Oceanography Notes for OPSC/UPSC: Part I

These oceanography notes for OPSC/UPSC (Part I) cover the water cycle, ocean floor relief, trenches and ridges, ocean zones, and the temperature, salinity and density of sea water. Prelims asks mostly facts and statement-based questions from these areas, while mains uses them for Blue Economy, climate and cyclone answers. Waves, currents, tides, coral reefs and marine deposits are in Part II.

Oceanography Notes for OPSC/UPSC: Hydrosphere and Water Cycle

Distribution of water on Earth

Oceans cover about 71% of the Earth’s surface and hold about 97% of all water. The Northern Hemisphere is about 61% water and the Southern Hemisphere about 81%, which is why the south is called the water hemisphere. The Pacific is the largest and deepest ocean, covering about one-third of the globe.

Hydrological cycle

• Evaporation is conversion of liquid water to vapour. It absorbs latent heat, which is why an open pond stays cool on a hot day. CDS 2009
• Transpiration is loss of water vapour from plants after water is drawn from the soil. Evapotranspiration is evaporation plus transpiration together.
• Sublimation is the direct change of ice to vapour.
• Condensation and precipitation form clouds and rain, snow or hail.
• Infiltration and runoff recharge groundwater and return water to rivers and the sea.

The standard sequence is evaporation, condensation, precipitation, infiltration. NDA 2010 Oceans lose more water by evaporation than they gain from rain, and the gap is balanced by runoff from land. Groundwater and surface water are connected through springs and seepage. CDS 2015

Ocean Relief and Ocean Floor Features

The ocean floor has relief as varied as the continents and more extreme in range. The mean depth of the oceans is about 3,700 m against a mean land height of about 840 m. The deepest point, Challenger Deep, is nearly 11 km below sea level, deeper than Mount Everest is high. The main divisions are the continental margin, the deep sea plain, the mid-oceanic ridges and the trenches.

Continental shelf, slope and rise

• Origin of shelves: deposition of marine sediment, submergence of the continental platform, and faulting of continental margins.
• Width: widest where land is low and plain-like (the Siberian shelf in the Arctic is over 1,000 km wide); narrow where young fold mountains run parallel to the coast, as along the Pacific coast of the Americas. The Atlantic has a larger share of shelf than the Pacific. NDA 2012
• Importance: shelves are rich fishing grounds because light and river-borne nutrients reach shallow water, and most offshore oil and gas comes from them, for example Bombay High in India.

Submarine canyons

Submarine canyons are deep, steep-sided valleys cut into the slope and sometimes the shelf. The chief agent is the turbidity current, a dense flow of sediment-laden water. Examples include the Hudson Canyon off the United States, the Congo Canyon and the Swatch of No Ground in the northern Bay of Bengal off the Ganga delta.

Abyssal plains, seamounts and guyots

• Abyssal plains lie at about 3,000-6,000 m depth and are among the flattest surfaces on Earth because thick sediment buries the relief. They are covered by pelagic deposits (oozes and red clay).
• Seamount: an underwater mountain, usually volcanic, that does not reach the surface.
• Guyot: a flat-topped seamount, believed to be a former volcanic island levelled by waves and then submerged.
• Atoll: a ring-shaped coral reef around a lagoon, often on a sunken volcano.

Mid-oceanic ridges

Mid-oceanic ridges form a global chain of about 65,000 km, and mark divergent plate boundaries where new oceanic crust forms. Standard MCQ The Mid-Atlantic Ridge is S-shaped, runs roughly north-south and rises above sea level at Iceland. Its length is quoted as about 14,000 km in the standard MCQ key, while some sources give about 16,000 km.

Ocean trenches

Trenches are the deepest parts of the ocean. They form at convergent boundaries where one plate subducts below another, and most lie around the Pacific rim.

Ocean Zones: Pelagic and Benthic Zones

Ocean life and light are divided by depth and by distance from the sea bed.

• Pelagic zone: open water neither near the bottom nor near the shore, and commercially the most important division. Pelagic life decreases with depth.
• Benthic zone: the sea bed; its organisms are called benthos and include many animals and plants, not only micro-organisms.
• Demersal zone: the water layer just above the benthic zone.

Temperature of Ocean Water

Factors and horizontal distribution

Ocean temperature depends on latitude, the distribution of land and water, ocean currents and prevailing winds. The average surface temperature is about 27°C in the tropics and falls toward the poles. The warmest water lies slightly north of the equator (the thermal equator). The Northern Hemisphere oceans are warmer than the Southern because of the greater land area, and Southern Hemisphere isotherms are more regular because the south is mostly ocean.

Vertical distribution and thermocline

The ocean has a warm surface mixed layer, a thermocline and a cold deep layer below.

