing point of 35 ppt: -1.91°C
• Boiling point: mote than 100°C
• Salinity and density of seawater are positively correlated
• Evaporation is controlled by salinity (salts in water lowers the rate of evaporation)
• Potent factor in the origin of ocean currents
• Affects the marine organisms and plant community

Horizontal distribution of salinity

To make life easier, I will remove the symbol o/oo and place only number

 The salinity for normal open ocean ranges between 33 and 37.
High salinity regions

 In the land locked Red Sea (don’t confuse this to Dead Sea which has much greater
salinity), it is as high as 41.
 In hot and dry regions, where evaporation is high, the salinity sometimes reaches to
70.

Comparatively Low salinity regions

 In the estuaries (enclosed mouth of a river where fresh and saline water get mixed)
and the Arctic, the salinity fluctuates from 0 – 35, seasonally (fresh water coming
from ice caps).

Pacific

 The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal
extent.

Atlantic

 The average salinity of the Atlantic Ocean is around 36-37.

 The equatorial region of the Atlantic Ocean has a salinity of about 35.
 Near the equator, there is heavy rainfall, high relative humidity, cloudiness and calm
air of the doldrums.
 The polar areas experience very little evaporation and receive large amounts of fresh
water from the melting of ice. This leads to low levels of salinity, ranging between 20
and 32.
 Maximum salinity (37) is observed between 20° N and 30° N and 20° W – 60° W.
It gradually decreases towards the north.
Indian Ocean

 The average salinity of the Indian Ocean is 35.

 The low salinity trend is observed in the Bay of Bengal due to influx of river water by
the river Ganga.
 On the contrary, the Arabian Sea shows higher salinity due to high evaporation and
low influx of fresh water.

Marginal seas

 The North Sea, in spite of its location in higher latitudes, records higher salinity due
to more saline water brought by the North Atlantic Drift.
 Baltic Sea records low salinity due to influx of river waters in large quantity.
 The Mediterranean Sea records higher salinity due to high evaporation.
 Salinity is, however, very low in Black Sea due to enormous fresh water influx by
rivers.

Inland seas and lakes

 The salinity of the inland Seas and lakes is very high because of the regular supply of
salt by ‘ the rivers falling into them.
 Their water becomes progressively more saline due to evaporation.
 For instance, the salinity of the Great Salt Lake , (Utah, USA), the Dead Sea and
the Lake Van in Turkey is 220, 240 and 330 respectively.
 The oceans and salt lakes are becoming more salty as time goes on because the rivers
dump more salt into them, while fresh water is lost due to evaporation.

Cold and warm water mixing zones

 Salinity decreases from 35 – 31 on the western parts of the northern hemisphere

because of the influx of melted water from the Arctic region.
Sub-Surface Salinity

 With depth, the salinity also varies, but this variation again is subject to latitudinal
difference. The decrease is also influenced by cold and warm currents.
 In high latitudes, salinity increases with depth. In the middle latitudes, it increases up
to 35 metres and then it decreases. At the equator, surface salinity is lower.

Vertical Distribution of Salinity

 Salinity changes with depth, but the way it changes depends upon the location of the
sea.
 Salinity at the surface increases by the loss of water to ice or evaporation, or
decreased by the input of fresh waters, such as from the rivers.
 Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the
salt is ‘added.’ There is a marked difference in the salinity between the surface zones
and the deep zones of the oceans.
 The lower salinity water rests above the higher salinity dense water.
 Salinity, generally, increases with depth and there is a distinct zone called
the halocline (compare this with thermocline), where salinity increases sharply.
 Other factors being constant, increasing salinity of seawater causes its density to
increase. High salinity seawater, generally, sinks below the lower salinity water. This
leads to stratification by salinity.

Why Salinity Is Important

Salinity can affect the density of ocean water: Water that has higher salinity is denser and
heavier and will sink underneath less saline, warmer water. This can affect the movement of
ocean currents. It can also affect marine life, which may need to regulate its intake of
saltwater.

Seabirds can drink salt water, and they release the extra salt via the salt glands in their nasal
cavities. Whales can't drink much saltwater; instead, the water they need comes from
whatever is stored in their prey. They do have kidneys that can process extra salt,
however. Sea otters can drink salt water because their kidneys are adapted to process the salt.

Deeper ocean water may be more saline, as is ocean water in regions with a warm climate,
little rainfall, and plenty of evaporation. In areas close to shore where there is more flow from
rivers and streams, or in polar regions where there is melting ice, the water may be less
saline.

Even so, according to the U.S. Geological Survey, there is enough salt in the world's oceans
that if you removed it and spread it evenly over the Earth's surface, it would create a layer
about 500 feet thick.

In 2011, NASA launched Aquarius, the agency's first satellite instrument designed to study
the salinity of the world's oceans and predict future climate conditions. NASA says the
instrument, launched aboard Argentine spacecraft Aquarius/Satélite de Aplicaciones
Científicas, measures the salinity in the surface—about the top inch—of the world's oceans.