• Division of ocean floor
  • Continental shelf
  • Continental slope
  • continental rise
  • Deep ocean floor
  • Oceanic ridge
  • Submarine Canyons
  • Trenches
  • Sea mounts
  • Hills
  • Guyots
  • Marginal sea
  • Bays, gulf, straits, and Isthmus
  • Continental shelf deposits
  • Abyssal plain deposits
  • UN Convention on Laws of Seas (UNCLOS) and sea division
  • Ocean zones
  • Intertidal, Photic, Neritic, Aphotic, Oceanic, Bentic
  • Marine deposits
  • Marine pollution
  • Types of islands
  • Ocean temperature
  • Salinity
  • Oceanic movement
  • Waves
  • Ocean current
  • Forces responsible
  • Gyres
  • Equatorial Countercurrents
  • North Indian Ocean Current
  • Effects of ocean currents
  • Upwelling and Downwelling
    • Ekman spiral and transport
    • Nutrients enrichment
  • Tide
    • Ebb and Flood/Flow
    • Moon position and tide (Syzygy and Quadrature)
  • Coral reefs
  • Relief features
  • Coral bleaching
  • Kelp Forest


The oceans are the largest and most prominent feature on Earth. In fact, they are the single most defining feature of our planet. Water covers roughly around 70% of Earth’s surface. The world ocean is a single inter-connected body of water, which is large in size and volume. It can be divided into five principal oceans– the Pacific, Atlantic, Indian, Southern or Antarctic Ocean, and Arctic Ocean.



The ocean floors can be divided into four major divisions

  • Continental-Oceanic margin
  • Continental Shelf
  • Continental Slope
  • Continental Rise
  • Deep ocean plains
  • Oceanic Ridges


Continental-Oceanic margin

Continental Shelf

  • Angle is 1º, depth is 120-150 meter, and it extends generally 70 km into the sea. But this varies a lot
  • Width=The continental shelf is virtually absent in west coast of South America.It is 120 km wide in east coast of North America. In Bay of Bengal too, it is very wide.
  • Depth =It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
  • The shelf typically ends at a very steep slope, called the shelf break.
  • Continental Shelf of all oceans together covers5% of the total area of the oceans.
  • The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers etc.
  • The shelf is formed mainly due to
  • Submergence of a part of continent
  • Relative rise in sea level
  • Sedimentary deposits brought down by rivers


Continental Slope

  • At the end of continental shelf slope steepens abruptly.
  • The continental slope connects the continental shelf and the ocean basins.
  • The gradient of the slope region varies between 2°-5°.
  • The depth of the slope region varies between 200 m and 3km.
  • The seaward edge of the continental slope loses gradient at this depth and gives rise to continental rise.
  • Canyons and trenches are observed in this region.


Continental Rise

  • The continental rise is a sediment underwater feature found between the continental slope and the abyssal plain.
  • Sediment, silt, and other material are picked-up by river when it travels over land, which is gradually carried out to sea. Some of these sediments settle on the continental shelf, but others drift down the continental slope to form the continental rise.
  • The formation of the continental rise is a constant and very slow process.


Deep Sea Plain (Abyssal Plain)

  • At the end of continental slope, slope becomes gentle again to 0.5º to 1º.
  • Its end marks the end of continental margin.
  • Undulating plain lies 2-3 miles below sea level and cover 40% of ocean floor.
  • Lying generally between the foot of a continental rise and a mid-ocean ridge.
  • Abyssal plains cover more than 50% of the Earth’s surface.
  • These plains are covered with fine-grained sediments like clay and silt.


Oceanic Ridges

  • A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. [Divergent Boundary]
  • The oceanic ridge system is a continuous underwater mountain range. It is created when magma rising between diverging plates of the lithosphere cools and forms a new layer of crust.


There are also minor relief features in the ocean floors like

  • Submarine Canyons
    • Canyons are deep concave gorges on continental shelf, slope or rise, often extending from the mouths of large rivers.
  • Trenches
    • Long narrow and steep depression on abyssal plain is called a trench.
    • The trenches lie along the fringes of the deep-sea plain at the bases of continental slopes and along island arcs.
    • They are of tectonic origin and are formed during Ocean-Ocean Convergence and Ocean-Continent Convergence.
    • They are some 3-5 km deeper than the surrounding ocean floor.
    • The deeper trenches (> 5500 meters) are called deeps.
    • The trenches run parallel to the bordering fold mountains or the island chains.
    • They are associated with active volcanoes and strong earthquakes (Deep Focus Earthquakes like in Japan).
  • Sea mounts
    • Sea hills on abyssal plains rising above 1000 meters from the floor are called sea mounts.
  • Hills
    • Sea hills on abyssal plains rising less than 1000 meters from the floor are called Abyssal hills.
  • Guyots
    • Guyots are seamounts which have flat tops. All of them are generally of volcanic origin.


Marginal Sea

A marginal sea is a division of an ocean, partially enclosed by islands, archipelagos, or peninsulas, adjacent to or widely opens to the open ocean at the surface, and/or bounded by submarine ridges on the sea floor.

