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The mobility of sea waters is divided into three parts: 1. Currents 2. Waves 3. Tides.
Part # 1. Currents:
Currents are defined as those waves on the surface of sea water which move horizontally. Water moves from one place to another like a river, actually it moves in a specific direction. Name of current is based on their direction. A current is actually that water which flows in specific direction only.
There are various types of currents in Oceans, which move and shift water from one place to another. These are affected by the size, depth and structure of sea. Water flowing as current remains stable at the both ends and moves faster than the river water in middle part and flow is much deeper and large also.
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Length of Ocean Currents:
The length of currents extends upto 1000 Kms and they may be 200 Kms broad. Their speed is faster than that of rivers. In each current speed of water varies from 2 to 10 Km/Hr. They move towards a particular direction for long time and collectively form a big flow called ‘Global Conveyor Belt’, which affects the Global climate.
The temperatures of Gulf Stream and Humboldt current make their surroundings warm and cold respectively. Ports of eastern North America do not freeze in winter due to warm effect of Gulf Stream whereas temperature in Lima and Peru is lower than their surrounding areas. Although these are situated on tropical latitude but still their temperature is low.
Oceanographers have classified currents on the basis of various methods. These are given below:
Types of Currents:
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(a) Periodic Currents:
Currents which change their speed and direction after particular fixed time are known as periodic currents.
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(b) Seasonal Currents:
Those currents which change their speed and direction with changing season.
(c) Coastal Currents:
Those currents which move on the outer side of the sea coast.
(d) Long Shore Current:
Those currents move along the sea coast as these are produced when the waves strike of the coast, these flows close to coast and upto long distances.
(e) Off Shore Current:
These are produced at a distance from the coast.
(f) Inshore Current:
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These types of currents are produced near to the coast.
(g) Drift:
Those currents which get broader under the effect of prevailing winds are known as drift. The velocity of the drift is less than 24 km a day. Namely, for example
(i) North Atlantic Drift
(ii) West Wind Drift
(h) Streams:
These are large size currents, having large volume of water moving.
Types of Currents on the Basis of Temperature:
(a) Hot Water Currents:
The currents which move from warm regions to cold regions are known as hot water currents.
(b) Cold Water Currents:
Those currents which move from cold regions to hot regions are known as cold water currents.
Causes of Origination of the Ocean Currents:
There are various reasons because of which ocean currents are formed; Rotation of Earth, Gravitation, Heat of Sun, Temperature difference, Salinity, Density of water, Melting of Ice, Instant change in weather, Direction and size of coast etc.
(i) Prevailing Winds:
These winds are permanent and always flow in one direction. With their friction they move sea water because of which currents are formed.
Trade winds blow towards west between 30° North and 30° South of Equator. Because of this currents move from east to west in the north and south of Equator i.e. in at Tropical region. Similarly, Western winds blow between 40° North to 65° North and 40° South to 60° South latitudes (Temperate region).
They move from west to east because of this they are known as Western Winds. Currents moving from west to east are not known as western currents. They are known by the name of that direction in which they are moving. In Indian Ocean direction of Monsoon winds change with the change in Weather.
(ii) Temperature:
Currents find their birth in variation of temperature of Oceanic waters. Currents move in North-South direction because of temperature difference between Equator and Polar regions. In other words we can say that when heavy and cold water of Polar Regions settle down, hot water starts moving from Equator to Poles for acquiring the empty space. Because of this hot water currents move from equator to poles causing birth of hot and cold water currents because of the difference in temperature.
(iii) Density of Water:
Density of saline water is more than the density of clean water. Highly saline water settles down because of its weight. To fill that space clean water moves toward the region, resulting in the formation of current. Salinity of Mediterranean Sea is higher than that of Atlantic Ocean because of this a current moves from Atlantic Ocean to Mediterranean sea alongside upper water.
(iv) Evaporation:
Evaporation reduces the sea level as water evaporates in this process. As soon the water level decreases, flow of water from other regions begins towards decreased level region. This forms a ‘current’.
(v) Rotation:
Earth rotates on its own axis because of which ‘centrifugal force’ is produced. Under the effect of this force, flowing water opts a circular path, which is known as Gyre. The movement is clockwise in Northern Hemisphere and anticlockwise in Southern Hemisphere. Currents in ‘N. Hemisphere’ move towards right and in ‘S. Hemisphere’ currents move towards their left. Winds also turn on the basis of this Farrel’s law. In Atlantic Ocean, Western Winds produce a current called ‘Gulf Stream’.
