Hydrosphere Notes 11th Geography for Tnpsc Exam
11th Geography Lesson 3 Notes in English
“World cannot survive without water and morality cannot exist without rain”
- As thirukkural quotes, water is the most important resource in the world.
- Over 90% of the world’s supply of fresh water is in Antarctica.
- You must know that 85% of the world population lives in the driest half of the planet. Now we shall learn about the hydrosphere in detail.
- Water is the most common substance found on earth.
- It is an important constituent of all life forms on the earth.
- Hydrosphere is one among the four spheres of the earth.
- The hydrosphere includes the water on the surface of the earth, the water below the surface called ground water and the water in the atmosphere above earth’s surface.
- Oceans, rivers, lakes and glaciers form part of surface water.
- There is substantial amount of water under the surface of the earth.
- The atmosphere has water in all the three forms.
- The total amount of water on the earth does not change over time.
- Water is constantly in motion within the spheres of the earth which is being transformed and reused all over the earth. The earth’s hydrosphere, thus, acts as a closed system.
Distribution of Land and Water in the Earth
- Earth is covered by land and water.
- About 70.8% of its area (361million sq km) is covered by water and 29.2% (148 million sq km) of its area by land.
- About 96.5% of water is salty found in seas and oceans.
- Fresh water occupies only 2.5%. Saline ground water and saline lakes together form 1%.
- Fresh water is defined as water with a salinity of less than1% compared to that of the oceans (i.e. below 0.35%).
- Water with salinity between 0.35‰ and 1‰ is typically referred to as marginal water because it is marginal for many uses by humans and animals.
- Considering the distribution of fresh water 68.6% of it is locked in Glaciers and icecaps.
- About 30.1% is stored as ground water and the remaining 1.5% is available as surface water.
- Surface water includes ice and snow on the land and sea, water in the lakes, rivers, swamps and marshes, moisture in soil, atmosphere and biosphere.
- Rivers and lakes are the major sources of fresh water around the world, and are vital to the communities they serve.
- Rivers generally have a source on a mountain either from a glacier, a spring or a lake.
- River Ganga has its source from Gangotri glacier in the Himalayas.
- River Cauvery has its source from a spring in Talacauvery located in Kodagu district of Karnataka.
- River Nile has its source near Lake Victoria in Uganda.
- The river flows through confined channel between two banks and ends up at the mouth which is either on a sea or lake.
- When rivers drain their water into a lake or an inland sea, it is said to be an inland drainage.
- The Nile River in Africa is the longest river in the world.
- The Nile River flows through Egypt, Uganda, Ethiopia, Kenya, Tanzania, Democratic Republic of the Congo, Rwanda, Burundi, Sudan and Eritrea drains and into the Mediterranean Sea forming a delta to the north of Cairo city.
- The river Amazon in South America, is the second longest river, and has the largest drainage basin of any river.
- The Amazon River flows through Peru, Colombia, and Brazil and drains into the Atlantic Ocean forming an estuarine delta.
RIVERS AND LAKES
- The Yangtze River, which flows in China, is the longest river in Asia, and the third longest river in the world.
- The longest river system in the United States, the Mississippi-Missouri system is considered the fourth longest river in the world.
- The total volume of water in rivers in the world is estimated at 2,120 km cube.
- Asia excluding Middle East, has the largest run off of 13,300 km3/year followed by North America with 12,000 km3 per year.
- Lakes are larger bodies of water with outlet through a river or stream.
- Lakes may have their origin through tectonic activity, volcanic activity, river, glacier and wave action or sometimes meteoric origin.
- Caspian Sea, Lake Baikal and Wular Lake have been formed by earth movements.
- Lake Baikal is the deepest freshwater lake in the world.
- Caspian Sea is the largest salt water lake in the world.
- Lagoon lakes are formed by wave deposition.
- Chilika lake is the largest lagoon lake in India.
- Lonar lake in Maharashtra is believed to be formed by depression created by meteor impact which hit during Pleistocene Epoch.
- Wetlands are areas of marshy, fen, peat land or water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which the low tide does not exceed the six meters.
- Marshes are swallow wetlands around lakes, streams, or the ocean where grasses are reeds are common, without trees.
- Rann of Kutch in India is a salt marsh.
- A swamp is a wetland with lush trees and vines found in low lying area beside slow-moving rivers.
- Pallikaranai wetland is a fresh water swamp adjacent to the Bay of Bengal situated in the Southern part of Chennai.
- Groundwater is the most valuable resource for any country.
- The rain water that falls on the earth earlier runs off as surface water or precolates into the ground to recharge the ground water.
- The permeable rocks that can hold water and allow water to pass through them are called aquifers.
- The upper part of the saturated zone of the aquifer is called the water table.
- The level of the water table fluctuates according to the season.
- Tmc ft, is the abbreviation for one thousand million cubic feet (1,000,000,000 = 1 billion), commonly used in India with reference to volume of water in a reservoir or river flow.
- Cryosphere includes the water in frozen state.
- Glaciers, ice sheets, ice caps, lake and river ice, permafrost, seasonal snow and ice crystals in the atmosphere together form cryosphere.