Examples (Locate these on Atlas)

  • Arabian Sea
  • Persian Gulf
  • Red Sea
  • Gulf of Oman
  • Gulf of Aden
  • Gulf of Kutch
  • Gulf of Khambat
  • Bay of Bengal
  • Andaman Sea
  • Malacca Strait
  • Mozambique Channel
  • Great Australian Bight
  • Gulf of Mannar
  • Laccadive Sea



Bays, Gulfs, Straits, and Isthmus
  • Bays, gulfs, and straits are types of water bodies that are contained within a larger body of water near land.
  • These three water bodies are usually located at important points of human activities; thus, conflicts with nature and neighbors are common.



  • Bay is a water body surrounded on three sides by land with the fourth side (mouth) wide open towards oceans (In Gulfs, the mouth is narrow).
  • A bay is usually smaller and less enclosed than a gulf.
  • Example: Hudson Bay (Canada), Bay of Bengal etc.
  • An example of a bay at a river’s mouth is New York Bay, at the mouth of the Hudson River (Hudson Estuary).



  • A gulf is a large body of water, sometimes with a narrow mouth, that is almost completely surrounded by land.
  • The world’s largest gulf is the Gulf of Mexico.
  • Other Examples = Gulf of California, Gulf of Aden (between the Red Sea and the Arabian Sea), and the Persian Gulf (between Saudi Arabia and Iran), Gulf of Mannar.



  • A strait is a narrow passageway of water between the landmass (continents or islands).
  • When a body of water such as a strait is capable of being blocked or even closed in order to control transportation routes, the body is called a “choke point”.



  • Isthmus is the land-equivalent of a strait. i.e., a narrow strip of land connecting two larger land masses.
  • Example: Isthmus of Panama and Isthmus of Suez.


Continental Shelf Deposits


  • They are responsible for preventing cold under-current from rising and also increases the height of tides.
  • They are excellent location for ports.
  • Rich in marine organisms:The sunlight reaches the shelves and so minute plankton grows on them. This causes fishes to reach the shelves. The shelves are thus known as the richest fishing grounds of the world.



  • 90% of petroleum reserves are found in shelves. E.g.: Bombay High, Gulf of Cambay, Persian Gulf, Strait of Hormuz, Arctic Ocean, and Gulf of Mexico.
  • Sulfur metal rarely found on land but is abundant in Gulf of Mexico continental shelf due to marine volcanism.
  • Concentration of heavy metals on shelf. E.g.: Monazite sand in Kerala (Thorium), gold, silver and diamonds.
  • Pearls.


Resources from an abyssal plain

  • Poly metallic nodules are a concentration of metals around a core. Found in all oceans. Some of the minerals are rare on land.


Polymetallic Nodules (PMNs)
  • Polymetallic nodules (also called as manganese nodules) are small potato-sized (from millimetres to tens of centimetres in diameter) lumps of minerals found in deep sea.
  • They contain nickel, copper, cobalt, lead, cadmium, vanadium, molybdenum, titanium in various proportions of which nickel, cobalt and copper are considered to be of economic and strategic importance.
  • They are found in abundance carpeting the sea floor of world oceans in deep sea.
  • India was the first country to receive the status of a pioneer investor for exploration and utilization of PMNs.
  • It was allocated an exclusive area in Central Indian Ocean Basin by United Nations (UN) in 1987.
  • National Institute of Ocean Technology (NIOT) will launch ‘Samudrayaan project’ by 2021-22 to explore the deep sea region. It is pilot project of Union Ministry of Earth Sciences as part the ‘Deep Ocean mission’ for deep ocean mining of rare minerals.
  • Samudrayaan project
  • It proposes to send indigenously developed submersible vehicle with three persons to a depth of about 6000 metres to carry out deep underwater studies.


Significance of Polymetallic Nodules

  • They contain Rare Earth Elements and metals which are important to high-tech industries.
  • The amount of copper contained in the CCZ nodules is estimated to be about 20% of that held in global land-based reserves.
  • These Rare earth minerals are considered as the great source of valuable minerals such as gold, silver and zinc.
  • Presently, China is controlling more than 95% of rare earth metals. The exploration by India will nullify the increasing influence of China.



  • The extraction of metals from the PMN is not yet found to be economically viable at this stage.
  • The deep sea mining without holistic approach may cause a disturbance in the aquatic ecosystem.


UN Convention on Laws of Seas

  • Decides deep sea mining, environment protection, maritime boundary and dispute settlement.


UNCLOS sections the oceans into:

Territorial waters

  • 12 Nautical Miles from the baseline.
  • Countries are free to set laws and use its resources.
  • Foreign vessels arenot given all rights to passage through except “Innocent Passage”.
  • Passing through the waters which are not prejudicial to peace and security.
  • Nations have right to suspend the innocent passage.
  • Submarine while passing through other country’s territorial waters has to navigate on the surface and show their flags.