Currents of Atlantic Ocean:
In the Equatorial region of Atlantic Ocean, Trade Winds give direction to important currents. In simple words, it is because of Trade Winds that water of Equatorial region starts flowing towards west, resulting in the formation of currents. These are hot water currents. Some of them move towards North from Equator and others move towards South.
An anti-current starts flowing from west to east named as Equatorial Counter Current between North Equatorial Current and South Equatorial Current.
Currents of Northern Atlantic Ocean:
North Atlantic Ocean Equatorial Current:
Flows from western coast of Africa to Central American Island along the equator due effect of Trade winds.
Antilles Warm Current:
Antilles Current is actually south Atlantic Current which finds its origin under the effect of Trade winds in Southern Atlantic Ocean near the Equator at Western African Coast. Flowing west wards it reaches upto Cape-de- Sao-Rogue i.e. eastern end of Brazil. Here this current is divided into two parts. Southern part starts flowing southwards while its northern arm contributes its warm water to North Atlantic Equatorial Current.
Florida Current:
This current moves along the South Eastern Coast of USA. This is a hot water current.
Hot Water Current of Gulf:
From Cape Hateras to Grand Bank this current is known as Gulf Stream. It is 45 Kilometer wide water channel and it moves at the speed of 6-7 km/hr. This hot water stream deflects towards east due to the effect of western winds and rotation of Earth and after crossing Atlantic Ocean it is known as Hot water current of North Atlantic.
Norway’s Hot Water Current:
In eastern part of Atlantic Ocean, North Atlantic current is divided into two parts. The part which deflects towards north enters into Arctic sea while moving along the coast of Norway. This is known as hot water current of Norway.
Canaries Cold Current:
The other part of North Atlantic current deflects towards west and reaches upto Canary Islands. Due to rotation of Earth and coastal obstacles/hindrances, second branch of hot water current of North Atlantic deflects towards south. It moves along the eastern coast upto Spain and Azores in the form of new stream. This is known as cold water current of Canary.
It moves further and conjugates with the hot Equatorial Waters. Thus North Atlantic Ocean currents complete their one circle flowing clockwise and ending up in equatorial waters at Sargasso Sea. The name of ‘Sargasso Sea’ is derived from the Oceanic Algae or Sea weeds which is brown in colour and known as ‘Sargassum’. The area of Sargasso Sea is taken 11000 square Kms.
Labrador Cold Current:
This current moves towards New Found land through Bay of Baffin. It moves along the eastern coast of Canada and conjugates with warm Gulf Stream. This conjugation causes dense fog near New found land.
Southern Equatorial Current:
As these current moves toward west, it gets divided into two parts at the coast of Brazil. One deflects towards north and other starts moving along the coast of Brazil. This is known as hot water current of Brazil.
Cold Water Current of Southern Atlantic Ocean:
Hot water current of Brazil starts moving towards east due to effect of Western winds. When this current moves toward east, it is known as cold water current.
Benguela Cold Current:
Due to the rotation of Earth, some part of current of southern Atlantic Ocean deflects towards north and when it strikes with the Western Coast of Africa, water starts flowing towards North. At that point it is known as Benguela Cold current. It conjugates with the Equatorial current and completes its cycle.
Currents of Pacific Ocean:
Currents of Pacific Ocean also move like the current of Atlantic Ocean.
Northern Equatorial Current:
Due to the effect of Trade winds these currents begin from the western coast of Central America and reach upto Philippines islands while moving from east to west.
Southern Equatorial Current:
In south, Southern Equatorial Current moves towards west. Between these two currents an opposite current moves towards east. This is also a hot water current. From Philippines islands, North Equatorial Current moves towards north along the coast of Taiwan and Japan. At this point, this is known as Kuroshio Current.
North Pacific Current:
Kuroshio Current is divided into two parts while moving along the coast of Japan. One of its branch moves along the eastern coast and another branch moves along the western coast. Flowing along the coast of Japan, both of these currents start flowing towards North-East jointly, under the name of Warm Water Drift of Pacific Ocean.
Cold Water Current of California:
North Pacific Current is divided into two parts at the western coast of North America. Northern part flows anti clockwise along the coast of British Colombia and is known as ‘Alaska Current’. This is a hot water current and due to this current water does not freeze at the coast of British Colombia and Alaska. Its second branch deflects towards south along the coast of California. This is also known as cold water current of California.