- Earth’s climate is highly influenced by the extent of cryosphere as it controls the energy budget of the earth Perennial ice cover is found in Greenland and Antarctica as ice sheets, as mountain glaciers and as permafrost in higher latitudes.
- Permafrost is the condition prevailing when water freezes above and below the ground, (including rock or soil) for more than two consecutive years.
- Most permafrost regions are located in high latitudes, but alpine permafrost may exist at high mountains in much lower latitudes.
- Mount Kilimanjaro (5895m) in Tanzania, Africa, located closer to the equator has permafrost.
- Seasonal snow and ice crystals are confined to middle latitudes and high mountains in lower latitude.
- Sea ice is frozen ocean water. Its formation, growth and melting are all confined to the ocean.
- An ice shelf is a thick, floating slab of ice that forms where a glacier or ice flows down a coastline.
- The world’s largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne ice shelf in Antarctica.
- An iceberg is ice floating in open water that has broken off from glaciers or ice shelf.
Interaction of cryosphere with other spheres
- Cryosphere is a climate indicator.
- Cryosphere with its high albedo influences the energy balance of the whole planet.
- Changes in cryosphere will alter land cover, surface temperature, soil moisture, air temperature, radiation, air circulation, clouds, precipitation, sea level, sea surface temperature, salinity, ocean current, fauna, flora and microbes.
- There is a complex interaction and balance among the spheres of the earth which makes life to flourish in the earth.
- If there is a change in one sphere it affects the other spheres as well.
- Nature maintains this balance.
- Understanding this complex interactions and living in harmony with nature will help to mitigate the environmental problems faced by the earth.
Oceans and Seas
- The water in the oceans and seas is termed as marine water.
- Continuous water body that surrounds the continents, created by earth’s internal force is known as Ocean.
- The term ocean takes its origin from the Greek word ‘Oceaonus’ meaning enormous river encircling the earth.
- The area of the World Ocean is 361 million square kilometre.
- The earth has at present five major oceans: The Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean, and the Southern ocean.
- All these oceans are interconnected to form one Global Ocean or World Ocean.
- This nature of water to level up quickly has made it as a reference point to measure the height of the land features and the depth of the sea features.
- Mean Sea Level (MSL) is the average height of the surface of the sea for all stages of the tide.
- MSL is reference point to measure the height of land features and depth of the sea features.
- Sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land.
- Marginal sea is a sea partially enclosed by islands, archipelagos, or peninsulas and extension of oceans towards land.
- They are generally shallow.
- Andaman Sea, Arabian Sea, Bay of Bengal, Java Sea, Persian Gulf and Red Sea are marginal seas of the Indian Ocean.
- Bay is a water body surrounded on three sides by land and the fourth side (mouth) wide open towards an ocean.
- Gulf is a large body of water, with a narrow mouth, that is almost completely surrounded by land.
- The world’s largest gulf is the Gulf of Mexico. Sound, creek, bight and cove are bays which vary in size and depth.
- Strait is a narrow channel of water, connecting two larger bodies of water.
- Palk Strait connects Gulf of Mannar and Bay of Bengal.
- Isthmus is a narrow strip of land connecting two larger land masses.
- Isthmus of Suez connects Africa and Asia.
- Enclosed seas are seas that reach very deep into the continent stay connected with one or the other ocean of the world through straits.
- Mediterranean Sea is the best example for enclosed sea.
- Partly Enclosed Seas are those types of seas that are connected to the oceans by a very wide opening and have similar characters of the adjacent ocean.
- A series of islands may also occur between a partly enclosed sea and the ocean to which it is connected. Caribbean Sea is a perfect example.
- 6 Landlocked Seas are completely surrounded by landmass on all sides without any natural outlet.
- They are actually hyper saline lakes.
- Dead Sea and Caspian Sea are good examples of landlocked seas.
- Jordon River and Volga River flow into Dead Sea and Caspian Sea respectively.
- Fjord is a long indented bay with steep slope that has been created by the submergence of U shaped glacial valley. Example: Sogne Fjord in Norway (203 km).
- Ria is an indented bay with gradual slope formed by the submergence of V shaped river valley. George River in Sydney is the best example for Ria.
Oceans of the world
1. The Pacific Ocean
- Pacific Ocean is the largest ocean in the world. It is bigger than all continents put together.
- Portuguese explorer Ferdinand Magellan in 1521 named the ocean Pacific Ocean meaning ‘peaceful’ because he felt the ocean to be calm after sailing from the Atlantic Ocean through the stormy and dangerous Strait of Magellan.
- Average depth of this ocean is 4,280 meters.
DISTRIBUTION OF THE OCEANS
2. The Atlantic Ocean
- Atlantic Ocean is the second largest ocean of the world. The Atlantic Ocean’s name refers to Atlas of Greek mythology.
- The North Atlantic Ocean was formed by the break-up of the supercontinent Pangaea and the south Atlantic was formed when the Gondwana land broke in the geological past.
- The Suez Canal, an artificial sea level waterway in Egypt, connecting the Mediterranean Sea to the Red Sea through the Isthmus of Suez was officially opened on November 17, 1869.
3. The Indian Ocean
- The Indian Ocean is the third-largest ocean in the world.
- It is named after India.
