Movements Of Ocean Water

A is correct. Tide refers to the upward and downward motion of ocean water, caused by the gravitational pull of the moon.

B is incorrect. Current refers to the onward movement of the water, caused by variations in temperature and density of water.

C is incorrect. Waves are generated by the winds that blow over the seas, and carried forward by these winds

As result of the moon and the sun pulling the earth gravitationally in the same direction.

Spring tides occur when the sun and the moon are in the same line, whereas neap tides occur when the sun and moon are perpendicular to each other, as shown in the photograph.

Spring tides are associated with full moon or new moon.

Other options are incorrect.

B is correct, as the farthest point on the elliptical trajectory of the moon is called the Perigee.

A is incorrect. Aphelion refers to the point on the trajectory of earth’s revolution, when it is farthest to the sun.

C is incorrect. Perihelion refers to the point on Earth’s trajectory, which is closest to the sun.

D is incorrect. It refers to the point on moon’s revolution trajectory, which is farthest from the earth.

The earth is at its perihelion in January, and after half year, it reaches the aphelion, as shown in the figure.

(i) Wave refers to the energy that flows through the water, making the water particles travel in circular, rising and falling motion. The energy is a result of the wind blowing over the water surface. Waves break at the shorelines and lose the energy to the landmass.

(ii) Waves in the ocean get their energy from the wind. When wind blows over water, it leads to the formation of ripples. On the surfaces of open seas, this forms waves. The wind propels the waves forward, while the gravity pulls the crest downward.

(iii) It refers to the periodic rise and fall of water twice a day, which occurs due to the gravitational effects of sun and moon on the free surface of earth’s water bodies.

(iv) Tides are caused by the gravitational pull exerted by the moon, and also by the sun. the free surface of water responds to the pulling effect of moon’s weight periodically rise twice a day. Another cause for tides is the centrifugal force of earth’s rotation, and tides are the result of the difference between these two forces.

(v) Tides help in navigation, as high tides raise the level of water. This helps the large ships to enter the harbour, as they would not be able to enter in at the shallow hours.

(i) Currents affect the ocean temperature. It varies with the type of current:

• Cold Currents bring cold water into the warm water oceans for example, Labrador Current. These are found along the middle latitudes in both the hemispheres, and in higher latitudes along the east coast in Northern Hemisphere.

• Warm Currents bring warm water to the colder oceans, for example Gulf Stream. They are found in the lower and middle latitudes along the east coasts of the continents in both the hemispheres. They are also found along the west coast of the continents in higher latitudes of Northern Hemisphere.

North Western Europe is dominated by the North Atlantic drift, a warm current broken away from Gulf Stream. This brings in warm waters and raises the temperature of the coastline.

(ii) The currents are caused by:

1) Primary Forces: They influence the movement of the water, and are responsible for formation of currents, like:

• Solar Energy: The expansion of water around the equator leads to the creation of a gradient, which makes the water flow.

• Wind propels the water to move along its direction.

• Gravitational pull of the earth creates gradients for the water to flow.

• The Coriolis Force interferes the moving water and leads to the formation of Gyres.

2) Secondary Forces: These influence the way the currents move in the oceans, as they affect the vertical mobility of water, like:

• Difference in the density of water

• Difference in the salinity of water

As we throw a stone, circular waves are generated around the area where the stone hits the surface of the lake. These ripples move outward, and carry the disturbance with them. However, the water does not move with the disturbance. It moves perpendicular to the disturbance, just rising and falling. Such waves are transverse waves- they do not carry the material of the medium with them.

Slowly, the energy of the wave dissipates and the ripples die out.

It is easy to see from the map that certain currents are warm, while others are cold. The reason for this is simple: the currents that move from equator towards the poles are warmer. They carry warm water towards the cold oceans. This warm water has been generated due to the insolation from the sun. eg. Gulf Stream, Kurushio etc.

The cold currents originate at the poles and descend towards the equator, as seen in the image. Eg. Oyashio, Labrador current.

The currents deflect due to the rotation of the earth and direction of the wind. The wind circles around low and high pressure areas, changing the direction of the current. Also, the Coriolis Force influences their direction.

The following currents exist in the Atlantic, Pacific and Indian oceans respectively:

• Atlantic Ocean: Gulf Stream, Brazilian Current, Falkland Current, Labrador Current, North Atlantic Drift.

• Pacific Ocean: North Equatorial Current, Equatorial Counter Current, West Wind Drift, California Current, Alaska Current, Kurushio, Oyashio

• Indian ocean: South Equatorial Current, West Australia Current, Auglhas Current.

• Winds influence the movement of the currents, as they drive their forward motion. Hence, the currents along the equator move linearly, whereas the currents in the lower and mid-latitudes move in the directions stipulated by the Coriolis force. In low pressure areas, there is an anti-clockwise motion, whereas in high pressure areas, the motion is clockwise.

The following currents exist in the Atlantic, Pacific and Indian oceans respectively:

• Atlantic Ocean: Gulf Stream, Brazilian Current, Falkland Current, Labrador Current, North Atlantic Drift.

• Pacific Ocean: North Equatorial Current, Equatorial Counter Current, West Wind Drift, California Current, Alaska Current, Kurushio, Oyashio

• Indian ocean: South Equatorial Current, West Australia Current, Auglhas Current.

• Winds influence the movement of the currents, as they drive their forward motion. Hence, the currents along the equator move linearly, whereas the currents in the lower and mid-latitudes move in the directions stipulated by the Coriolis force. In low pressure areas, there is an anti-clockwise motion, whereas in high pressure areas, the motion is clockwise.