Oceanography, Meaning, History, Significance, Laws

Oceanography is the study of the oceans, submarine reliefs, and the physical, chemical and biological properties of oceanic water. Oceans comprise a single, enormous, continuous body of water that surrounds all of the earth's landmasses. Oceanic waters play a very important role in controlling the global weather and climate, interactions between the atmosphere and the hydrosphere, and maintaining the water balance of the globe.

Many of the processes that occur in the oceans depend on the physical properties of seawater. Temperature and salinity are the most important physical characteristics of the oceanic water. In this article, we will focus on these physical characteristics.

About Oceans

The oceans are limited to the vast depressions of the earth's outer layer. They contain 97.2% of the world's total water and more than 70% of the Earth's surface. Geographers have classified the oceanic portion of the world into five oceans - the Pacific, the Atlantic, the Indian, the Southern Ocean, and the Arctic oceans.

• Pacific Ocean: It is the largest ocean in the world, bigger than all continents put together. The average depth of this ocean is 4,280 meters.
  • The Pacific Plate is by far the largest tectonic plate on Earth.
  • It is the only major oceanic plate.
• Atlantic Ocean: It is the second-largest ocean in the world. 
  • The North Atlantic Ocean emerged as a result of the breakup of the supercontinent Pangaea, while the South Atlantic arose after the Gondwana land broke in the geological past.
• Indian Ocean: It is the third-largest in the world. It is named after India. Its calm open water encouraged the sea trade earlier than the Atlantic or the Pacific Ocean. 
  • The opening of the Indian Ocean began 156 Million years ago when Africa separated from East Gondwana.
• Southern Ocean (Antarctic Ocean): It is the world’s fourth-largest ocean and youngest ocean, formed when South America moved away from Antarctica, opening the Drake Passage.
  • It has a boundary where cold, northward-flowing water from Antarctica meets warmer sub-Antarctic water.
• Arctic Ocean: It is the shallower and smallest ocean. 
  • It is bordered by Eurasia and North America. 
  • Its surface temperature and salinity fluctuate seasonally due to the melting and freezing of its ice cover.

Relief of Ocean floor

The relief features found on the ocean floor are called submarine relief. A 'Hypsometric curve' is used to depict the layout of an ocean floor. It is a graph that shows the percentage of a continent that is above or below sea level. The Ocean basins are broadly divided into four major sub-divisions: Continental Shelf, Continental Slope, Abyssal Plains, and Oceanic Deeps.

Physical Properties of Ocean Waters

Physical properties like temperature and salinity directly affect the density, buoyancy, and stability of seawater and consequently the motion of water in the ocean basins.

Temperature of Ocean Water

The temperature of the ocean's surface water varies similarly to that of the land surface. The major source of heat energy for ocean water is the radiation from the sun.

• Horizontal distribution of ocean temperature and its factors: The average temperature of surface water of the oceans is about 27°C and it gradually decreases from the equator towards the poles. 
  • Latitude: The temperature of surface water decreases from the equator to the poles as insolation decreases poleward.
  • Unequal distribution of land and water: The northern hemisphere's oceans receive more heat because they come into contact with a wider area of land than the southern hemisphere's oceans.
  • Prevailing wind: Winds moving from the land to the oceans drive warm surface water away from the coast, causing cold water from underneath to rise to the surface. This causes temperature to vary longitudinally.
    • On the contrary, onshore winds accumulate warm water along the shoreline, raising the temperature.
  • Ocean currents: Warm ocean currents raise the temperature in cold areas while cold currents decrease the temperature in warm ocean areas.
• Vertical distribution of ocean temperature:
  • Mixed layer (epipelagic zone): The warm upper layer of ocean water, extending up to 200 meters in tropical regions, has an average temperature of 20°C-25°C. This is due to the intermixing of water.
  • Thermocline layer: It is the transition layer between warmer mixed water and cooler deep water below. This layer extends from 200 metres to 1000 metres in the tropical region (500m to 1000m at mid-latitudes) and varies according to the seasons. The temperature decreases rapidly in the thermocline layer.
  • Deep water layer: Below the thermocline, temperature decrease is gradual up to 4000m. Beneath this depth, the temperature of ocean water is constant at 4°C. 

Salinity of Ocean Water

Salinity corresponds to the number of grams of dissolved salts per kilogramme of seawater and is expressed as parts per thousand (o/oo) or ppt. Together with temperature, salinity is an important property of seawater. 

• Distribution of salinity:
  • The salinity decreases horizontally from the equator towards the poles. The highest salinity is found between 20° and 40° north latitudes, which is characterised by high temperatures and evaporation but less rain than the equatorial region.
  • Salinity changes with depth, but how it changes is determined by the location of the sea. Salinity at the surface increases by the loss of water to ice or evaporation, or decreases by the input of fresh waters, such as from the rivers.
  • Areas with the highest salinity: Don Juan Pond (Antarctica), Lake Van (Turkey), Dead Sea (Israel, Jordan), Great Salt Lake (United States), etc.
• Factors affecting ocean salinity are:
  • Evaporation and Precipitation: Salinity is high where evaporation is high, and low where rainfall is high. The salinity of water is due to the combined effect of these two factors. 
    • For example, water at mid-latitudes is more saline than in the tropical region. 
  • Freshwater flow: At the same latitude, salinity depends on the influx of fresh water from rivers, streams, etc. The inflow of freshwater makes the water less saline. 
    • For example, the Bay of Bengal experiences low salinity due to river water influx, while the Arabian Sea experiences higher salinity due to high evaporation and low freshwater influx.
  • Freezing and melting of ice: In the Polar regions, the surface salinity is influenced by the processes of freezing and melting of ice.
    • Example: Antarctica and the Arctic have a low salinity. Thawing icebergs adds freshwater; icebergs that have broken off ice sheets developed over land do not contain salt, and freezing seawater into ice removes even more salt.
• Wind: Winds can decrease salinity by blowing in storm fronts that carry heavy rains and increase salinity by accelerating evaporation at the sea surface.
  • Ocean Currents: The ocean currents contribute to the salinity variations. 
    • For example, the North Atlantic Drift brings saltier water to the North Pole. 