Contiguous Zone

  • Area 12 Nautical Miles beyond the Territorial waters (i.e. 24 Nautical Miles from the baseline limit).
  • Country can enforce laws only in 4 areas viz. pollution, taxation, customs, and immigration.


Exclusive Economic Zones (EEZs)

  • Area from the edge of the territorial sea out to 200 nautical miles from the baseline.
  • Country has sole exploitation rights over all natural resources.
  • The most important reason to introduce EEZ was to halt the clashes over the fishing rights and oil rights.
  • Foreign vessels have freedom of navigation and over flight, subject to the regulation of the coastal states.
  • Foreign states are allowed to lay submarine pipes and cables.


International Seabed Authority
  • It is an intergovernmental body that organizes, regulates and controls all mineral (non-living resources) related activities in the international seabed area beyond the limits of national jurisdiction and ensures that marine ecosystem remains protected.
  • It was established in 1994by the Law of the Sea Convention and functions under the aegis of the United Nations Convention on Law of the Sea (UNCLOS).
  • HQ at Jamaica.
  • It has an observer status to UN.
  • The total area under its regulation is more than 54% of the total area of the world’s oceans.
  • India is its member.
  • ISA has registered & voluntary commitments towards Agenda 2030 for sustainable development.


Marine Deposits


  • River brought sediments
  • Weathering due to wave actions
  • Wind Blown Dust
  • Submarine erosion
  • Marine life decomposition
  • Extraterrestrial (Meteorites)
  • Volcanic


Marine Pollution

Marine pollution is a combination of chemicals and trash, most of which comes from land sources and is washed or blown into the ocean. This pollution results in damage to the environment, to the health of all organisms, and to economic structures worldwide.


Types of Islands


Type Description Diagram
Continental Island
  • Continental islands are simply unsubmerged parts of the continental shelf that are entirely surrounded by water.
  • Once a part of large continent separated by geomorphologic processes.
  • Many of the larger islands of the world are of the continental type.
  • Example=SriLanka, Madagascar, Vancouver, Baffin Island
Volcanic Island
  • Volcanic islands occur in ocean basins (such as the Hawaiian Islands) or on or near ocean ridges (e.g., St. Paul Rocks and Ascension Island in the Atlantic Ocean) or on hotspot.
  • They are large volcanoes erupted on the seafloor whose tops have emerged above sea level.
  • Example = Japan, Philippines, Kurile, Aleutian
Sand-bar Island
  • A sandbar is an area of sand, gravel or fine sediment that sits above the water.
  • It may be connected to the shoreline, or it may be offshore.
  • It is generally narrow and straight.
  • A sandbar is also known as a shoal or sandbank.
  • A large sandbar is called a barrier island.
  • Example = Long Island (New York)
Coral Island
  • Island built of organic material derived from skeletons of corals and numerous other animals and plants associated with corals.
  • Coral islands consist of low land perhaps only a few meters above sea level.
  • Example=Lakshadweep, Maldives,Mauritius, Bahama, Bermuda



  • The study of the temperature of the oceans is important for determining the
    • Movement of large volumes of water (vertical and horizontal ocean currents),
    • Type and distribution of marine organisms at various depths of oceans,
    • Climate of coastal lands, etc.
  • Main energy source for ocean temperature – Insolation (Incoming Solar Radiation)
  • Oceans play an important role in energy and temperature regulation on earth (it warms slowly in comparison of land due to its specific heat of the water).
  • Average temperature of ocean = 3-5 degree Celsius
  • But average surface temperature of ocean water = 25 degree

Factors affecting temperature distribution



  • Surface water temperature decreases from the equator towards the pole.
  • It is due to the declining intensity of insolation per unit area while moving towards pole.
  • But highest temperature is not at the equator insteadat the tropics (high rainfall, cloud cover = high albedo/ reflection of sunrays)


Hemispheric variation

  • Unequal distribution of land and water.
  • Northern hemisphere is warmer than southern because of large land mass in northern hemisphere (High specific heat of water causes slower heating of it).


Prevailing winds (Longitudinal variation of temperature)

  • The offshore winds (Winds blow towards the ocean from the land) drive warm surface water away from coast.
  • This results in upwelling of cold water from below.
  • The onshore winds raise the temperature at coast by piling up warm water near it.


Ocean currents

  • Oceanic currents describe the movement of water from one location to another.
  • Warm ocean currents = Increases the temperature of cold areas (like Gulf stream)
  • Cold ocean currents = Lowers the temperature in cold areas (Like Canary current)


Enclosed and open sea

  • Temperature at Lower latitudes = Enclosed sea > Open sea
  • Temperature at Higher latitude = Open sea > Enclosed sea


Physical characteristics of the sea surface: 

  • Boiling point of the sea water increases with increasing salinity and vice versa.
  • Salinity increased –> Boiling point increased –> Evaporation decreased


Diurnal range of temperature

  • Maximum temperature in day and minimum temperature in night time.
  • Tropical water has higher diurnal range (due to less cloud) than equatorial waters.
  • Because, heating and cooling of water rapid under clear sky.


