Movements of Ocean Water | Ocean Water Movements

Ocean Water Movements, Movements of Ocean Water, Waves, Tides, Ocean Currents, Currents of Pacific Ocean, Currents of Atlantic Ocean

Movements of Ocean Water

Ocean water is dynamic. The horizontal motion refers to the ocean currents and waves. The vertical motion refers to tides. The upwelling of cold water from subsurface and the sinking of surface water are also forms of vertical motion of ocean water.

Waves

Waves are actually the energy, not the water as such, which moves across the ocean surface. Water particles only travel in a small circle as a wave passes. Wind provides energy to the waves. Wind causes waves to travel in the ocean and the energy is released on shorelines. (Movements of Ocean Water)

The motion of the surface water seldom affects the stagnant deep bottom water of the oceans. As a wave approaches the beach, it slows down. This is due to the friction occurring between the dynamic water and the sea floor and when the depth of water is less than half the wavelength of the wave, the wave breaks. The largest waves are found in the open oceans. Waves continue to grow larger as they move and absorb energy from the wind.

A wave’s size and shape reveal its origin. Steep waves are fairly young ones and are probably formed by local wind. Slow and steady waves originate from faraway places, possibly from another hemisphere. The maximum wave height is determined by the strength of the wind, i.e. how long it blows and the area over which it blows in a single direction.

1. Characteristics of Waves

Important terms associated with waves are:

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.
The Wave amplitude: It is one-half of the wave height.
Wave period: It is the time interval between two successive wave crests or troughs as they pass a fixed point.
Wavelength: It is the horizontal distance between two successive crests.
Wave speed: It is the rate at which the wave moves through the water, and is measured in knots.
Wave frequency: It is the number of waves passing a given point during a one second time interval.

2. Wave Motion

Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward. The falling water pushes the former troughs upward, and the wave moves to a new position. The actual motion of the water beneath the waves is circular. It indicates that things are carried up and forward as the wave approaches, and down and back as it passes. (Movements of Ocean Water)

Tides

The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of sun and the moon, is called a tide. Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges. Surges are not regular like tides. The study of tides is very complex, spatially and temporally, as it has great variations in frequency, magnitude and height.

1. Causes of Tides

The moon’s gravitational pull to a great extent and to a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides. Another factor is centrifugal force, which is the force that acts to counterbalance the gravity. Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth.

The ‘tide-generating’ force is the difference between the gravitational attraction of the moon and the centrifugal force. On the surface of the earth, nearest the moon, pull or the attractive force of the moon is greater than the centrifugal force, and so there is a net force causing a bulge towards the moon. On the opposite side of the earth, the attractive force is less, as it is farther away from the moon, the centrifugal force is dominant. Hence, there is a net force away from the moon. It creates the second bulge away from the moon. (Movements of Ocean Water)

2. Types of Tides

Tides vary in their frequency, direction and movement from place to place and also from time to time. Tides may be grouped into various types based on their frequency of occurrence in one day or based on their height.

3. Tides Based on Frequency

1. Semi-diurnal tide: The most common tidal pattern, featuring two high tides and two low tides each day. The successive high or low tides are approximately of the same height.

2. Diurnal tide: There is only one high tide and one low tide during each day. The successive high or low tides are approximately of the same height.

3. Mixed tide: Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean. (Movements of Ocean Water)

4. Tides Based on Height

The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth:

1. Spring tides: On full moon and new moon days, the Sun, the Moon and the Earth are almost in the same line. On these days, the Sun and the Moon exert their combined gravitational force on the Earth. Thus on these two days the high tides are the highest and are known as spring tides. The height of a spring tide is about 20 per cent more than the normal high tide. They occur twice every month.

2. Neap tides: On half Moon days (i.e. first and last quarter phases of the Moon), the Sun and the Moon are at right angles to the centre of the Earth. The tide producing forces of the Moon and the Sun, work in opposite directions and they partly cancel each other’s force. In such cases, the high tide is lower than the normal and low tide is higher than the normal. The difference is about 20 per cent. This is known as the neap tide. (Movements of Ocean Water)

5. Characteristics of Tides

1. Tidal range: The difference between the high tide water and the low tide water is called the tidal range. The time between the high tide and low tide, when the water level is falling, is called the ebb. The time between the low tide and high tide, when the tide is rising, is called the flow or flood.