Movement of Ocean Waters

The ocean water is dynamic, caused by differences in ocean water density due to temperature and salinity, and external influences such as the gravitational pull of the sun, moon, and the winds.

• The movement of ocean water takes place in three different ways:
  • Waves: Waves on the ocean's water are created by wind friction on the water's surface or other disturbances on the seabed.
  • Tides: The rhythmic rise and fall of the seawater due to the gravitational pull of the moon and the sun is called a Tide.
  • Ocean Currents: A large mass of moving water from one part of the ocean to another in a definite direction is called an ocean current.
    • The movement is produced due to the earth’s rotation, a temperature difference of ocean water, salinity, density, direction of winds, etc. 

Significance of Oceans

Oceans form a major part of our environment and have an overwhelming influence on humans and their activities.

• Ocean as a modifier of climate: The ocean plays a crucial role in climate modification by storing heat, supplying water vapour, and regulating temperature through ocean currents, which maintain global energy balance and facilitate heat exchange between latitudes.
• Oceans and Resources: The animals and plants of the sea constitute a vast resource from which man can derive food, fertilisers for agriculture, and raw materials for industry. 
  • “Deep Ocean Mission” is launched by India to explore the deep ocean for resources and develop deep-sea technologies for sustainable use of ocean resources.
• Oceans and Mineral Resources: Oceans are rich in various minerals, including petroleum, magnesium, salt, copper, lead, silver, and gold, as well as metals like copper, lead, silver etc. 
  • India has the exclusive rights to explore polymetallic nodules from the seabed in the Central Indian Ocean Basin. 
• Ocean Transportation and Trade: Oceans provide natural highways at a low cost for international trade. 
  • Around 80 per cent of India's merchandise trade with Europe passes through the Red Sea.
• Ocean and Energy: Changes in salinity, thermal gradients, tidal currents or ocean waves can be used to generate electricity - and provide reliable, sustainable and cost-competitive energy.

Conventions and Laws on Ocean

There are several international conventions and laws to help countries preserve order, productivity, and peaceful interactions at sea.

• United Nations Convention on the Law of the Sea (UNCLOS): This 1994 Law regulates all uses of the seas and provides the legal framework for international ocean governance. 
• High Seas Treaty: Adopted in 2023, it aims to address crucial challenges such as rising sea surface temperatures, overexploitation of marine biodiversity, overfishing, coastal pollution etc.
• UN Environment Programme: The UNEP’s Regional Seas Conventions and Action Plans is the world's only legal framework for protecting the oceans and seas at the regional level.
• UN Decade of Ocean Science for Sustainable Development (2021-2030): It aims to promote ocean science and knowledge generation to improve the state of the ocean system and facilitate the sustainable development of this vast marine ecosystem.

Maritime Zones

Maritime zonеs rеfеr to specific areas of thе ocean that arе dеfinеd by intеrnational law and rеgulations. Thеsе zones are established to govern various aspects of maritime activities, rеsourcе managеmеnt, еnvironmеntal protеction, and national sеcurity.

Oceanography UPSC PYQs

Question 1: Tides occur in the oceans and seas due to which among the following (UPSC Prelims 2015)

1. Gravitational force of the Sun
2. Gravitational force of the Moon
3. Centrifugal force of the Earth

Select the correct answer using the codes given below.

1. 1 only
2. 2 and 3 only
3. 1 and 3 only
4. 1, 2 and 3

Answer: (d)

Question 2: Consider the following factors: (UPSC Prelims 2012)

1. Rotation of the Earth
2. Air pressure and wind
3. Density of ocean water
4. Revolution of the earth

Which of the above factors influences the ocean currents?

1. 1 and 2 only
2. 1, 2 and 3 only
3. 1 and 4 only
4. 2, 3, and 4 only

Answer: (b) 

Question 3: Account for variations in oceanic salinity and discuss its multidimensional effects. (UPSC Mains 2017)

Oceanography FAQs

Q1. What are ocean deposits?

Ans. Ocean deposits are the unconsolidated sediments (Inorganic or Organic ) derived from various sources deposited on the oceanic floor.

Q2. What causеs ocеan tidеs?

Ans. Ocеan tidеs arе primarily causеd by thе gravitational pull of thе moon and thе sun on Earth's watеrs. The moon's gravitational forcе is strongеr as it is closеr to Earth, lеading to high and low tidеs.

Q3. What is ocean floor configuration?

Ans. The configuration of Ocean basins includes Continental margins encompassing Continental shelf & Continental slope, and Deep ocean basins encompassing the Abyssal plains, abyssal hills, ocean trenches, Mid-oceanic ridges and deep ocean floor. 

Q4. Why are oceans blue?

Ans. The oceans are blue because water absorbs colours in the red portion of the light spectrum. This acts as a filter, leaving colours in the blue section of the light spectrum visible to us.

Q5. What are the sources of salt in the ocean?

Ans. Most of the ocean salts are derived from weathering and erosion of the earth’s crust by the rivers. Some of the ocean salts have been dissolved from rocks and sediments below the sea floor, while others have escaped from the earth’s crust through volcanic vents as solid and gaseous materials.