Annual range of temperature

  • Bigger the size of ocean =Better mixing of water and heat + Slow heating
  • Hence, lower annual range
  • Pacific ocean –> Lower annual range than Atlantic Ocean


Variationin ocean temperature

  • Earth’s surface at equator receives about four times more average incoming solar energy than at pole.
  • Radiation penetrates some distance below the surface due to the transparency of water.
  • Shorter wavelengths (high energy) penetrate deeper than longer wavelengths. Heat is carried further to the deeper levels by mixing.
  • Diurnal and seasonal temperature variations are relatively small in water than on land due to the high specific heat of water.
  • Most solar energy is absorbed within a few meters of the ocean surface by directly heating the surface water. This provides the energy for photosynthesis for marine plants and algae.


Vertical variation in oceanic temperature

  • Vertical distribution of temperature in the deep ocean is controlled by density driven water movements.
  • The maximum temperature of oceans is always at their surface due to the direct incidence of solar energy.
  • Heat conduction by itself is extremely slow, so only a small proportion of heat is transferred downwards by this process.
  • The heat is transmitted to the lower sections of the oceans through the process of convection.


Thermal layer distribution in ocean

  • 1stlayer
    • Top layer consists of warm oceanic water.
    • It is about 500 meter thick with temperature range between 20-25°C.
    • This layer exists throughout the year in tropical region but develops only during summer in mid-latitude.
  • 2ndlayer
    • Temperature decline rapidly between the depths of about 200 meters to 1000 meters.
    • This region of steep temperature gradient is known as the permanent thermocline.
    • About 90 per cent of the total volume of water is found below the thermocline in the deep ocean. In this zone, temperatures approach 0°C.
    • The thermocline is less steep (almost absent) in Polar Regions as the surface temperatures are close to 0°C which results in small variations in temperature with depth.
  • 3rdlayer
    • There is virtually no seasonal variation beyond 1000 meters (beneath permanent thermocline) and the temperatures are around 2°C.
    • This layer extends up to the deep ocean floor.
    • This narrow range is maintained throughout the deep oceans, both geographically and seasonally, because it is determined by the temperature of the cold, dense water that sinks at the polar-regions and   flows towards the Equator.


Horizontal variation in oceanic temperature

  • Average temperature of surface water of the ocean is about 27 °C.
  • Average temperature gradually decreases from the equator towards the poles.
  • The ocean temperature records,relativity lower temperature in southern hemisphere then the northern hemisphere.
  • This anomaly is a result of unequal distribution of land and water in the northern and southern hemispheres.
  • The highest temperature is recorded slightly away from equator in the northern direction.



Throughout Earth’s history, certain processes have served to make the ocean salty. The weathering of rocks delivers minerals, including salt, into the ocean.

  • Salinity is a vital property of sea water. It is determined by the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.
  • It is usually expressed as parts per thousand (o/oo) orppt.
  • Salinityof 7 o/oo has been considered as the upper limit to demarcate ‘brackish water’.
  • Even small variations in ocean surface salinity (i.e., concentration of dissolved salts) can have dramatic effects on the water cycle and ocean circulation.
  • Factors that affect the salinity of the oceans can be broadly grouped into two categories:

1.Factors that increase salinity(due to increase in salt concentration by extracting fresh water from the ocean)

        • Evaporation from the ocean’s surface waters removes water molecules, leaving the salt behind.
        • Ice formation as freezing of ice leaves salt in the water.
        • Advection of more saline water
        • Mixing with more saline deep water(Due to the ocean currents)
        • Solution of salt deposits

2. Factors that decrease salinity (due to the decrease in salt concentration by incorporating freshwater into the ocean)

        • Precipitation on the ocean surface waters adds water molecules.
        • Melting of ice which dilutes the concentration of salt in the water.
        • Advection of less saline water
        • Mixing with less saline deep water(Due to the ocean currents)
        • Inflow of fresh water from land
  • Surface salinity majorly affected by three processes
    • Decrease of salinity by precipitation
    • Increase of salinity by evaporation
    • Change of salinity by mixing processes (Due to the ocean currents)


Salinity of the oceans

  • Standard salinity of ocean water is = 35.5 ppt – salinity of Atlantic Ocean
  • Dead Sea (350 salinity), Lake van (400), Lake Urmia
  • Man seldom drowns in sea with high salinity
  • Because, high salinity = high density


Sources of salts in ocean water

  • Sediments carried by rivers.
  • Submarine volcanism at Oceanic Ridge.
  • Chemical reaction between rocks of geothermal vent of volcano and cold water.
  • Erosion of oceanic rocks.


Share of different salts is as shown below

Removal of Salts in Ocean water 

  • Physical removal – waves break at the beaches, salt-spray
  • Biological removal – marine lifeforms extract calcium from sea water for their bones.


Distribution of salinity

 Vertical (Change with depth)

  • Salinity changes with depth, but the way it changes depend upon the location of the sea.
  • Other factors being constant, increasing salinity of seawater causes its density to increase. High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.
  • There is a distinct zone called the halocline(compare this with thermocline) where salinity increases sharply.



