NCERT Book Class 11 Geography Movements Of Ocean Water

The ocean water is dynamic. Its physical characteristics like temperature, salinity, density and the external forces like of the sun, moon and the winds influence the movement of ocean water. The  orizontal and vertical motions are common in ocean water bodies. The horizontal motion refers to the ocean currents and waves. The vertical motion refers to tides. Ocean currents are the continuous flow of huge amount of water in a definite direction while the waves are the horizontal motion of water. Water moves ahead from one place to another through ocean currents while the water in the waves does not move, but the wave trains move ahead. The vertical motion refers to the rise and fall of water n the oceans and seas. Due to attraction of  the sun and the moon, the ocean water is raised up and falls down twice a day. 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. 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. Most of the waves are caused by the wind driving against water. When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until white caps appear in the breaking waves. Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf.

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 far away 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.  Waves travel because wind pushes the water body in its course while gravity pulls thecrests of the waves downward. The falling water pushes the former troughs upward, and the wave moves to a new position (Figure 14.1). 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.

TIDES
The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of  he 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 egular like tides. The study of tides is very complex, spatially and temporally, as it has great variations in frequency, magnitude and height. 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 counter the balance the gravity. Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth. On the side of the earth facing the moon, a tidal bulge occurs while on the opposite side though the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes tidal bulge on the other side (Figure 14.2). The ‘tide-generating’ force is the difference between these two forces; i.e. the  ravitational 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. On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.

Excercise

1. Multiple choice questions.
(i) Upward and downward movement of ocean water is known as the :
(a) tide (c) wave
(b) current (d) none of the above
(ii) Spring tides are caused :
(a) As result of the moon and the sun pulling the earth gravitationally in the same direction.
(b) As result of the moon and the sun pulling the earth gravitationally in the opposite direction.
(c) Indention in the coast line.
(d) None of the above.
(iii) The distance between the earth and the moon is minimum when the moon is in :
(a) Aphelion (c) Perihelion
(b) Perigee (d) Apogee
(iv) The earth reaches its perihelion in:
(a) October (c) July
(b) September (d) January
2. Answer the following questions in about 30 words.
(i) What are waves?
(ii) Where do waves in the ocean get their energy from?
(iii) What are tides?
(iv) How are tides caused?
(v) How are tides related to navigation?
3. Answer the following questions in about 150 words.
(i) How do currents affect the temperature? How does it affect the temperature of coastal areas in the N. W. Europe?
(ii) What are the causes of currents?

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