Once in a month, when the moon’s orbit is closest to the earth (perigee), unusually high and low tides occur. During this time the tidal range is greater than normal. Two weeks later, when the moon is farthest from earth (apogee),the moon’s gravitation force is limited and the tidal ranges are less than their average heights.

2. Tidal current: Tidal currents (a horizontal motion) are a result of the rise and fall of the  water level due to tides (a vertical motion). The effects of tidal currents on the movement of water in and out of bays and harbours can be substantial.

The tidal bulges on wide continental shelves, have greater height. When tidal bulge shit the mid-oceanic islands they become low. The shape of bays and estuaries along a coastline can also magnify the intensity of tides. Funnel-shaped bays greatly change tidal magnitudes. The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal bulge is 15 – 16 m.

3. Tidal bore: When the tide enters the narrow and shallow estuary of a river, the front of the tidal wave appears to be vertical due to the piling up of the river water against the tidal wave and the friction of the river bed. It looks as if a vertical wall of water is moving upstream. This is called a tidal bore. In India tidal bores are common in the Hugli River.

6. Importance of Tides

Since tides are caused by the earth-moon-sun positions which are known accurately, the tides can be predicted well in advance. This helps the navigators and fishermen plan their activities. Some of the important activities associated with tides are:

1. Tidal flows are of great importance in navigation. Tidal heights are very important, especially harbours near rivers and within estuaries having shallow ‘bars’ at the entrance, which prevent ships and boats from entering into the harbour. Large ships enter the harbour of a shallow sea during high tide and they go back also at the time of high tide. London and I have become important ports due to the tidal nature of the mouths of the Thames and Hugli rivers respectively.
2. The river mouths and estuaries are kept clean of sedimentation due to the action of tidal currents. The force of the outgoing tide and the river current carries the silt away to the open sea. This helps in navigation.
3. The tidal force can also be used as a source for generating electricity. A 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is under way.
4. The inflow of the salty tidal water, especially along the coast of cold countries, retards the process of freezing and prevents the harbours from becoming ice-bound.
5. The fishing industry is helped by the rhythm of high and low tides. The fishermen mostly sail out to the open sea during low tides and return to the coast at high tides.

Movements of Ocean Water
Ocean Currents

Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction.

1. Causes of Ocean Currents

Ocean currents are influenced by two types of forces namely:
Primary forces that initiate the movement of water.

Secondary forces that influence the currents to flow. The forces that influence the currents are:

1. Heating by solar energy causes water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope. There is much difference in the temperature of ocean waters at the equator and at the poles. As warm water is lighter and rises, and cold water is denser and sinks, warm equatorial waters move slowly along the surface polewards, while the heavier cold waters of the polar regions creep slowly along the bottom of the sea equatorwards.

2. Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course. Most of the ocean currents of the world follow the direction of the prevailing winds.

3. Coriolis force causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere. These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins.

4. Salinity of ocean water varies from place to place. Waters of high salinity are denser than waters of low salinity. Hence on the surface, waters of low salinity flow towards waters of high salinity while at the bottom, waters of high salinity flow towards waters of low salinity.

2. Types of Ocean Currents

The ocean currents may be classified based on their depth or temperature. (Movements of Ocean Water)

3. Currents Based on Depth

1. Surface currents constitute about10 per cent of all the water in the ocean, these waters are the upper 400 m of the ocean.

2. Deep water currents make up the other 90 per cent of the ocean water. Deep waters sink into the deep ocean basins at high latitudes, where the temperatures are cold enough to cause the density to increase.

4. Currents Based on Temperature

1. 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.

2. 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.

5. Characteristics of Ocean Currents

The currents are strongest near the surface and may attain speeds over five knots. At depths, currents are generally slow with speeds less than 0.5knots. The speed of a current is known as its drift. Drift is measured in terms of knots. The strength of a current refers to the speed of the current. A fast current is considered strong. A current is usually strongest at the surface and decreases in strength (speed) with depth. Most currents have speeds less than or equal to 5 knots.