  • Highest salinity is found near tropics.
  • It decreases towards equator and pole
    • Equator = Heavier rains causes incorporation of freshwater
    • Poles = Less evaporation prevents removal of water molecule from the surface



Temperature and density share an inverse relationship.

    • As temperature increases, the space between water molecules increases (hence volume increases) which therefore decreases the salinity.
    • If the temperature of water decreases its density increases, but only to a point. 
    • At a temperature of 4°C pure water reaches its maximum or peak density, cooled further it expands and becomes less dense than the surrounding water which is why when water freezes at 0°C it floats.


Density = Mass/Volume

  • Example = Let the mass of a definite water body is 10 and volume is 5, which gives density is equal to 2. If the volume increases to 10 keeping mass constant, the density reduces to 1.
  • Salinity and density share a positive relationship.
    • As density increases, the amount of salts in the water (also known as salinity), increases.
  • The ocean water is constantly churning underneath, bringing nutrients up to the top.
    • The difference in density of cold water versus density of warmer water is responsible for ocean currents and upwelling.
    • Warm seawater floats and cold (4° C), dense (1 g/cm3) seawater sinks, so ocean temperatures also vary across the surface and into the depths.


Variation of density, salinity, and temperature with the oceanic depth


  • When the temperature, density or salinity of a layer changes rapidly, this region is referred to as a cline.
  • Thermoclines are areas of rapid change in temperature. Areas of rapid change in density are pycnoclines and areas of rapid change in salinity are haloclines.


  • The ocean water is dynamic and its movement is influenced by physical characteristics like temperature, salinity, density and the external forces like of the sun, moon and the winds.
  • The horizontal and vertical motions are common in ocean water bodies.
  • The horizontal motion refers to the ocean currents and waves.
  • Water moves ahead from one place to another through ocean currents while the water in the waves does not move, but the wave trains move ahead.
  • The vertical motion refers to tides (rise and fall of water in the oceans and seas) and upwelling.
  • Due to attraction of the sun and the moon, the ocean water is raised up and falls down twice a day.
  • The upwelling of cold water from subsurface and the sinking of surface water are also forms of vertical motion of ocean water.
  • Vertical currentsarise mainlydue todensity differencescausedby temperatureandsalinity changes.


  • Waves are formed due to friction between wind and surface water layer.
  • The stronger the wind, the bigger the wave.
  • They die out quickly on reaching shore or shallow waters.
  • Actual motion of water beneath the wave is circular.


Important terminologies related to wave

  • Crest and Trough: The highest and lowest points of a wave are called the crest and trough respectively.
  • Wave height: It is the vertical distance from the bottom of a trough to the top of a crest of a wave.
  • Wave amplitude: It is one-half of the wave height.
  • Wave period: It is the time interval between two successive wave crests or troughs.
  • Wavelength: It is the horizontal distance between two successive crests.
  • Wave frequency: It is the number of waves passing a given point during one second time interval.
  • Wave speed: It is the rate at which the wave moves through the water. It is measured in knots.


Breaking of wave

When the depth of water is less than half the wavelength, the wave breaks.



  • The size and force of the wave depends on
    • Velocity of the wind
    • Duration of the wind
    • Distance over which the wind blows unhindered (the fetch)
  • If the water is deep (so that bottom doesn’t interfere with the wave), winds blow over large distances for longer time, waves are bigger.

Important effects by waves

  • Sea: Normally several trains of differing wave lengths and directional movements of waves occurs in seas. This overall irregular and chaotic wave pattern is called sea.
  • Swell: If the wave moves away from the disturbance zone into a smooth zone, there appears a uniform pattern of equivalent wavelength and amplitude. This is called swell.
  • Surf: As the swell reaches shore, the pattern gets disturbed due to friction from the shallow sea floor and the wave breaks. The lower part of the wave travels more slowly than the upper part so its height increases, they crowd together and eventually breaks. This breaking is called surf.
  • Swash: Once the wave breaks, the top fallen over part is carried over to the shore. This is called swash.
  • Backwash: As the water goes back to the sea, it is called backwash.



Ocean current is a continuous, directed movement of ocean water masses that flow from one place to another circumnavigating the earth. The forces that initiate the movement of water are:


  • The major ocean currents are wind-driven currents, though some ocean currents result from density and salinity variations of water.
    • The subtropical high pressure cells are responsible for many of the Earth’s great ocean currents.


Heating by solar energy

  • Water expands under high temperature.
    • Near equator, water is lighter than at the poles.
    • Near poles, water is cold hence denser and sinks down causing movement of water from equator towards the pole.
    • Cold water from pole flow towards equator at subsurface level, to balance loss of water at equator.