- Its calm open water has encouraged the sea trade earlier than the Atlantic of the Pacific Ocean.
[1953 –British, 2002-Draft, Great Australian Bight]
4. The Southern Ocean
- The Southern Ocean is the world’s fourth-largest ocean.
- The Southern Ocean is the youngest ocean and formed 30 million years ago when the South America moved from the Antartica, opening the drake Passage.
- This ocean has the boundary where cold, northward flowing water from the Antartica mixes the warmer sub Antartica water.
- During summer in southern hemisphere over half of the southern ocean is covered with ice and icebergs.
5. The Arctic Ocean
- The Arctic Ocean is shallower and smaller than the other oceans.
- It is completely surrounded by Eurasia and North America.
- It is covered by the ice completely in the winter.
- The Arctic Ocean’s surface temperature and the salinity vary seasonally as the ice cover melts and freezes alternatively.
- Its salinity is the lowest on an average of the five major oceans.
- Bering Strait connects the Arctic Ocean with the Pacific Ocean while the Greenland Sea and the Labrador Sea connects it with the Atlantic.
- The deepest point in Litke Deep in the Eurasian Basin, or 5,450 m.
- The International Hydrographic Organisation (IHO) is the inter-governmental organisation that surveys and produces charts for the world’s sea, oceans and navigable waters
- The low-tide line forms the base line for marking maritime zones.
- Water landward of the baseline in defined as internal waters over which the state has complete sovereignty.
- A country’s territorial sea extends up to 12 nautical miles (22.2 km) from its baseline.
- The contiguous zone is a zone of water extending from the outer edge of the territorial sea up to 24 nautical miles (44.4 km) from the baseline.
- An Exclusive Economic Zone (EEZ) extends from the base line to a maximum of 200 nautical miles (370.4 km).
- A coastal nation has control of all economic resources within its exclusive economic zone, including fishing, mining and oil exploration.
- Everything beyond EEZ is called International Waters or the High Seas. No nation has sovereign rights over this area.
- A nautical mile is based on the circumference of the earth, and is equal to one minute of latitude which is equivalent to one sixtieth of a degree of latitude.
- A nautical mile is a unit of measurement defined as 1,852 metres.
- Nautical miles are used in Navigational charts.
Relief of ocean
- The bottom of the ocean has a variety of landforms just as it is seen on the earth’s surface.
- There are large mountain ridges, deep depressions, flat plains, basins and volcanoes.
- The configuration of an ocean floor is shown with the help of a ‘Hypsometric curve’ or ‘Hypsographic curve’.
- It is a graph denoting the proportion of a landmass standing above or below the sea level .
Indian National Centre for Ocean
- Information Services (INCOIS) with its Marine Satellite Information Services uses the remotely sensed sea surface temperature (SST) to identify the locations of fish aggregation.
- The details of the Potential Fishing Zones (PFZ) are then disseminated to the fishermen once in every three days along the Indian Coast by displaying the details in the Lighthouse in their respective regional language.
India’s Exclusive Economic Ocean
[North Indian Ocean with Arabian Sea and Bay of Bengal, The dashed lines democratic India’s EEZ, which covers about 2 million sq.km . Which is roughly 60% of India’s land area India’s coastline including islands is about 7000 km long]
- Continental shelf is the seaward extension of land that lies under the sea water. It occupies 7% of the sea floor.
- The continental shelf slopes gently away from the land and is covered with shallow seas with an average depth of 200 fathoms.
- The width of the continental shelf varies according to the nature of the rock beneath the crust.
- If the crust is dynamic then the shelf would be narrow and vice versa.
- Continental shelves are formed due to either any one or combination of the factors like fluvial deposits, marine erosion, tectonic forces, and the fluctuations in sea level in the past.
- Continental shelves are well known for oil, natural gas, mineral deposits and coral reefs.
- World famous fishing grounds like Grand Bank are situated here.
- The world’s widest continental shelf (1210 km long) is located along the coast of Siberia, in Russia.
- Continental shelf on the east coast of India is formed by deltas of the Ganga, the Godavari, the Krishna and the Cauvery.
- On the West coast of India the continental shelves are formed due to faulting and consequent submergence.
- The zone of steep slope extending from the continental shelf to the deep sea plain or abyssal plain is called continental slope.
- The slope angle varies from 5° to 60°.
- It occupies 9% of sea floor.
- This is the region in oceans where landslides, turbid currents, large sediment slumps, under water canyons, gorges cut by the currents and rivers occur.
- The deposit from the continental shelves immediately falls down here.
- The origin of continental slope is believed to be due to erosional, tectonic and aggradational processes.
MAJOR RELIEF FEATURES OF OCEAN FLOOR
[Continental Shelf, Seamount, Abyssal plain, Guyot, Volcanic island, Mid-Ocean ridge, Continental Slope, Trench]
- The area between the continental slope and the sea floor is known as the continental rise.
- This part is noted for the accumulation of sediments similar to the alluvial fans near the foot hills in the land.
- It represents the boundary between continents and abyssal plain. It constitutes about 5% of the oceanic area.
- The Abyssal plain is the vast area of flat terrain in the bottom of the oceans.
- It is the largest part of ocean relief covering more than 50% of the total area.
- There is an accumulation of very fine sediments on the floor.
