Causes of Salinity of Ocean Water

Causes of Salinity of Ocean Water

Posted by Manisha on June 06, 2020

Water is tasteless

Our oceans comprise around 97% of all water on, in and above Earth, and only 3% of water our lands have. 
Whenever you are on a  beach, while swimming, surfing, and playing with ocean water you must have experienced the taste of ocean water. How’s it taste like? Salty! Isn’t so? 

Yes, ocean water is salty in taste but why? What is the salinity of ocean water? What makes it salty? What are the causes of salinity of ocean water? 
Here, in this post, you will get all answers to these questions. So let’s begin with- 

What is the salinity of ocean water?

The concentration of dissolved salt in a given volume of water is called salinity and the amount of dissolved salt like sodium chloride- table salt, magnesium, calcium, and potassium, present in the oceans, is called salinity of ocean water. Due to salinity, the ocean water is undrinkable and remains unusable (without process). 

Average Salinity of Ocean or Seawater

The salinity is expressed in parts per thousand (ppt) or in percentage (%) (according to Scripps Institution of Oceanography at San Diego).
According to the oceanographer, the average salinity of ocean water is around 35 grams per thousand grams of seawater or ocean water, which means there are 35 grams of salt present in one kilogram of ocean water. 

(1 kg ocean water = 35 gram salt)

However, all-natural waters contain some amount of dissolved salt into it. 
All five oceans are having different amounts of salinity into it. The Atlantic ocean has the highest salinity.

average salinity of ocean water
Average Salinity of Ocean Water

Causes of Salinity of Ocean Water

Factors, processes, and causes of salinity of ocean water, which affects the amount of salt present in oceans are—

1. Continental Drainage System

Our continents have a well-developed network of a drainage system, includes rivers, channels, canals, and streams that dilute and erode salts, and minerals like sodium chloride, magnesium, potassium, calcium from rocks and weathered rock material, carried along with them and finally drained into the oceans.
This causes the concentration of salts into oceans over a period of time.

The salinity on the mouth of rivers (where the river falls into the ocean), is very less. This way rivers and streams deliver salts and minerals to the ocean and raise its salinity.

2. Evaporation of Ocean Water

There is a direct relationship between the evaporation and salinity of ocean water.
Higher evaporation leads to higher salinity.
Freshwater in the form of water vapor, moves from ocean to the atmosphere through evaporation, causing higher salinity. 
Evaporation depends upon the temperature, dry winds, and clouds.
That is why ocean water near tropical deserts has high salinity.
Studies said that subtropical regions are of high evaporation.
About 86% of global evaporation happens over oceans (according to science.nasa.gov). 

For example, The saltiest ocean water is in the Red Sea and in the Persian Gulf Region (around 40%) due to high evaporation and less inflow of freshwater.

3.  Amount of Rainfall

There is an inverse relationship between rainfall and salinity.
Higher precipitation in the form of rainfall and snowfall leads to lower down the salinity of ocean water. 
As rainfall is a rich source of fresh water. It also increases the volume of ocean water and reduces its density which causes low salinity of ocean water.
Around 78% of global precipitation occurs over oceans.

For example, areas near the Equator and Tropical Convergence Zone (ITCZ), receives high rainfall. Therefore the salinity of ocean water is comparatively low here.

4.  Melting of Glacier Ice

Great glaciers like Antarctica, Arctic, Greenland, etc, are rich sources of freshwater. They hold water in the form of ice but when these ice melts and releases melted water into the ocean, it causes less salinity and vice versa.
That is why polar areas have lower salinity.

5. Ocean Currents

Do you know, ocean water is not still. It is moving or flowing from one place to another like river, built inside the oceans due to variation in temperature of ocean water, salinity, the pressure of winds, Coriolis effects.
This movement of ocean water is known as ocean currents.
These ocean currents keep on circulating ocean water from the equator, towards poles and from poles towards the equator, and so on. 
This way these ocean currents cause the mixing of salinity and help in balancing salinity levels of ocean water.

6. Ocean Surface Vector Winds (OSVW) or Sea Surface Winds

The winds which blow over the surface of the ocean are called sea surface winds, also known as Ocean Surface Vector Winds (OSVW).
On the surface of the ocean, the speed of wind blows very high as there is no barrier on its way.
These winds blow away the upper layer of ocean water along with them, due to this process the new layer of ocean water gets exposed and prone to evaporate, ultimately causing salinity. 

7. Latitudes and Longitude 

Location i.e. latitude and longitude of the ocean also affects the salinity of ocean water. 
Generally, the ocean situated on higher latitudes, near northern and southern poles like the Arctic Ocean and Antarctic Ocean, has less salinity. 
Whereas the ocean situated 20-30 degree latitudes have a higher salinity. 
The Atlantic Ocean has the highest salinity of ocean water i.e. more than 35% or 35 grams per thousand (ppt). (According to Aquarius satellite latest image, released by NASA)

global map of ocean saltiness
Global Map of Ocean Saltiness by NASA's Aquarius Instrument
Picture Credit: NASA/GSFC/JPL-Caltech | science.nasa.gov | salinity


8. Open Vents  and cracks on ocean beds

The presence of open cracks and vents which are found inside the ocean in its bed also causes the salinity of ocean water.
Through these vents and cracks, hot minerals like sodium, magnesium, calcium, potassium, magma, smoke, smog, continuously erupt, and releases into the ocean water. 
After contamination of ocean water with this erupted material, some chemical reactions take place which dissolves the minerals like sodium, calcium, potassium, magnesium, iron, etc, with ocean water, causing an increase in salinity.

For example, The Mariana Arc, part of Ring of Fire, lies to the North of Guam in the Western Pacific, was explored in 2004 by scientists and oceanographers.

causes of salinity of ocean water
Causes of Salinity of Ocean Water

Further details regarding this, you may visit the usgs.gov official website.

Nowadays, our technology grows tremendously, everything gets automated with the help of these technologies.
In the field of Oceanography, one such technology brought new dimension and revolution into this field i.e. NASA’S Aquarius/SAC-D Satellite.
It is an instrument in the satellite that has been orbiting our Planet Earth since June 10th, 2011 used for measuring changes in salinity or salt concentration, on the surface of the ocean. 

The salinity sensor detects the microwave emissivity of the top 1-2 centimeters (about 2 inches) of ocean water.
One great example of working in the field of oceanography is by - San Diego, scientists, and their colleagues measuring the global distribution of ocean salinity to better understand how it changes with climate over seasons and years.

We can see how our scientists and geographers together worked and researched hard, indulged into different aspects and fields or branches of geography to reveal the truth behind each and everything that exists on Earth.