Currents of the Atlantic Ocean

(a) North and South Equatorial Current

To the north and south of the equator, there are two westward moving currents-the North Equatorial Current and the South Equatorial Current.
Due to the rotation of the Earth (Coriolis Effect), these currents move almost due west along the equator.
–  The North Equatorial Current moves northwards due to the presence of the South American continent and the Coriolis force, and takes the north-west direction. It enters the Gulf of Mexico to form the Gulf Stream.
The South Equatorial Current originates from the western coast of Africa, from where it moves towards South America.
The east coast of Brazil obstructs the South Equatorial Current which then bifurcates in two branches.
The northward branch merges with the North
Equatorial Current, while the second branch flows along the east coast of Brazil and is known as the Brazilian Current.
The North Equatorial Current and the South Equatorial Current are warm currents.

(b) Gulf Stream

The Gulf Stream is one of the largest warm currents. It originates from the Gulf of Mexico (about 20° N) and moves in a north-easterly direction along the eastern coast of North America.
The average speed is about 33 km per day and its average width is about 70 km.
–  Under the impact of the Westerlies, this warm current reaches the western coast of Europe (about 70° N latitude).
The general direction of flow of the Gulf Stream, north of 30° N latitude, is northward.
Near Newfoundland, its water mixes with the cold water of the Labrador Current, which forms very dense fog. The foggy conditions around Newfoundland hamper the navigation of ships.
From here, the Gulf Stream moves northeastwards.
This current gradually widens and its speed decreases. It becomes a prominent, slow moving current known as the North Atlantic Drift.
–  Near Western Europe, it splits into two parts. One part moves northwards, past UK and Norway, while the other part is deflected southwards as the cold Canary Current.
The warm water of the Gulf Stream modifies the weather conditions off the eastern coast of North America and the western coast of Europe.
On the western coast of Europe, the seaports remain open even in the severe winter season due to the warm water of the Gulf Stream.

(c) Labrador Current

–  The cold Labrador Current of the North Atlantic Ocean, has its origin in the Arctic Ocean.
This current flows from north to south between Greenland and the Baffin islands.
The Labrador Current merges with the Gulf Stream near Newfoundland.
This helps in the growth of plankton- a feed for fish. Thus the Grand Banks near Newfoundland have become the ideal fishing ground in the world.
The average speed of the Labrador Current is about 25 km per day.
This current brings huge icebergs with it from the Arctic Ocean.

(d) Canary Current

The Canary Current is a cold current and flows along the western coast of Spain and Portugal and the north-west coast of North Africa.
The average speed of this current is about 45 km per day.
The relative coolness of the Canary Current reduces the relative humidity and thus causes scanty rainfall in the greater parts of the Sahara Desert.

(e) Brazil Current

The Brazil Current is a warm current and flows southward along the east coast of South America (about 40° S latitude).
The average speed of the Brazil Current is about 30 km per day.
– From 40° S, it is deflected eastwards due to the Earth’s rotation and flows in easterly direction.
–  It modifies the weather conditions along the eastern coasts of Brazil and Argentina. (Movements of Ocean Water)

(f) Falkland Current

The cold waters of the Antarctic Sea flow as Falkland Current from south to north along the eastern coast of South America up to Argentina.
The Falkland Current brings huge icebergs from the Antarctic region to the South American coast. Benguela Current.
The Benguela Current is a cold current which originates in the Antarctic region and flows along the coast of south-west Africa.
The Benguela Current helps in reducing the relative humidity of the eastward moving warm and moist air masses.
The Kalahari Desert is largely formed under the influence of this current.
Further northwards, the Benguela Current merges with the South Equatorial Current. South Atlantic Drift
The eastward continuation of the Brazil Current is called the South Atlantic Drift or the West Wind Drift.
It develops at about 40° S latitude due to the impact of the Westerlies.
The eastward movement is due to the Earth’s rotation

Currents of the Pacific Ocean

(a) North Equatorial Current

The North Equatorial Current is a warm current which originates off the western coast of Mexico and flows in the westerly direction.
–  It runs parallel to the equator and reaches the islands of Philippines after covering a distance of about 12,000 km.
Near Philippines, under the impact of Coriolis force, it turns northwards.
–  One branch of the North Equatorial Current flows northward to join the Kuroshio Current, while the southern branch turns eastwards to form the Counter Equatorial Current.