- The sediments are combinations of fine particles of clay and microorganisms.
- As in the case of sedimentary rocks of earth’s surface these sediments are in layers and are used to trace geological events in the past.
Mid oceanic ridges
- The mid-ocean ridges are submarine mountains.
- They are continuous and are connected to form a single global mid-oceanic ridge system.
- They are formed by the tectonic forces acting from within the earth.
- Mid oceanic ridges are located on the divergent plate boundaries where magma flows through the fissure to form new oceanic crust.
- They form the longest mountain range in the world extending for more than 56,000 km long and has a maximum width of 800–1,500 km.
- The long, narrow, steep-sided depressions formed by tectonic forces beneath the abyssal plain are called Ocean trenches.
- Oceanic trenches actually extend 3 to 4 km below the level of the abyssal plain.
- There are 26 oceanic trenches in the world: 22 in the Pacific Ocean, 3 in the Atlantic Ocean and only one in the Indian Ocean.
- The Challenger Deep in the Mariana Trench, (10,994 m) in the Pacific Ocean is the deepest part of the earth.
- A trench forms along the convergent boundary where one plate subducts below the other.
- An island is a landmass surrounded by water on all sides.
- Islands may be formed on the continental shelf or as oceanic islands.
- Most of the oceanic islands are volcanic in origin.
- Group of islands formed by subduction of ocean plate are known as archipelago.
- Islands of Japan form an archipelago.
- Marine organisms, the coral polyps colonize the tropical warm water and form islands known as coral islands.
- Lakshadweep Island in Indian Territory is made of corals. Andaman Nicobar islands are of volcanic origin.
- Ocean deep is grouped into two categories based on their size.
- Very deep but less extensive depression are called deeps.
- Long narrow linear and more extensive depressions are called ‘trenches’.
- Flat topped volcanic hills submerged under the sea water are called guyots.
- It is a part of an underwater chain of volcanic mountains produced by slow plate movement.
- Seamounts are conical, volcanic hills submerged under ocean water. It does not reach to the water’s surface.
- It is an isolated rise with an elevation of thousand metres or more from the surrounding sea floor and with a limited summit area.
- It occupies 4.39 percent of ocean region. Seamounts and guyots are most abundant in the North Pacific Ocean.
Ridges and Trenches of the World
[ Trenches: Aleutian , Puerto Rico, Peru Chile, Java, Kermadec, Tonga, Mariana, Phillipine, Kuril, Japan, KyuKyu
Ridges: Mid Atlantic, East Pacific, Pacific Antarctic, South West Indian, Mid Indian, South East Indian ]
Bottom relief of Pacific Ocean
- Continental shelf of the Eastern Pacific Ocean is very narrow due to the presence of trenches while those on the western coast are wide.
- Continental shelf adjoining coasts of Australia and Indonesia varies in width from 160 to 1,600 km.
- In the Pacific Ocean, the abyssal plains are very vast.
- Absence of mid oceanic ridges is the main reason for deep sea plains.
- Prominent submarine ridges of the Pacific Ocean are Albatross plateau, Cocas ridge and Aleutian ridge.
- Tasmania basin (New Zealand ) and east pacific basin are major basins of Pacific Ocean.
- Pacific Ocean has about 25,000 islands. There are number of archipelagos both in North and South Pacific Ocean.
- The Hawaii islands were formed by hotspot.
- The challenger deep in Mariana trench is the deepest part of Pacific Ocean (10994m).
Bottom relief of Atlantic Ocean
- In the North Atlantic Ocean, extensive continental shelves are found around the shores of Newfoundland (Grand bank) and British islands (Dogger Bank).
- In the South Atlantic Ocean, a very extensive continental shelf is found between Bahia Blanca and Antarctica .
- The most striking relief feature which is the ‘S’ shaped Mid–Atlantic ridge which extends for 16,000 km from Iceland in the north to Bouvet Island in the south.
- The ridge separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic.
- Iceland and Faroe are the few peaks of the Mid-Atlantic ridge.
[Bermuda trench, Canary islands, Puerto Rico trench, Cape Verde islands, Falkand islands, South Sandwich trench]
- The mid-Atlantic ridge divides the Atlantic Ocean into two major basins, i.e., East and West Atlantic basins.
- Other basins are Spanish basin, north and south Canary basin, Guinea basin, Brazilian basin and Labrador basin.
- Puerto Rico Deep (8,380 m) is the deepest of all deeps in the Atlantic Ocean.
- Other deeps are Romanche Deep and South Sandwich Trench.
- The West Indies is an island archipelago near the main land of North America.
- British Isles and Newfoundland are famous islands, formed on the continental shelf in the North Atlantic Ocean.
- Sandwich island, Georgia Island, Falkland and Shetland islands are islands in the South Atlantic Ocean.
Bottom Relief of the Indian Ocean
- The Indian Ocean has continental shelf of varying width.
- Continental shelf along the coast of Arabian Sea, the Bay of Bengal and Andaman varies in width from 192km to 280km.
- A variety of coral reefs thrive in the warm tropical water of the Indian Ocean.
- Indian Ocean has a continuous central ridge called the Arabic Indian ridge.