  • Salinity increases density of water which sinks at depth.
    • Less saline water move towards high saline water on the surface.
    • High saline water move towards less saline water at sub-surface
  • Equator → rainfall (less saline water)
  • Poles → iceberg (high saline water)


Thermohaline differences

  • Thermohaline circulations are due to changes in density (result of changes in temperature and salinity).
  • Thermohaline circulations can be both –
    • Vertical (i.e. upwelling or downwelling)
    • Horizontal (Ocean current movement)


Ocean currents can be classified based on temperature as cold currents and warm currents:  

Warm currents bring warm water into cold water areas and are usually observed on the east coast of continents in the low and middle latitudes (true in both hemispheres). In the northern hemisphere, they are found on the west coasts of continents in high latitudes.

Cold currents bring cold water into warm water areas. These currents are usually found on the west coast of the continents in the low and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere.



Subtropical Gyres

  • The large, circular moving loops of water that are driven by the major wind belts of the world are called gyres which generally comprises of four main currents.
  • They rotate clockwise in the Northern Hemisphere and counter clockwise in the Southern Hemisphere. World’s five subtropical gyres:
    • The North Atlantic Subtropical Gyre or Sargasso sea (due to presence of seaweed called Sargasso on the surface)
    • The South Atlantic Subtropical Gyre or Navigator gyre (after the Portuguese prince Henry, the navigator)
    • The North Pacific Subtropical Gyre or Turtle gyre
    • The South Pacific Subtropical Gyre or Hyerdahl gyre
    • The Indian Ocean Subtropical or Majid gyre


Sargasso Sea

  • The Sargasso Sea is a motionless sea confined to the sub-tropical north Atlantic gyre.
    • The gyral system formed by the anti-cyclonic circulation of oceanic water.
    • Atmospheric stability due to it being located in the transition zone of the trade winds and the westerlies which is characterized by anti-cyclonic conditions.
    • Hence, there are feeble winds which allow little intermixing with waters outside the gyre.
  • It is located entirely within the Atlantic Ocean, is the only sea without a land boundary.


Equatorial Countercurrents

Equatorial currents moves large volume of water westward. As a result water piles up along the western margin of an ocean basin, which raises sea level on the western side of the basin.

  • The water on the western margins then flows downhill under the influence of gravity, creating narrow equatorial counter currents that flow to the east counter to and between the adjoining equatorial currents.


North Indian Ocean Currents

  • Indian Ocean is half an ocean; hence the behavior of the North Indian Ocean Currents is different from that of Atlantic Ocean Currents or the Pacific Ocean Currents.
  • Seasonal monsoon winds peculiar to the Northern Indian Ocean region directly influence the ocean surface water movement.
  • Due to the seasonal change of ocean current circulation, North Indian Ocean Currents is important for sea-trade, cultural interaction.
  • During summer, wind and current flow towards India from Arabia in influence of strong south-west monsoon winds.
    • Sea vessel moves from Arabia to India by utilizing wind and ocean current.
  • During winter, wind and current flow towards Arabia from India in influence of prevailing trade winds (easterly trade winds)
    • Sea vessel moves from India to Arabia by utilizing wind and ocean current.


Effects of ocean currents

  1. Meeting of cold and warm currents
    • Creates excellent fishing zones.
      • Kurishio (warm) + Oyashio (cold)
      • Labrador (cold) + Gulf Stream(warm)
      • Falkland (cold) + Brazilian(warm)
    • Mixing of cold and warm water creates fog.
  2. Warming effect by warm current
    • This makes port operable near the adjacent areas in comparison of areas lying on the same latitude but at the locations where cold current flows.
    • Norway ports are operable in winter whereas ports in Russia freeze.
  3. Dessicating effect by cold currents (an element for desert formation)
    • Peru or Humboldt current = Atacama Desert
    • Benguela current = Namib Desert
    • Canary current = Sahara Desert
    • California current = Mojave Desert


Upwelling and Downwelling

 Ekman spiral and transport

  • When the wind pushes the water in its direction, the surface layer begins to move but gets deflected towards the right (in North hemisphere) due to Coriolis Effect.
  • This surface layer pulls along the layer immediately below with it. But as that layer moves it gets deflected further to right.
  • Successive layers move progressively right until a layer moves 90º to the direction of the wind. This is called Ekman spiral.
  • Ekman transport is the movement of the 90º layer.


  • Upwelling occurs where water from the deeper parts of the ocean is allowed to travel upwards to the surface.


  • Downwelling is the vertical movement of surface water to deeper parts of the ocean which occurs in the areas where waters converge and “pile up”.


Upwelling and nutrient enrichment at surface water

  • 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.
    • Cause of Tide = Gravitational Pull of Moon (major effect) and Sun (minor effect) + Centrifugal Force
  • The gravitational pull and the centrifugal force (due to inertia) working together are responsible for creating the two major tidal bulges on the earth.
  • Tsunami, storm surge, and tides are tidal waves (waves with large wavelengths that have greater intensity and destructive power).


Ebb and Flood/Flow

In open sea

  • Ebb is the tidal phase (time between high tide and low tide) during which the water level is falling.
  • Flood is the tidal phase (time between high tide and low tide) during which the water level is rising.

