Temperature and Salinity of Ocean Water


  1. Ocean waters get heated up by solar energy just as land.
  2. The process of heating and cooling the oceanic water is slower than on land.

Factors Affecting Temperature Distribution The factors which affect the distribution of temperature of ocean water are:

1. Latitude

The temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.

2. Unequal distribution of land and water

The oceans in the northern hemisphere receive more heat due to their contact with a larger extent of land than the oceans in the southern hemisphere.

3. Prevailing wind

The winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below. It results in the longitudinal variation in the temperature.

4. Ocean currents

Warm ocean currents raise the temperature in cold areas while cold currents decrease the temperature in warm ocean areas. The Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.

Horizontal and Vertical Distribution of Temperature

  1. The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth.
  2. The boundary usually begins around 100 – 400 m below the sea surface and extends several hundred meters downward.
  3. This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.
  4. About 90 percent of the total volume of water is found below the thermocline in the deep ocean.
  5. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from the surface to the bottom.
  6. The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C.
  7. The second layer called the thermocline layer lies below the first layer and is characterized by a rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
  8. The third layer is very cold and extends up to the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight.
  9. The average temperature of surface water of the oceans is about 27°C and it gradually decreases from the equator towards the poles.
  10. The highest temperature is not recorded at the equator but slightly towards the north of it.
  11. Decrease of temperature with the increasing depth, but the rate of decrease is not uniform throughout.
  12. The temperature falls very rapidly up to the depth of 200 m and thereafter, the rate of decrease of temperature is slowed down.


  1. Salinity is the term used to define the total content of dissolved salts in seawater.
  2. It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.
  3. It is usually expressed as parts per thousand (o/oo) or ppt. Salinity is an important property of seawater.
  4. The salinity of 24.7 (o/oo) has been considered as the upper limit to demarcate ‘brackish water’.
average salinity of different oceans

Factors affecting ocean salinity are mentioned below:

  1. The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.
  2. 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.
  3. Wind also influences the salinity of an area by transferring water to other areas.
  4. The ocean currents contribute to the salinity variations. Salinity, temperature, and density of water are interrelated.

Horizontal Distribution of Salinity

  1. The salinity for normal open ocean ranges between 33o/oo and 37 o/oo.
  2. In the landlocked Red Sea, it is as high as 41o/oo, while in the estuaries and the Arctic, the salinity fluctuates from 0 – 35 o/oo, seasonally.
  3. In hot and dry regions, where evaporation is high, the salinity sometimes reaches 70o/oo.
  4. The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
  5. The highest salinity is recorded between 15° and 20° latitudes.
  6. The average salinity of the Indian Ocean is 35 o/oo. The low salinity trend is observed in the Bay of Bengal due to the influx of river water.

Vertical Distribution of Salinity

  1. Salinity changes with depth, but the way it changes depends upon the location of the sea.
  2. Surface salinity of the World’s Oceans water to ice or evaporation, or decreased by the input of freshwaters, such as from the rivers.
  3. Salinity at depth is very much fixed because there is no way that water is ‘lost’, or the salt is ‘added.’
  4. Waves


  1. The horizontal and vertical motions are common in ocean water bodies.
  2. The horizontal motion refers to ocean currents and waves.
  3. The vertical motion refers to tides.
  4. Ocean currents are the continuous flow of huge amounts of water in a definite direction.
  5. Due to the attraction of the sun and the moon, the ocean water rises up and falls down twice a day. This is what leads to the formation of tides.


  1. Waves are the energy, not the water as such, which moves across the ocean surface.
  2. Water particles only travel in a small circle as a wave passes. Wind provides energy to the waves.
  3. Wind causes waves to travel in the ocean and the energy is released on the shorelines.
  4. As a wave approaches the beach, it slows down. This is due to the friction occurring between the dynamic water and the seafloor.
  5. The largest waves are found in the open oceans.
  6. Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf.
  7. Steep waves are fairly young ones and are probably formed by local wind.
  8. Slow and steady waves originate from faraway places, possibly from another hemisphere.
  9. Waves travel because the wind pushes the water body in its course while gravity pulls the crests of the waves downward.
  10. The actual motion of the water beneath the waves is circular.