- Other important ridges include the East Indian ridge, West Australian ridge, South Madagascar ridge.
[Chagos Laccadive Ridge, Sunda or Java Trench, Diamantina Trench, Reunion Hotspot]
- Basins of Indian Ocean include Comoro basin, North Australian basin, South Indian basin and the Arab basin.
- The average depth of the Indian Ocean is 3890m.
- Sunda deep near Java is the deepest part of this ocean (7450m).
- Madagascar and Sri Lanka are the most prominent islands present in Indian Ocean.
- Andaman and Nicobar islands in the Bay of Bengal are the raised part of mountains that are the extension of Arakan Yoma which forms a part of Himalayas.
- Reunion Island is located on a Hot spot.
- The measurement of degree of hotness or coldness of ocean water is referred to as ocean temperature.
- Temperature is normally measured in the unit of degree Celsius by thermometers.
- The major source of heat energy for ocean water is the radiation from sun.
- The heating and cooling capacity of water differs significantly from that of land.
Factors affecting horizontal distribution of ocean temperature.
- The factors affecting distribution of ocean temperature are latitude, prevailing winds, ocean currents and local weather.
- The temperature of surface water decreases from equator towards the poles because of the slanting rays of the Sun pole ward.
2. Prevailing wind:
- Direction of the wind affects the distribution of temperature of ocean water.
- The off shore winds blowing from the land towards ocean or sea raise the temperature of ocean water.
- Winds blowing from snow covered regions in winter lower the surface temperature.
- In trade wind belt, the off shore winds initiate upwelling of cooler water from beneath and on shore winds pile up warm water to increase the temperature to certain extent.
3. Ocean currents:
- Warm currents raise the temperature of the oceans where they flow whereas cold currents lower down the temperature.
- Gulf Stream (warm current) increases the temperature of the eastern part of North America and the west coast of Europe.
- Labrador cold current reduces the temperature near north eastern coast of North America.
- Apart from these, some minor factors like submarine ridges, local weather conditions like storms, cyclones, hurricanes, fog, cloudiness, evaporation and condensation also affect the surface temperature of ocean water.
- These images show the sea surface temperature in Celsius shows the sea surface temperature in July and in January.
- Cold temperatures are shown in purple, moderate temperatures in aquatic green and warm temperatures in yellow to red.
- Landmass is shown by black colour. The diurnal range and annual range of temperature of ocean is much less than that of the land.
Sea Surface Temperature in July 1997
- The temperature of the sea surface is highest (27°C to 30°C) not near Equator but few degrees north of the Equator.
- The lowest temperature recorded is -1.9°C near the poles.
- The maximum and minimum annual temperatures of ocean water are recorded in August and February in the Northern hemisphere and reverse in case of the southern hemisphere.
Sea surface Temperature in January 1997
Vertical distribution of temperature in oceans
- The uppermost layer of ocean water is warm and well mixed surface layer with average temperature between 20° and 25°C.
- The depth of this layer varies according to seasons.
- On an average this layer extends up to 200 m in tropical region.
- Beneath this layer lies the thermocline layer.
- This layer varies in depth between 200 metre to 1000 metre.
- This layer is unique that the temperature decreases rapidly with increasing depth.
- Below the thermocline temperature decrease is gradual up to 4000m.
- Beneath this depth the temperature of ocean water is constant at 4°C.
Vertical Distributions of temperature in Oceans
Salinity of the ocean
- Salinity is defined as the ratio between the weights of dissolved salts (in grams) per 1000 grams of water.
- It is expressed as part per thousand (‰) and has no units.
- Example: 30‰ means 30 grams in 1,000 grams of sea water.
- The average ocean salinity is 35‰.
- Sources of salt in the ocean: Sea water is a weak but complex solution made up of many things including mineral salts and decayed biological marine organisms.
- 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.
- Depth of water is measured in the unit ‘Fathom’. One fathom is equal to 1.8 metre (six feet)
Factors affecting the salinity of ocean water
- The salinity of ocean water depends upon
- The rate of evaporation
- Amount of precipitation,
- Addition of fresh water flow from rivers
- Ice in Polar Regions
- Upwelling of deep water initiated by prevailing winds and
- Mixing of water by ocean currents.
Distribution of salinity
- On an average the salinity decreases from equator towards the poles.
- The highest salinity is observed between 20° and 40° north latitudes because this zone is characterized by high temperature, high evaporation but less rain than the equatorial region.
- The marginal areas of the oceans bordering the continents have lower salinity than their interior due to addition of fresh water to the marginal areas through the rivers .
- Very high salinity is recorded in Lake Von, Turkey (330‰) , Dead Sea (238‰) and Great Salt Lake, Utah, USA (220‰).
- Isohaline is an imaginary line drawn to join places having equal salinity.
- Salinity of Dead Sea is 8.6 times saltier than other oceans.
- The shore of Dead Sea is 423m below sea level.
- It has the lowest elevation on land. The sea is 377m deep.
- The high salt content will make people float on the sea.
- The high salt content has made the Dead Sea devoid of life in it.
Salinity of the Oceans
- Water in the ocean is never in a state of rest.
- Ocean water is always in motion. It moves horizontally as well as vertically.
- The movement of ocean water takes place in three different ways as waves, tides and ocean currents.
