Salinity of Ocean Waters; Factors affecting salinity, horizontal & vertical distribution

Salinity is the ratio between the weight of dissolved salt and weight of sea water. Hence it is defined as the total amount of solid materials in grams contained in 1 kg of sea water and is expressed as parts per thousand. The normal range of ocean salinity ranges between 33‰ – 37‰. Salinity is one of the most important physical properties of the ocean as it affects other properties like density, temperature, pressure, waves, currents, etc. The freezing point of sea water also depends on salinity. Also, higher the salinity, lower would be the evaporation rate. Salinity also increases the temperature of the oceans by scattering the solar radiation received at the ocean surface. It also increases the density of ocean water.

DISSOLVED SALTS IN WATER BODIES (GM OF SALT PER KG OF WATER)

Chlorine	18.97
Sodium	10.47
Sulphate	2.65
Magnesium	1.28
Calcium	0.41
Potassium	0.38
Bicarbonate	0.14
Bromine	0.06
Borate	0.02
Strontium	0.01

FACTORS AFFECTING SALINITY

• Evaporation: There exists a positive correlation between rate of evaporation and amount of salinity. Due to evaporation, water content is lost, and salts are left behind, which in turns increases the salinity.
• Precipitation: Higher the precipitation, lower is the salinity. The equatorial regions receive high rainfall throughout the year have lower salinity compared to the regions of lower rainfall in sub-tropical higher belts.
• Influx of freshwater from rivers: Influx of freshwater from rivers like Ganges, Amazon, etc reduces the salinity at places where these rivers enter the sea, that is, their mouth.
• Atmospheric pressure and wind direction: Sub-tropical high-pressure belts experiencing anti cyclonic conditions with stable air and high temperature has higher salinity. Prevailing winds pile up water from one part of the ocean to the other thereby changing and redistributing salinity.
• Ocean currents: They play a major role in the distribution of salinity by thorough mixing of sea water. For instance, equatorial warm current brings saline water from western coastal areas of the continents to eastern coastal areas thereby reducing salinity in the former and increasing it in the latter part.

HORIZONTAL DISTRIBUTION OF SALINITY

It can be understood both in terms of latitudinal as well as regional distribution considering major oceans as separate regions.

LATITUDINAL DISTRIBUTION

• Latitude shows a decreasing trend in salinity from equator to the poles.
• Equatorial regions have high temperature throughout the year, but highest salinity is not found in the equatorial zone due to excessive rainfall which overtakes the rate of evaporation in this zone.
• The highest salinity in the northern hemisphere is recorded between 20-40° latitude and in southern hemisphere maximum salinity is recorded between 10–30° latitude. Salinity again has a decreasing trend beyond these latitudes in both hemispheres.
• In Polar regions salinity further decreases due to abundance of freshwater in the oceans which results from the melting of snow.
• The average salinity of Atlantic Ocean is 36 ‰. The highest salinity is recorded between 15° and 20° latitudes. Maximum salinity (37 ‰) is observed between 20° N and 30° N and 20° W – 60° W. It gradually decreases towards the north.

REGIONAL DISTRIBUTION

Regional distribution of salinity is expressed in the form of Isohalines (are imaginary lines drawn on map joining places of equal salinity).

• Salinity in Atlantic Ocean is affected by the presence of ocean currents like warm Gulf Stream which transports saline water from western margin of Atlantic Ocean and increases the salinity along eastern margin.
• Highest salinity is observed in Lake Van Turkey (330‰), Dead Sea (230‰) and Great Salt lake (220‰).
• Indian Ocean shows more complex salinity conditions due the presence of Indian Sub-continent and monsoons. The eastern part of Indian Ocean has comparatively low salinity especially towards Bay of Bengal due to influx of enormous volumes of freshwater by rivers like Ganga, Godavari etc.
• Arabian Sea records higher salinity due to less influx of freshwater from rivers and arid conditions prevailing there (due to high evaporation).

VERTICAL DISTRIBUTION OF SALINITY

Salinity generally changes with depth, but it depends on the location of the sea.

• Salinity, generally, increases with depth and there is a distinct zone called the halocline, where salinity increases sharply.
• High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.
• Increasing salinity of seawater causes its density to increase.