  1. The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.
  2. The movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges.
  3. The study of tides is very complex, spatially and temporally, as it has great variations in frequency, magnitude, and height.
  4. The moon’s gravitational pull to a great extent and a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides.
  5. Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth.
  6. On the side of the earth facing the moon, a tidal bulge occurs while on the opposite side the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes a tidal bulge on the other side.
  7. The ‘tide-generating’ force is the difference between these two forces; i.e. the gravitational attraction of the moon and the centrifugal force.
  8. On the surface of the earth, nearest the moon, pull or the attractive force of the moon is greater than the centrifugal force, and so there is a net force causing a bulge towards the moon.
  9. On the opposite side of the earth, the attractive force is less, as it is farther away from the moon, the centrifugal force is dominant.
  10. The tidal bulges on wide continental shelves have greater height.
  11. The shape of bays and estuaries along a coastline can also magnify the intensity of tides.
  12. When the tide is channeled between islands or into bays and estuaries, they are called tidal currents.

Types of Tides

  1. Tides vary in their frequency, direction, and movement from place to place and also from time to time.
  2. Tides may be grouped into various types based on their frequency of occurrence in one day or 24 hours or based on their height.

1. Tides based on Frequency

Semi-diurnal tide

The most common tidal pattern, featuring two high tides and two low tides each day. The successive high or low tides are approximate the same height.

Diurnal tide

There is only one high tide and one low tide each day. The successive high and low tides are approximately the same height.

Mixed tide

Tides having variations in height are known as mixed tides.

2. Tides based on the Sun, Moon, and the Earth’s Positions

  1. Tides vary in their frequency, direction, and movement from place to place and also from time to time.
  2. Tides may be grouped into various types based on their frequency of occurrence in one day or 24 hours or based on their height.

Spring tides

When the sun, the moon, and the earth are in a straight line, the height of the tide will be higher. They occur twice a month, one on full moon period and another during the new moon period.

Neap tides

  1. The sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another.
  2. Normally, there is a seven-day interval between the spring tides and neap tides.
  3. When the moon is farthest from Earth (apogee), the moon’s gravitational force is limited and the tidal ranges are less than their average heights.
  4. When the earth is closest to the sun (perihelion), around 3rd January each year, tidal ranges are also much greater.
  5. When the earth is farthest from the sun (aphelion), around 4th July each year, tidal ranges are much less than average.


Ocean currents are like river flow in oceans.
They represent a regular volume of water in a definite path and direction.
Ocean currents are influenced by two types of forces namely:
1. Primary forces that initiate the movement of water;
2. Secondary forces that influence the currents to flow.
Ocean Currents
The primary forces that influence the currents are:
    1. Heating by solar energy
    2. Wind
    3. Gravity
    4. Coriolis force
  • Heating by solar energy causes the water to expand.
  • Wind blowing on the surface of the ocean pushes the water to move.
  • Gravity tends to pull the water down the pile and create gradient variation.
  • The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere.
  • The large accumulations of water and the flow around them are called Gyres.
  • Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.
  • Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator.
  • Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water.

Types of Ocean Currents

1. Indian Ocean currents

There are two types of currents in the Indian oceans
  1. North Indian Ocean currents
  2. South Indian ocean currents

2. South Indian ocean currents

There is no influence of monsoon on the south Indian ocean currents
There are two types of warm currents are found in this ocean
    1. Mozambique’s warm currents
    2. Madagascar’s r warm current
These two warm currents meet in the eastern part of South Africa and form Agullahas warm currents.
Currents Of the South Indian ocean

2. Atlantic Ocean Currents

There are two types of currents in the Indian oceans
    1. North Atlantic Ocean currents
    2. South Atlantic Ocean currents

1. North Atlantic Ocean currents

Warm currents
  • North Equatorial Current
  • North Atlantic drift
  • Gulf Stream
Cold currents 
  • Canary Current
  • Labrador Current
  • Norwegian current
North Atlantic Ocean currents

2. South Atlantic Ocean currents

Warm currents
  • Brazil current
  • South equatorial current
Cold currents
  • Benguela
  • Palk land current
south Atlantic ocean currents

3. Pacific Ocean currents

  1. North Pacific Ocean currents
  2. South Pacific Ocean currents

1. North Pacific Ocean currents

Warm currents
  •  Kuroshio warm current
  • North pacific equatorial currents
  • Alaska
Cold currents
  • Oyashio 
  • California
  • Okhotsk
north pacific ocean currents

2. South Pacific Ocean currents

Warm currents
  • East Australian currents
Cold currents
  • Peru/ham bolt  
south pacific ocean currents

Effects of Ocean Currents

  1. West coasts of the continents in tropical and subtropical latitudes (except close to the equator) are bordered by cool waters.
  2. West coasts of the continents in the middle and higher latitudes are bordered by warm waters which cause a distinct marine climate.
  3. The mixing of warm and cold currents helps to replenish the oxygen and favor the growth of planktons, the primary food for the fish population.
  4. The best fishing grounds in the world exist mainly in these mixing zones.