- The waves are oscillating movements in the ocean water which transfer energy from place to place.
- They are caused by friction of wind on the surface of water or any other disturbances’ on the sea bottom.
Parts of Waves
- Crest: The upper or highest part of a wave is called the crest.
- Trough: The lowest part of a wave is called the trough.
- Wave height: The vertical distance between the crest and the trough is known as wave height.
- Wave length: The horizontal distance between two crests or two troughs is known as wave length.
- Wave amplitude: Wave amplitude is one-half of the wave height.
- Fetch: The distance of open water across which the wind can blow without interruption is called fetch.
- Frequency: The number of wavelengths that pass a fixed point per unit of time is frequency. Example, 100 waves per sec per cm.
- Period: The time taken by one wavelength to pass a fixed point is known as period.
- Velocity: Refers to speed and direction.
- Steepness: Steepness of the wave is equal to the height divided by length.(H/L)
Parts of a Wave
- The rhythmic rise and fall of the sea water due to gravitational pull of the moon and the sun is called a Tide.
- Isaac Newton (1642– 1727) was the first person to explain tides scientifically.
- The rise of seawater towards the land is known as High tide or flow tide.
- The fall of seawater more towards sea is known as ‘Low tide water’ or ebb tide.
- On any day there will be two high tides and two low tides.
- The highest high tide occurs on full moon day and new moon day. It is known as spring tide.
- Spring tide happens when the sun, earth and moon aligned in straight line.
- The lowest low tide is known as neap tide. It happens when the sun, earth and moon are positioned at right angles.
- The movement of ocean water as a result of tidal action is known as a tidal current.
- In places of narrow coastal inlet these tidal currents flow rapidly through the mouth with greater height and velocity.
- For example in the Bay of Fundy, between Nova Scotia and New Brunswick of Canada, the difference between high and low tides is as high as 14m.
- Ports which utilize the tidal current for entry and exit of ships from the harbour are known as tidal ports.
- In India Kolkatta and Kandla are examples of tidal harbours.
- The Gulf of Cambay and the Gulf of Kutch in Gujarat on the west coast have the maximum tidal range of 11m and 8m with average tidal range of 6.77m and 5.23m respectively.
- Tides help to clear the sediments deposited by rivers on their bed and thus prevent siltation of harbours.
- The energy of the tides is used to generate electricity.
- Tidal power stations have been set up in UK, Canada, France and Japan.
- In India Gulf of Khambhat, Gulf of Kutch and Sundarbans have scope for tidal energy production.
- A harbour is a sheltered water body where ships are anchored.
- A port is the area at the edge of a water body where boats and ships are docked, where transfer of goods and passengers take place and where trading is facilitated.
- Large mass of moving water from one part of the ocean to another in a definite direction is called as ocean current.
- The movement is produced due to earth’s rotation, temperature difference of ocean water, salinity, density and some extent due to air pressure and winds.
- Ocean currents can be classified on the basis of mode of origin, volume and velocity and boundaries.
- In the order of velocity ocean currents can be classified as drifts, currents and streams.
- Drifts are movement of surface water of low velocity influenced by prevailing winds, currents are movement of oceanic water in definite direction and greater velocity and streams are larger mass of water moving in a definite direction and much greater velocity than the drifts and currents.
World Ocean Currents
- Ocean currents are distinguished by the temperature they possess.
- When ocean currents originate from equator it is termed as warm current.
- Likewise when a current starts from polar region it is termed as cold current.
- Vertical circulation of ocean water takes place due to difference in salinity and temperature between the surface and the water deep below.
- Upwelling is an oceanographic phenomenon that involves movement of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water.
- Down welling is the process of accumulation and sinking of cold high saline water beneath warmer or fresher water.
Major ocean currents of the world
- In every ocean, there is circulation of ocean water from Equator to pole and from pole to equator.
- The warm currents from the equator flows over the surface of ocean towards the pole and sink to the bottom of the ocean floor in the higher latitudes due to high density and flow towards the equator to complete the circulation.
- This large scale circulation is known as gyre.
- The gyre circulates is clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.
a) Ocean currents of the Pacific Ocean
1. North Equatorial current.
- North equatorial current originates from Revilla Gigedo island west of Mexico and flows towards the Philippines Island covering a distance of about 12,000 km from east west.
- It is a warm current.
- It derives from its water from the Californian current and the South east Monsoon drift which flows north along the Mexican coast.
- The volume of water increases from east to west as many small currents join it from right.
- It gets divided into two and the northern branch joins the Kuroshio Current and the southern branch abruptly turns and forms the Pacific counter current.
2. South equatorial current.
- South equatorial current is originated due the action of the trade winds from east to west.
- It is a warm current.
- It extends for about 13,600km from east to west.
- It is stronger than the North equatorial current.
- It is further divided into many branches due to the presence of many islands and uneven surface topography.
3. Kuroshio current ( Black Tide)
- It is a warm ocean current flowing in north easterly direction up to 30o N latitude and it carries warm water off the Formosa coast.
- It flows towards north and meets Oyashio cold current off the Kuril Islands.
- It is also called as Japan current.
4. Oyashio Current( Parental Tide)
- It originates from the Bering Strait and flows towards south carrying cold water.
