Movements Of Ocean Water Class 11 Notes Geography Chapter 13 - CBSE

Chapter : 13

What Are Movements Of Ocean Water ?

https://drive.google.com/file/d/1D6miT1kM7PbVF7TdH_Hpbg2qmxB1CRKA/view

The ocean water is dynamic. Its physical characteristics such as temperature, salinity, density, and the external forces of the sun, moon, and the winds influence the movement of ocean water.
The horizontal 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.

Waves

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. 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 and causes waves to travel in the ocean and the energy is released on shorelines.
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.
Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward.

Tides

The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.
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. 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 these two forces, i.e., the gravitational attraction of the moon and the centrifugal force.
The tidal bulges on wide continental shelves have greater height. When tidal bulges hit the mid-oceanic islands they become low.
When the tide is channelled between islands or into bays and estuaries they are called tidal currents.
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 24 hours or based on their height.

Tides Based On Frequency

Semi-diurnal tide: This is 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.
Diurnal tide: There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height.
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.

Tides Based On The Sun, Moon, And The Earth Positions

The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth. Spring tides and neap tides come under this category.
Spring tides: The position of both the sun and the moon in relation to the earth has direct bearing on tide height. When the sun, moon, and earth are in a straight line, the height of the tide will be higher. These are called spring tides and they occur twice a month, one on full moon period and another during new moon
period.
Neap tides: Normally, there is a seven-day interval between the spring tides and neap tides. At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another. The Moon’s attraction, though more than twice as strong as the sun’s, is diminished by the
counteracting force of the sun’s gravitational pull.
- Once in a month, when the moon’s orbit is closest to the earth called 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 is called apogee, the moon’s gravitational force is limited and the tidal ranges are less than their average heights.
- When the earth is closest to the sun (is called perihelion), around 3rd January each year, tidal ranges are
  also much greater, with unusually high and unusually low tides. When the earth is farthest from the sun (is called aphelion), around 4th July each year, tidal ranges are much less than average.
- 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.

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.
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.
Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.
Tides are used to generate electrical power (in Canada, France, Russia, and China). A 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is underway.

Ocean Currents

Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction.
Ocean currents are influenced by two types of forces: (i) primary forces that initiate the movement of water, and (ii) secondary forces that influence the currents to flow.
The primary forces that influence the currents are: (i) heating by solar energy, (ii) wind, (iii) gravity; (iv) coriolis force.
Friction between the wind and the water surface affects the movement of the water body in its course.
Gravity tends to pull the water down to pile and create gradient variation.
The Coriolis force intervenes and 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. They produce large circular currents in all the ocean basins.

The ocean currents may be classified based on their depth as surface currents and deep water currents:

Surface currents constitute about 10% of all the water in the ocean, these waters are the upper 400 m of the ocean,
Deep water currents make up the other 90% of the ocean water.

Ocean currents can also be classified based on temperature - cold currents and warm currents:

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.
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).
Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and Coriolis force. The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern. Ocean currents have a number of direct and indirect influences on human activities.