In tidal inlet system

  • Ebb is the tidal phase (time between high tide and low tide) during which the tidal current is flowing seaward (ebb current).
  • Flood is the tidal phase (time between high tide and low tide) during which the tidal current is flowing inland (flood current).



Magnitude of tides based on Perigee and Apogee of earth

Perigee tides are higher than apogee tides

  • When the earth is closest to the moon (Perigee), tidal ranges are also much greater, with unusually high and unusually low tides.
  • When the earth is farthest from the moon (apogee), tidal ranges are much less than average.



  • Syzygy refers to the positions of the sun, earth, and moon in a straight line.
  • It occurs when the sun, moon, and earth aligned in a same line as shown in figure.
  • occur twice a month
    • Conjuction – New moon
    • opposition – full moon
  • It causes spring tides (maximum tides due to the combined effect of gravitational force by sun and moon gravitation).



  • Quadrature refers to the orthogonal positions of the sun and moon.
  • The forces of the sun and moon – counteract
  • Occurs on 7/8th Day of every fortnight.
  • It causes Neap tides are (minimum tides due to the counter effect of  gravitational force by sun and moon).
  • There is a 7 Days interval between the spring tides and neap tides.


  • Tidal bore: When a tide enters a narrow and shallow estuary of a river, the front of the tide appears to be vertical (due to piling up of water because of bottom friction). This is called tidal bore.
  • Tidal current: When a gulf is connected with a sea through a narrow channel, during tides water flows in and out. This is called tidal current.
  • Kallakkadal: These are ocean swells which occur in Indian Ocean during April-May. They originate in South Indian Ocean due to storms and propagate northward with ocean currents. They get amplified as they reach shores. The flooding is most severe on spring tides.


Tidal range

  • Difference between High tide and Low Tide.
  • It depends upon:
  • Depth of ocean water
  • Configuration of coastline
  • Openness and closeness of the sea


Highest tidal range

  • World: Bay of Fundy
  • India: Gulf of Khambhat



  • Underwater structures made from calcium carbonate secreted by corals.
  • It often called “rainforests of the sea” as they form some of the most diverse ecosystems on Earth.
  • An individual coral is known as a polyp which has a symbiotic relationship with plant like cells called zooxanthellae.
    • Polyp ingests tiny organisms called plankton & other small creatures.
    • Thousands of identical polyps live together and form a coral colony.
  • Each polyp excretes a calcium carbonate exoskeleton beneath it. Over long periods of time, the skeletons of many coral colonies add up to build the structure of a coral reef.
  • Many other species – fish, invertebrates, algae and microorganisms – make their homes on and around this reef.
  • Coral reefs over a period of time transform or evolve into coral Islands (Lakshadweep).
    • In India, they are present around A&N, Lakshadweep, Gulf of Kutch and the Gulf of Mannar.


Coral Reef Relief Features

Fringing reef (Shore reefs), barrier reef and atoll (coral islands are formed on atolls) are the most important relief features.


Fringing Reef

  • It is a coral platform attached to a continental coast or an island, sometimes separated by a narrow, shallow lagoon known as Boat Channel.
  • They grow from the deep bottom of the sea and have their seaward side sloping steeply into the deep sea.
  • Coral polyps do not extend outwards because of sudden and large increase in depth.
  • Fringing Reefs are the most commonly found coral reefs among the three.
  • Example = Sakau Island in New Hebrides, South Florida Reef.


Barrier Reef

  • Barrier Reefs develop off the coast and parallel to the shore as a broken and irregular ring.
  • They are separated from their adjacent land mass by a lagoon of open, often deep water.
  • They are considered as the largest, highest and widest reefs among the three coral reefs.
    • They run for 100 kilometres and is several kilometres wide
  • Barrier reefs are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific.
  • Example = The Great Barrier Reef of Australia which is 1200 mile long.



  • An atoll can be defined as a reef that is roughly circular and surrounds a large central lagoon.
    • If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms.
  • This lagoon is mostly deep having a depth of 80-150 metres and may be joined with sea water through a number of channels cutting across the reef.
  • Atolls are located at great distances from deep see platforms, where the submarine features may help in formation of atolls, such as a submerged island or a volcanic cone which may reach a level suitable for coral growth.
  • Example = Fiji Atolls, Suvadivo in Maldives and Funafoothis Atoll of Ellice.


Conditions for survival

  • Shallow water: Where sunlight can reach them. Rarely develop in water deeper than 165 feet (50 meters).
  • Clear water: That lets sunlight through. They don’t thrive well when the water is opaque.
  • Warm water: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand water temperatures of 68–90° F or 20–32° C.
  • Pollution-free water: Corals are sensitive to pollution and sediments. Wastewater discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef.
  • Salinity: Corals need saltwater (salinity almost 27 ppt) to survive and require a certain balance in the ratio of salt to water. This is why corals don’t live in areas where rivers drain fresh water into the ocean (“estuaries”).