- It is a cold current.
- It meets with Kuroshio warm current and Aleutian current.
5. Californian Current.
- Californian current is flowing towards south along the west coast of U.S.A between 48o N and 23o N latitudes.
- It is cold current which exhibits great amount of up welled water.
- When it enters the region of Trade winds, it is deflected to the right and joins the equatorial current.
6. Peru Current.
- Peru Current is perhaps the best studied ocean current of the Pacific Ocean.
- Alexander Von Humboldt in 1802 noted the details of the Peru Current.
- Hence, it is also known as Humboldt Current.
- It is a cold current.
- It is flowing towards north along the west coast of South America carrying cold water from northerly deflection of the Sub-Antarctica water moving in 40o S.
7. El Nino or Counter current.
- It is a warm counter ocean current of the pacific equatorial waters flowing south ward at 400 m depth to a distance about 180 km.
8. West Wind Drift.
- It is an easterly moving drift in the Pacific Ocean extending from Tasmania to the South American coast.
- It is a cold current.
- The speed of the drift is greater under the influence of Roaring Forties.
- It splits into two branches and one moves south around the Cape Horn into the Atlantic Ocean and the Other one moves northward along the Peruvian coast due to deflection and joins the Peru Current.
b) Currents of the Atlantic Ocean
1. North equatorial current.
- North equatorial current is flowing from east to west.
- It is a warm current. It is situated between 5o – 20o N latitudes.
- After leaving the west coast of Africa, it attains its main characteristics.
- When it reaches the east coast of South America, it splits into two branches and one branch called Antilles current is moving along the coast of West Indies and other branch is diverted into the Caribbean sea.
2. South Equatorial current.
- It is flowing south of equator within 0degree– 12degree S latitude in between the coast of Africa and South America.
- It is a warm current. It is a northern continuation of Benguela current.
- It is stronger than the North equatorial ocean current.
- It is caused by the action of Trade winds.
3. Gulf Stream.
- Gulf Stream starts from the Gulf of Mexico and carries warm waters into the colder latitudes.
- It is a warm current.
- It bends with the coastline up to 40th parallel after which the direction is almost to the east, due to the force and the direction of the westerlies and the deflective force of the earth.
- It joins the Labrador cold current near New Found land, Canada after passing through the strait of Florida.
- The Gulf Stream was discovered by Ponce de Leon in 1513.
4. Canaries Current.
- The ocean current flowing along the Western coast of North Africa between Maderia and Cape verde is known as the Canaries Current.
- It is a cold current.
- It is flowing towards south and merging with the North equatorial current.
5. Labrador Current.
- In the north Atlantic, a cold current flows from the Baffin Bay and Davis Strait towards south.
- It brings cold waters from polar zone and moves along the coast of green land.
6. Benguela current.
- It is a cold current flowing northward along the western coast of Africa is known as the Benguela current.
- It carries cold waters from sub-Antarctica surface water and mixes with south equatorial current.
The Sargasso Sea – Sea with landless border
- The Sargasso Sea occupies about two thirds of the North Atlantic Ocean, stretching seven hundred miles wide and two thousand miles long.
- The only “sea” with absolutely no land around it, the Sargasso Sea got its name from common brown seaweed called Sargassum that floats in vast mats in its waters.
- The Sargasso Sea is surrounded only by ocean currents.
- It lies within the Northern Atlantic Subtropical Gyre.
- The Gulf Stream establishes the Sargasso Sea’s western boundary, while the Sea is further defined to the north by the North Atlantic Current, to the east by the Canary Current, and to the south by the North Atlantic Equatorial Current.
- Since this area is defined by boundary currents, its borders are dynamic.
C) Currents of the Indian Ocean
- The south Indian gyre is formed by south equatorial current, Madagascar current west wind drift and west Australian current.
- To the north of equator the currents in the Arabian Sea and Bay of Bengal flow in the clockwise direction as southwest monsoon drift and in the anti-clockwise direction as northeast monsoon drift due to the influence of monsoon winds.
- The Antarctic circumpolar current flows between 40 to 60° S latitude.
- It flows from west to east influenced by the westerly and circles around entire Antarctica.
- There is a counter west ward current within this circum polar current.
d) Currents of the Southern ocean
- The southern ocean surrounds the continent of Antarctica.
- The large oceans, the pacific, the Atlantic and the Indian Ocean merge into this circum global zone of water to their south.
- The movement of water in the southern ocean is in one sense a relatively simple, generally west-east circumpolar drift caused under the influence of north westerly winds.
- This general flow sends offshoots to the three major oceans to its north.
- The Peru or Humboldt Current in the Pacific Ocean, the Falkland Current and the Benguela Current in the Atlantic Ocean and the West Australian Current in the Indian Ocean recieve a part of their cold waters from the Southern Ocean.
- Besides the surface currents, there is also a very complex system of sub-surface currents between the southern Ocean and the oceans into the north.
- Generally the water moves from this ocean towards the equator on the surface and at the great depths but at in remediate depth, there is a movement of water from the equatorial areas towards the Southern Ocean.
Wind circulations during Normal and El Nino conditions
The significance of the Ocean Currents
- Ocean currents play an important role in the earth’s climate. They distribute energy and nutrients within the ocean.