(b) South Equatorial Current.

The South Equatorial Current is a warm current which originates due to the influence of South-east Trade winds and flows from east to west.
It bifurcates into northern and southern branches near New Guinea.
The northern branch turns eastward and joins the Counter Equatorial Current, while the southern branch flows along the north-eastern coast of Australia.

(c) Kuroshio Current

–  Kuroshio Current is an important warm current, which develops partly due to the Coriolis force and partly due to the obstruction by the Philippines in the flow of the North Equatorial Current.
The average velocity is about 30 km per day and the average surface temperature is about 20°C.
This current keeps the eastern coast of Japan warm even in the coldest month (January), when it is snowing heavily in Honshu and Hokkaido.
A branch of Kuroshio Current enters the Sea of Japan as Tsushima Current and keeps the western coast of Japan comparatively warm.
–  Around 35° N, the Kuroshio current comes under the impact of the Westerlies and flows in the north-east direction to reach the western coast of North America.
Further northwards, it is known as the Aleutian Current

(d) Kurile or Oyashio Current

–  The Kurile or Oyashio Current is a cold current which originates from the Bering Strait and moves southwards along the coast of the Kamchatka peninsula to touch the island of Kurile.
It carries with it the cold water and icebergs from the Arctic Ocean to the coast of eastern Russia and Japan.
–  Near 50° N latitude, it is bifurcated into two branches. One of them merges with Kuroshio Current and creates dense fog which is hazardous to navigation, but ideal for abundant growth of plankton.
Thus the north-eastern coast of the Japanese islands is an important fishing ground in the world.
The second branch moves up to the Japanese coast.
–  The Oyashio Current is comparable to the Labrador Current of the North Atlantic Ocean.

(e) California Current

The California Currents is a cold current which flows southwards along the Pacific coastline of USA, and is comparable to the Canary Current of the Atlantic Ocean in most of its characteristics (Movements of Ocean Water).
After reaching the Mexican coast, it turns westward and merges with the North Equatorial Current.
Dense sea fogs are experienced off the coast of San Francisco.

(f) Peru Current

The Peru Current is a cold current, also known as the Humboldt Current, which flows along the western coast of South America.
It flows from south to north along the coast of Peru and is caused by the northward deflection of the West Wind Drift.
It affects the coastal climate of Chile and Peru.

(g) East Australian Current

The East Australian Current is a warm current which is the southern branch of the South Equatorial Current, which flows from north to south along the eastern coast of Australia.
–  New Zealand is surrounded by this current.
–  It raises the temperature along the east Australian and the New Zealand coasts for considerable distance southwards.

(h) West Wind Drift

It is a strong, cold current, flowing from between Tasmania and South American coast.
It flows under the influence of the Westerlies and is largely confined between 40° Sand 50° S latitudes.
This current becomes very strong due to large volume of water and high velocity winds (Roaring Forties).
One of its branch enters the Atlantic Ocean through Cape Hom, and the other branch turns northwards and joins the Peru Current

Effects of Ocean Currents

1. The oceanic circulation transports heat from one latitude belt to another in a manner similar to the heat transported by the general circulation of the atmosphere. The cold waters of the Arctic and Antarctic circles move towards warmer water in tropical and equatorial regions, while the warm waters of the lower latitudes move polewards.

2. West coasts of the continents in tropical and subtropical latitudes (except close to the equator) are bordered by cool waters. Their average temperatures are relatively low with narrow diurnal and annual ranges. There is fog, but generally the areas are arid.

3. West coasts of the continents in the middle and higher latitudes are bordered by warm waters which cause a distinct marine climate. They are characterised by cool summers and relatively mild winters with a narrow annual range of temperatures.

4. Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.

5.  The mixing of warm and cold currents help to replenish the oxygen and favour the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.

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