Section 15 3 Ocean Movements Continued Answers

Ocean water is always in a state of motion. There are majorly three forms of ocean water movements. In this article, you can read all about the movement of the ocean water for the UPSC exam geography segment.

Ocean Water Movements

The movement of ocean water is influenced by its physical characteristics (temperature, salinity and density) and the external forces (sun, moon and  winds). The movement of ocean water takes place in three ways-

1. Waves
2. Ocean currents, and
3. Tides.

Waves and ocean currents are horizontal movements of ocean waters while the tides are due to vertical movement of the ocean waters.

Waves

Waves are a kind of horizontal movement of ocean water.

• Waves are oscillatory movements that result in the rise and fall of the water surface.
• 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.
• The energy for the waves is provided by the wind.
• As the wave approaches the beach, it slows down due to the friction occurring between the dynamic water and the seafloor.

Characteristics of Waves

1. Wave crest – The upper or highest part of a wave is called the crest.
2. Wave trough- The lowest part of a wave is called the trough.
3. Wave height – It is the vertical distance from the bottom of a trough to the top of a crest of a wave.
4. Wavelength – It is the horizontal distance between two successive crests or troughs.
5. Wave amplitude – It is one-half of the wave height.
6. Wave period – It is the time interval between two successive wave crests or troughs as they pass a fixed point.
7. Wave frequency – It is the number of waves passing a given point during one second time interval.
8. Wave speed – It is the rate at which the wave moves through the water and is measured in knots.

Image source – NCERT

Tides

Tides are the periodical rise and fall of the ocean levels once or twice a day mainly due to the gravitational forces exerted by the sun, the moon and the rotation of the earth.

• Tides occur due to the vertical movement of the ocean waters and are different from the movement of water caused by meteorological effects like winds and atmospheric pressure changes. Such water movements are called surges and are not regular like tides.
• Moon's gravitational pull is the major cause of the occurrence of tides, as it is more effective than that of the sun. The sun's gravitational pull and the centrifugal force due to the rotation of the earth are the other forces that act along with the moon's gravitational pull.

Image source – NCERT

• The gravitational pull and the centrifugal force create two major tidal bulges on the earth. On the side of the earth facing the moon, a tidal bulge is created because the attractive force of the moon is greater than the centrifugal force. On the opposite side of the earth, the attractive force is less as it is farther away from the moon, and the centrifugal force is dominant. It results in a second bulge away from the moon. The tide generating force is the difference between the gravitation of the moon and the centrifugal force.
• When the tide is channelled between islands or into bays and estuaries, they are called tidal currents.
• The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada.

Flow tide and ebb tide

Flow tide/High tide – The time between the low tide and high tide, when the tide is rising is called the flow or flood tide. It is the rising tide.

Ebb tide/low tide – The time between the high tide and low tide, when the tide is falling is called the ebb. It is the receding tide.

Types of Tides

Based on their frequency of occurrence –

• Semi-diurnal – The most common tidal pattern with two high tides and two low tides each day. The successive high or low tides are approximately of the same height.
• Diurnal tide – Only one high tide and one low tide each day. The successive high or low tides are approximately of the same height.
• Mixed tide – Tides varying in height are known as mixed tides. These generally occur along the west coast of North America and on many islands of the Pacific Ocean.

Based on the sun, the moon and the earth's positions –

• Spring tide – When the sun, the moon and the earth are in a straight line, the height of the tide will be higher than normal and are called spring tides. Spring tides occur twice a month – one on the full moon and the other on the new moon.
• Neap tides – When the sun and the moon are positioned at right angles to each other, the height of the tide will be lower than the normal and are called neap tides. This happens normally after seven days of a spring tide. The gravitational forces of the sun and the moon tend to counteract each other. Neap tides also occur twice a month (during the first quarter moon and the last quarter moon).

Magnitude of Tides

• Perigee – When the moon's orbit is closest to the earth, it is called perigee. During this time, unusually high and low tides occur. This happens once in a month.
• Apogee – When the moon is farthest from the earth, it is called apogee. During this time, tidal ranges are less than their average heights. Perigee and apogee are two weeks apart.
• Perihelion – When the earth is closest to the sun, around 3rd Jan each year, it is called the perihelion position. During this time, there are unusually high and unusually low tides.
• Aphelion – When the earth is farthest from the sun, around 4th July, this position is called aphelion. During this time, tidal ranges are much less than average.

Ocean Currents

Ocean currents are just like a river flowing in an ocean. A large volume of water moving in a fairly defined direction over great distances is called an ocean current.