Oya Siwo Current:
This is cold water current which begins from Bearing strait and moves from north to south upto east of ‘Kamchatka Peninsula’
Okhotsk Current or Cold Kurile Current:
This current moves along the eastern coast of Sakhalin islands and conjugates with the Oya Siwo Current near Hakkaido island of Japan. Here Oya Siwo Current joins it and its water starts flowing beneath warm water of Oya Siwo because of which fog is formed at this point. Joining of hot and cold currents results into production of plank tine, food for fish.
Southern Equatorial Current:
This is a hot water current which moves along the coast of Central America in East to the eastern coast of Australia in west. It flows south wards along eastern coast of Australia and is known as warm current.
Southern Pacific Drift:
Eastern Australia Currents move from west to east near Tasmania. Here, these are known as Southern Pacific Currents. This current deflects towards north at the south western coast of South America and moves further along the coast of Peru. This is also known as cold water current of Peru. In this way a cycle is completed. Because of this cold water current Chile and Peru receives very low rainfall at their coastal regions.
Currents of Indian Ocean:
Currents of Indian Ocean are highly affected by Monsoon winds. Direction of Monsoon winds changes in summer and winter season because of which direction of currents also change. In the north of Indian Ocean, more land mass is found and in southern portion of Indian Ocean, more open sea is found. This snatches an opportunity from northern region to have a cycle of currents like that in other two oceans while Southern Indian Ocean enjoys same current cycle.
Currents of Northern Indian Ocean:
In winters, northern equatorial current deflects southwards while moving from east to west. This is known as North Eastern Monsoon drift. This drift begins from Malacca strait enters into Arabian Sea while moving along the coast of Bay of Bengal. It deflects southward near Bay of Aden and starts moving from west to east and completes its cycle by conjugating with Opposite Equatorial Current.
In summer, monsoon winds flow in south-east direction because of which some part of Southern Equatorial Current and Northern Equatorial Current start moving along the coast of Africa. This is also known as ‘Current of Somali’
South Western Monsoon Drift:
Currents of Somali create a cycle around Indian sub- continent due to the effect of South West Monsoon Winds. These are known as South Western Monsoon Drift.
Currents of Southern Indian Ocean:
South Equatorial Current:
South Equatorial Current moves from east to west as that of South Pacific Current.
While moving further it gets divided into two parts. One of its branches deflects towards the south of Madagaskar Island and other moves along the coast of Mozambique. This is also known as hot water current of Mozambique. When this current moves along the eastern coast of Madagaskar it is known as Madagaskar hot water current.
Agulhas Current:
This hot water current has been formed by the conjugation of hot water currents of Madagascar and Mozambique.
West Wind Drift:
Because of western winds, Agulhas current, deflects towards north at the southern end of Africa. While moving further it conjugates with cold Antarctic current.
West Australian Current:
It moves from west to east. It moves along the southern coast of Australia and its second branch deflects towards north from the western coast. The second part is known as cold water Current of Western Coast. While moving further it conjugates with the Southern Equatorial Current and completes the cycle of Southern Indian Ocean.
Effects of Currents:
Ocean Currents leave intense effect in coastal regions, islands, economical activities, weather, agriculture etc.
Hot water currents help in rising the temperature. Kuroshiwo current and Gulf Stream modify the weather of Southern Japan and Eastern America respectively. Ports of Western Europe are used as trade points for whole of the year because hot water does allow water to freeze in any season. Cold water currents decrease the temperature coast of Labrador is freezingly covered by ice because of cold water current.
Currents also affect the amount of rainfall. Hot water currents increase the rainfall because they bring winds along with them, laden with water vapours. North America, Ireland, Britain and south India receive rainfall because of this process. On the other hand Atacama Desert remains dry because cold water currents do not play any role in occurrence of rainfall. Due to this various deserts are situated on the western coast of Australia, South America and Africa.
Conjugation of hot and cold water currents helps in increasing the development rate of ‘Plankton’, which is a diet of fish. This conjugation takes place of eastern coast of North America, where New Found land current and Labrador Current enjoined making it a major fishing bank of world.
Superior quality fish are found in cold water currents. When these currents move towards regions with high temperature, they carry fish with them. This movement boosts the fishing trade. The direction of currents helps in saving fuel and time also while similar direction encourages speed of moving ships.
On the other hand conjugation of currents forms dense fog which creates problem for marine navigation. Sometimes ships are not able to move because of this fog. This whole situation affects the economic activities of respective areas.
Part # 2. Waves:
Water of Oceans never stops. It moves in the form of waves in the direction of winds. Waves are the forward movement of ocean’s water due to the oscillation of water particles by the frictional drag of wind over the water’s surface. The peak of the wave is known as crest and lowest point is known as trough.