• It is a permanent feature in the tropics, seasonal in mid-latitudes, and almost absent in polar seas where surface and deep water are equally cold.
• Below the thermocline water is close to freezing or a few degrees above.

Ocean Mean Temperature and the monsoon

Ocean Mean Temperature (OMT) is measured up to the depth of the 26°C isotherm. OMT collected in January-March can be used to assess whether monsoon rainfall will be above or below the long-term mean, and it is a more stable indicator than sea surface temperature. The mean depth of the 26°C isotherm in the south-western Indian Ocean in January-March is about 59 m, not 129 m. UPSC 2020

Salinity of Ocean Water

Factors affecting salinity

• Raise salinity: high evaporation, low precipitation, anticyclonic stable conditions and enclosed sea basins.
• Lower salinity: river inflow, heavy rain, melting ice and mixing with open ocean.
• Higher salinity lowers the freezing point and lowers the rate of evaporation, and raises density.

Distribution of salinity

Salinity rises from the equator to about 20-30° latitude and then falls toward the poles, so the maximum lies in the tropics and subtropics, not at the equator, where heavy rain dilutes the water. Polar surface water is fresh overall because of low evaporation and melting ice, even though freezing leaves local brine.

The usual increasing order is Baltic Sea, Arctic Sea, Gulf of California, Red Sea. CDS 2012

Halocline, thermocline and pycnocline

Salts in sea water

Density and Thermohaline Circulation

The average density of sea water is about 1.027 g/cm³. Density increases with salinity, with cooling and with pressure, and decreases when water is heated. Sea water of 35‰ freezes at about −1.9°C, and unlike fresh water it keeps getting denser until it freezes. In polar areas temperature varies little, so salinity becomes the main control on density. Density is lowest in the warm surface layer and highest in cold polar water.

The pycnocline is the layer of rapid density change. Swimming is easier in sea water because it is denser, so it gives more buoyancy. CDS 2008

Thermohaline circulation

Thermohaline circulation is the density-driven, large-scale circulation also called the ocean conveyor belt or meridional overturning circulation. Cold, salty water sinks in the North Atlantic and around Antarctica, flows slowly along the deep ocean and rises in other regions. It redistributes heat and strongly affects global climate. For mains: weakening of the Atlantic circulation is studied as a climate risk, because it would change heat transport to Europe and tropical rainfall patterns.

Maritime Zones and Ocean Resources for Mains

• India’s zones are defined by the Territorial Waters, Continental Shelf, Exclusive Economic Zone and other Maritime Zones Act, 1976, and the EEZ is about 2.02 million sq km.
• India made a submission to the Commission on the Limits of the Continental Shelf in 2009 for an extended shelf.
• The Deep Ocean Mission was approved by the Union Cabinet in June 2021 with an outlay of ₹4,077 crore over five years under the Ministry of Earth Sciences. It includes a manned submersible, Matsya 6000, planned to reach 6,000 m, and deep-sea mining technology.
• India holds a 75,000 sq km site in the Central Indian Ocean Basin allotted by the International Seabed Authority for polymetallic nodules.

Mains analysis points

• Why ocean floor mapping matters: fishing, cables, hydrocarbons, nodules, disaster modelling and defence.
• How salinity and temperature interact to shape stratification, cyclones and monsoon behaviour.
• Role of the thermohaline circulation in climate regulation and what its weakening would imply.
• Blue Economy: opportunities from fisheries, offshore energy and seabed minerals against risks of overfishing, pollution and conflict.

Oceanography in Odisha: Bay of Bengal and Chilika

• Odisha has a coastline of about 480 km on the Bay of Bengal, where surface waters stay warm enough for cyclones to intensify. Cyclones such as the 1999 Super Cyclone, Phailin (2013) and Fani (2019) crossed the Odisha coast.
• The Bay of Bengal has lower surface salinity than the Arabian Sea because the Ganga, Brahmaputra, Mahanadi and other rivers discharge large volumes of fresh water, and monsoon rainfall adds to it.
• Chilika Lake is the largest brackish water lagoon in Asia and the first Indian wetland designated a Ramsar site (1981). Its salinity changes with season and has a gradient: fresher in the northern sector fed by Mahanadi distributaries and saltier toward the outer channel and sea mouth. The Chilika Development Authority opened a new mouth in 2000 to restore tidal exchange and salinity balance.
• The offshore Mahanadi basin lies in the Bay of Bengal off Odisha and is a hydrocarbon exploration area on the shelf and slope.
• The Bengal Fan, the largest submarine fan in the world, lies in the Bay of Bengal and is built from river sediment.

Oceanography Practice Questions and PYQs