- Fog is formed where warm current. For example, when the Gulf Stream and Labrador Current meet near New Found land one of the densest fogs is formed.
- The warm ocean current increases the temperature of an area where it flows to and Cold Ocean current decreases the temperature of the area.
- The warm current brings heavy rainfall when the wind blows over it becomes warm while the cold current brings drought when the wind blows over it becomes cold and dry. For example, the wind blowing over the Peru Current is cold and dry causing the formation of the Atacama Desert located on the west coast of Peru.
- It regulates the global temperature. It gives free navigation.
- The Gulf Stream keeps ports and harbours of Russia and Scandinavia navigable throughout the year.
- The Kuroshio Current makes port on Japan navigable during the winter.
- It distributes minerals and pollution added to it becomes highly diluted and later negligible.
- It helps in the growth of juveniles of certain fish and its distributions to other countries-from its place of origin.
- Some up welling and down welling are due to currents which bring minerals to photic zone used by phytoplankton.
- Major fishing grounds are located in the zones where cold current and warm current meet.
- El Nino is a phenomenon that occurs in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (1971-2000 base period) in the region lying within the latitude 5°N to 5°S and longitudes 120° W to 170°W . This phenomenon occurs every two to seven years.
El Nino happens when
- Sea surface temperature increases between the central and eastern equatorial Pacific Ocean between the country Ecuador and the International Date Line The increase in temperature is sustained for a period of eighteen months to Two years.
- The temperature increase is up to 30 m beneath the ocean surface.
- When there is a modified vertical air circulation above the Pacific Ocean
Global influence of El Nino
- El Nino effect is experienced at Global level.
- The change in air circulation affects the economy of different countries also.
- Global weather patterns are altered to such an extent that they affect eco system, agriculture, tropical cyclone, drought, forest fire, floods and flood related health hazards.
- El Nino influences the jet streams.
- Due to this phenomenon California experiences heavy rainfall, northern Europe experiences dry winter, Southern Europe experiences mild wet winters, there are less number of cyclones in Sea of Japan, and heavy rain in East Africa.
- South East Asia experiences severe drought and forest fire.
- Peru in South America receives heavy rainfall during El Nino.
- Increase of temperature in the east Pacific Ocean is correlated with normal monsoon conditions in India while the increase of temperature in the central Pacific has high correlation with drought conditions in India.
- When temperature increases further to the west it suppresses the Indian Monsoon.
- La Nina is just the opposite to the condition of El Nino.
- When trade winds are strong, colder water up wells on the East Pacific Ocean, walker air circulation is confined to the west Pacific, wet condition in Southeast Asia and dry weather in South America is observed.
- The difference in the atmospheric pressure between the west and east tropical Southern Pacific Ocean is referred to as Southern Oscillation.
- Meteorologists have established a close inter link between Southern Oscillation and occurrences of El Nino and La Nina events.
- The acronym ‘ENSO’ (El Nino Southern Oscillation) is often used to study both the phenomena.
- Peruvian fishermen named the weather phenomenon El Nino meaning ‘little boy’ or ‘New born Christ’ and La Nina meaning ‘Little girl’ as the phenomenon was first noticed during Christmas time.
- As the name indicates there is a large scale churning of ocean water due to difference in temperature and salinity.
- The down welling of ocean water occurs in the extreme ends of Atlantic Ocean one near the Norwegian coast and another at Weddell Sea.
- Upwelling of cold water occurs in the North Pacific Ocean and in the Indian Ocean.
- This cycle of water movement within the Global Ocean is also known as Conveyor Belt .
- The slow, steady and three dimensional flow of water in the conveyor belt distributes dissolved gases and solids, mixes nutrients and carries it to various ocean basins.
- This cycle provides a stabilizing effect on climate of the earth.
- If it is disturbed, it is capable of causing sudden climatic change within the period of a few decades.
- The conveyor belt is a simplified version of actual circulation in the oceans.
MORE TO KNOW:
- 70% of human brain is water.
- 263 rivers either cross or demarcate international political boundaries.
- A nationwide water resources information system, “Generation of Database and Implementation of Web Enabled Water Resources Information System (India-WRIS) in the country” contain all aspects of water resources and related data provide data and information in public domain through India-WRIS Web GIS portal.
- If excessive water is taken from the aquifers along the coast, the sea water enters the coastal aquifer.
- This process is termed as saltwater intrusion.
- Carbon is removed from the atmospheric cycle by cryosphere during the formation of ice and is released when the ice melts.
- There is life cycle for oceans too! It is known as Wilson cycle.
Ocean and Sea
- In partially enclosed seas, their bottom relief and the submarine ridges with shallow water do not allow free mixing of open sea water.
- The temperature at the depth of 1800m in the Red Sea is higher than the temperature recorded at the same depth in the Indian Ocean.
- Raking refers to the use of a rake, a traditional wooden tool with the long handle and long pointed wooden toothed spade at the bottom for collecting salt.
- International Research Institute Climate Prediction Centre predicts and forecasts El Nino occurrences.
- Scientists are in the opinion that El Nino can cause Global Warming and it also increases the frequency of El Nino occurrence.