Waves do not move the water horizontally. We can prove this with an experiment. Fill a tub with water and make a disturbance in it with your hand; now put a cork in it. Notice it for some time, you will find out that cork moves up and down without changing its position. With the help of this experiment we can say that water does not move horizontally.
The wave length or horizontal size of wave is determined by the horizontal distance two crests or two troughs. The vertical size or height of wave is determined by the vertical distance between crest while direction and speed of waves is not same always and it depends upon the speed of winds. Wave period is a length of time it takes for a wave to pass a fixed point (crest to crest).
We can find the speed of wind with the following method:
The effect of winds reduces over the waves and currents with increase of depth. Waves depend on the speed of winds. High speed winds increase the power and height of waves. Sometimes speed of wind reduces due to barriers, this also effect the waves. When a wave breaks water moves with turbulent speed like a river towards coast which is known as swash/surge. Sometimes it moves sand, rock particles towards coast. The descending of this surge is known as back wash.
High speed waves which move for long time are known as swell. These are also known as capillary waves. Their wavelength is more than 100 meters. As waves approach land masses, the wave base begins to contact the sea floor. The friction slows the circular motion of wave’s base. This is known as breaker. These type of waves when move towards shore these are known as ‘surf’.
‘Erosion’ is always high and fast at soft sea coasts, which forms various types of land forms.
Tsunamis:
These are also known as seismic sea waves or long wavelength sea waves. These are formed due to volcanic activity or Earthquakes in sea, Asteroid, Displacement on the surface of water because of breakdown of ice bergs etc. They may have wave length of 100 km and their speed may vary from 600 to 800 km/hr. Their height may grow more than 15 meters. (50 feet).
In 2004 these waves resulted in the huge loss of life and property in south India. A large plate (hence large wavelength) of the earth’s crust creates these waves by displacing water and the energy liberated is huge. Wave amplitudes are small (0.1 – 1 m) but the wavelength is large, typically 100 to 1,000 km.
Oscillations of water particles are not just restricted to the water surface but are distributed in the entire depth. We cannot create such waves by a paddle. Because of the large volume of water involved, energy of tsunami waves is large even though the amplitudes are small. This is why it is difficult to observe the height of tsunami waves in the open sea but close to shore they become disastrous.
Part # 3. Tides:
Tides refer to the periodic rise and fall of Oceanic water. This movement takes place because of gravitational pull of Sun, Moon and Earth. Height of tide depends on the position of Moon. Effect of gravitation is high because water particles are present in Sea in liquid form. Tides rise twice in a day with the gap of twelve hours approximately.
To be more exact the difference may be put as 12 hours and 25 minutes. Basic reason behind this time difference is the difference of time taken by rotation of Earth and one revaluation of Moon. The Earth takes 24 hours to complete one rotation on its axis while moon takes 29 days and 12 hours to complete its one rotation around the Earth.
Result of this time difference is that any place on Earth faces the same to moon not after each 24 hours but 24 hours and 50 minutes. Hence, tides occur after 12 hours and 25 minutes each. It is the gravity of Earth which gives birth to the tides along with effect of rotation. When a tide rises on the portion of Earth facing moon, the opposite side of Earth also gives rise to tide due to rotational effect of Earth.
The average height of tide is 0.55 meters but sometimes it may vary from 2 to 3 meters. Sun also produces tides; these are known as Sun tides. Their height is less than lunar tides (Tides produced by Moon)
Tides based on the location of Earth, Moon and Sun:
1. Spring Tides:
These tides occur when the Sun and Moon are directly in line with Earth. These tides occur twice a month, at full Moon and new Moon.
2. Neap Tides:
Spring tides and Neap tides occur with the difference of seven day. That means the days between full moon and new moon. Basically Sun, Earth and Moon are positioned so on the 7th and 21st day of lunar month that these celestial bodies form a right angle. The effect of these bodies is seen against each other which results into Neap tide as compared to normal tidal activity.
For once in a month moon draws nearest to the Earth during its revaluation which is known as ‘Perigee’. While after two weeks it is situated farthest to the Earth known as ‘Apogee’.
Importance of Tides:
Depth of water increases near the coastal area because of spring tides. Because of this big/heavy ships can easily move towards port. Kandla of Gujarat and Diamond Harbour of Bengal are its major examples. Tides act as barriers in the deposition of soil and helps in desolation at mouth of rivers. Power is generated with the help of tides. Fishing boats also take help of tidal activity while entering the ports or getting out of it.