  • Coral lives a symbiotic life. Inside the sac of each coral polyp lives zooxanthellae algae.
    • Algae gives off oxygen and other nutrients (for food) that the coral polyp needs to live and in return the polyp provides the zooxanthellae with shelter and nutrients.
    • That is why coral reefs grow so near the surface of the water where it is the sunniest–the algae need sunshine for photosynthesis.
    • Corals also capture food. At night, they stretched out their stinging tentacles and catch the microscopic organisms that flow in the water and digest them in their stomachs.
  • Hence, corals have two way of getting food – through zooxanthellae and capturing microscopic organisms.
  • Despite occupying less than 1% of the world’s ocean surface, they provide a home for 25% of all marine species.
  • In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of many stony corals.


  • The majority of reef building corals are found within tropical and subtropical waters. These typically occur between 30º north and 30º south latitudes.
  • The Indonesian/Philippines archipelago has the world’s greatest concentration of reefs and the greatest coral diversity. Other area of reef concentration are the Great Barrier Reef of Australia, the Red Sea and the Caribbean, the latter having a much lower diversity than all major Indo- Pacific regions.


  • Coral bleaching occurs when the relationship between the coral host and marine algae, which give coral much of their colour, breaks
  • Without the marine algae, the tissue of the coral animal appears transparent and the coral’s bright white skeleton is revealed.
  • Coral reef bleaching is a common stress response of corals to many of the various disturbances caused by anthropogenic and natural events.
  • When temperature increases, the algae in the corals are killed and lose their colour. Hence, the process is known as coral bleaching, which leads to their death
  • Hurricanes, Predators, Over Fishing, oil pollution and fishing methods (Cyanide Fishing and Blast Fishing), climate change, El Nino, Coral diseases and local factors are responsible.


Causes of coral bleaching


  • Rises in seawater temperature as a result of global warming.
  • Changes in water heat affect zooxanthellae photosynthesis, forcing it out of coral polyps and depriving coral of essential resources.

Solar Irradiance

  • During summer months, coral bleaching can occur due to solar radiation.
  • Both photosynthetically active radiation and UV radiation have been associated with bleaching.

Subaerial Exposure

  • Low tides, sea level drops and tectonic uplift can result in sudden exposure of coral to the atmosphere.
  • Whether due to exposure to high/low temperature, increased solar radiation or desiccation, subaerial exposure often results in bleaching and consequent coral death due to sudden changes in atmospheric conditions.


  • Though uncommon, coral bleaching can sometimes be linked to sediment.
  • Activities such as land clearing and coastal construction can result in high rates of erosion, increasing sediment content in water and disrupting the natural process of photosynthesis.

Inorganic Nutrients

  • Increases in inorganic nutrients such as ammonia and nitrate do not deplete coral of zooxanthellae, but rather cause it to multiply, increasing its content by 2-3 times.
  • Though this does not directly cause bleaching, secondary adverse effects such as lower coral resistance and increased disease susceptibility can occur.


  • Exposure of coral to chemical contaminants such as copper, herbicides and oils can cause zooxanthellae loss.
  • Such loss is often highly localised, only occurring where xenobiotics (foreign substances) come into contact with coral.


  • Epizootics are diseases that are temporarily prevalent, often becoming widespread through animal populations.
  • Pathogen-induced bleaching commonly results in sporadic or whole-colony death amongst coral populations, leaving behind a white skeleton (Not to be mistaken for bleaching).


Kelp Forest

  • Kelps are the large brown algae seaweedswhich absorbs carbon emissions.
  • It grows in “underwater forests” in shallow oceans and live further from the tropics than coral reefs, mangrove forests, and warm-water seagrass beds.
    • Although kelp forests are unknown in tropical surface waters, a few species have been known to occur exclusively in tropical deep waterso.
    • Kelps and coral reefs are composed of algae that grow in the shallow parts of the ocean in warm and sunny waters. However, kelp forest grows in nutrient-rich waters while corals can develop in low nutrient waters.
    • Not present in Antarctica
  • Recognized as one of the most productive and dynamic ecosystems on Earth.
  • Smaller areas of anchored kelp are called kelp beds.

Environmental factors necessary for kelp to survive

  • Hard substrate (usually rock)
  • High nutrients
  • Clear shallow coastal waters
  • Light


  • Considered as Keystone Species.
  • Provides as an important source of food for many marine species (up to 60% of carbon found in coastal invertebrates is attributable to kelp productivity).
  • It slows down the flow of the water.
  • Natural breakwaters and prevent coastal erosion.
  • Can influence coastal oceanographic patterns.
  • Iimportant source of potash and iodine.
  • Produce algin (complex carbohydrate useful in industries such as tire manufacturing, ice-cream industry).


Speices classification

  • Keystone species refer to species that has a disproportionately large effect on its environment relative to its abundance.
  • Foundation species refer to species that has a strong role in structuring a community.
  • Indicator species defines a trait or characteristic of the enviornment.
  • Flagship species is a species selected to act as an ambassador, icon or symbol for a defined habitat, issue, campaign or environmental cause.