• The ocean currents are produced due to the earth's rotation – Coriolis Force (Coriolis force causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere), density differences in ocean waters due to differences in temperature and salinity, gravity and wind.

Gyre, Drift and Stream

Gyre – A gyre is any large system of circulating ocean currents, particularly involved with large wind movements. They are caused by the coriolis force.

Drift – When the ocean water moves forward under the influence of prevailing wind, it is called a drift. The term drift is also used to refer to the speed of an ocean current. It is measured in terms of knots (kn). Most currents have speeds less than or equal to 5 knots (1knot =1.852 km per hour).

Stream – Streams are larger masses of water moving in a definite direction and have much greater velocity than the drifts. For example, Gulf Stream.

Types of ocean currents

1. Based on the temperature –

Warm currents –

• These currents originate from the equator and flow towards the poles.
• These currents bring warm water into cold areas.
• These are usually observed on the east coast of continents in the low and middle latitudes of both hemispheres.
• In the northern hemisphere, these currents are found on the west coasts of continents in high latitudes (e.g., Alaska currents).

Cold currents –

• These currents start from the polar region and flow towards the equator.
• 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 of both hemispheres.
• In the northern hemisphere, these currents are found on the east coast in the higher latitudes (e.g., Labrador currents).

1. Based on the depth-

Surface currents – These constitute about 10% of all the water in the ocean. These waters occupy the upper 400m of an ocean. These currents are driven due to the blowing wind.

Deep water currents – These constitute about 90% of the total ocean water. These waters move around the ocean basins due to variation in the density and gravity.

Know more about ocean currents in the linked article.

Major Ocean Currents

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 warm waters of the lower latitudes move polewards while the cold waters of the Arctic and Antarctic circles move towards warmer waters in tropical and equatorial regions.

Ocean Currents of the Pacific Ocean

1. North Equatorial current – It is a warm current. It originates from RevillaGigedo island-west of Mexico and flows towards Philippines island.
2. South Equatorial current – It is a warm current which originates due to the action of trade winds from east to west. It is stronger than the North Equatorial current.
3. Kuroshio current (Black tide) – It is a warm current which flows in the north-easterly direction up to 30° latitude. It meets the Oyashio cold current off the Kuril islands. It is also called Japan current.
4. Oyashio current (Parental tide) – It is a cold current which originates from Bering Strait and flows southwards. It meets Kuroshio warm current and Aleutian current.
5. Californian current – It is a cold current which flows towards south along the west coast of the U.S.A. In the region of trade winds, it gets deflected to the right and joins the equatorial current.
6. Peru current – Also called Humboldt current as Alexander Von Humboldt noted the details of the Peru current. It is a cold current which flows along the west coast of South America towards the north.
7. West Wind Drift – It is a cold current. It is an easterly moving drift in the Pacific Ocean extending from Tasmania to the South American coast. It splits into two branches – one moves south around Cape Horn into the Atlantic Ocean and the other one moves northwards along the Peruvian coast and joins the Peru current.

Ocean Currents of the Atlantic Ocean –

1. North Equatorial current – It is a warm current which flows from east to the west. On reaching the east coast of South America it splits into two branches – one branch called Antilles current moves along the coast of the West Indies and the other branch is diverted into the Caribbean Sea.
2. South Equatorial current – It is a warm current which flows between the coast of Africa and South America. It is caused by the action of trade winds and is stronger than the north equatorial current.
3. Gulf Stream – It was discovered by Ponce de Leon in 1513. It is a warm current which starts from the Gulf of Mexico. It joins the Labrador cold current near Newfoundland, Canada after passing through the Strait of Florida.
4. Canaries current – It is a cold current which flows along the western coast of North Africa between Madeira and Cape Verde. It merges with the north equatorial current.
5. Labrador current – It is a cold current and brings cold waters from the polar zone. It moves along the coast of Greenland.
6. Benguela current – It is a cold current which flows northward along the western coast of Africa. It brings cold water from sub-Antarctic surface water. It merges with the south equatorial current.

Ocean Currents of the Indian Ocean –

The south Indian gyre is formed by the south equatorial current, Madagascar current, west wind drift and west Australian current. To the north of the 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 influence of monsoon winds.

Ocean Currents of the Southern Ocean –

The continent of Antarctica is surrounded by the Southern Ocean. The Pacific, the Atlantic and the Indian oceans merge into this circum-global zone of water to their south. The movement of water in the southern ocean is generally west-east circum polar drift caused under the influence of northwesterly winds. This 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 receive a part of their cold waters from the Southern Ocean.

Image source – NCERT

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