NCERT Textbook - Movements of Ocean Water | Geography (Prelims) by Valor Academy - UPSC PDF Download

 Page 1


MOVEMENTS OF OCEAN WATER
T
he 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
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. Ocean currents
are the continuous flow of huge amount of
water in a definite direction while the waves
are the horizontal motion of water. W ater 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
in 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 the
crests of the waves downward.  The falling water
pushes the former troughs upward, and the
Figure13.1 : Motion of waves and water molecules
CHAPTER
2024-25
Page 2


MOVEMENTS OF OCEAN WATER
T
he 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
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. Ocean currents
are the continuous flow of huge amount of
water in a definite direction while the waves
are the horizontal motion of water. W ater 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
in 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 the
crests of the waves downward.  The falling water
pushes the former troughs upward, and the
Figure13.1 : Motion of waves and water molecules
CHAPTER
2024-25
MOVEMENTS OF OCEAN WATER 109
wave moves to a new position (Figure 13.1).
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.
Characteristics of Waves
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.
Wave amplitude : It is one-half of the wave
height.
Wave period : It is merely 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.
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.
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.
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
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 13.2).
The ‘tide-generating’ force is the difference
between these two forces; i.e. 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. On the
surface of the earth, the horizontal tide
generating forces are more important than the
vertical forces in generating the tidal bulges.
Figure13.2 : Relation between gravitational
forces and tides
The tidal bulges on wide continental
shelves, have greater height. When tidal bulges
hit 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. When the tide is channelled
between islands or into bays and estuaries
they are called tidal currents.
2024-25
Page 3


MOVEMENTS OF OCEAN WATER
T
he 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
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. Ocean currents
are the continuous flow of huge amount of
water in a definite direction while the waves
are the horizontal motion of water. W ater 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
in 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 the
crests of the waves downward.  The falling water
pushes the former troughs upward, and the
Figure13.1 : Motion of waves and water molecules
CHAPTER
2024-25
MOVEMENTS OF OCEAN WATER 109
wave moves to a new position (Figure 13.1).
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.
Characteristics of Waves
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.
Wave amplitude : It is one-half of the wave
height.
Wave period : It is merely 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.
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.
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.
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
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 13.2).
The ‘tide-generating’ force is the difference
between these two forces; i.e. 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. On the
surface of the earth, the horizontal tide
generating forces are more important than the
vertical forces in generating the tidal bulges.
Figure13.2 : Relation between gravitational
forces and tides
The tidal bulges on wide continental
shelves, have greater height. When tidal bulges
hit 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. When the tide is channelled
between islands or into bays and estuaries
they are called tidal currents.
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 110
Tides of Bay of Fundy, Canada
The highest tides in the world occur in
the Bay of Fundy in Nova Scotia, Canada.
The tidal bulge is 15 - 16 m. Because
there are two high tides and two low tides
every day (roughly a 24 hour period); then
a tide must come in within about a six
hour period. As a rough estimate, the tide
rises about 240 cm an hour (1,440 cm
divided by 6 hours). If you have walked
down a beach with a steep cliff alongside
(which is common there), make sure you
watch the tides. If you walk for about an
hour and then notice that the tide is
coming in, the water will be over your
head before you get back to where you
started!
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 24 hours or based on their height.
Tides based on Frequency
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.
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, the moon
and the 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 (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 gravitational force is limited and
the tidal ranges are less than their average
heights.
When the earth is closest to the sun
(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 (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
2024-25
Page 4


MOVEMENTS OF OCEAN WATER
T
he 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
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. Ocean currents
are the continuous flow of huge amount of
water in a definite direction while the waves
are the horizontal motion of water. W ater 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
in 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 the
crests of the waves downward.  The falling water
pushes the former troughs upward, and the
Figure13.1 : Motion of waves and water molecules
CHAPTER
2024-25
MOVEMENTS OF OCEAN WATER 109
wave moves to a new position (Figure 13.1).
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.
Characteristics of Waves
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.
Wave amplitude : It is one-half of the wave
height.
Wave period : It is merely 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.
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.
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.
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
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 13.2).
The ‘tide-generating’ force is the difference
between these two forces; i.e. 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. On the
surface of the earth, the horizontal tide
generating forces are more important than the
vertical forces in generating the tidal bulges.
Figure13.2 : Relation between gravitational
forces and tides
The tidal bulges on wide continental
shelves, have greater height. When tidal bulges
hit 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. When the tide is channelled
between islands or into bays and estuaries
they are called tidal currents.
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 110
Tides of Bay of Fundy, Canada
The highest tides in the world occur in
the Bay of Fundy in Nova Scotia, Canada.
The tidal bulge is 15 - 16 m. Because
there are two high tides and two low tides
every day (roughly a 24 hour period); then
a tide must come in within about a six
hour period. As a rough estimate, the tide
rises about 240 cm an hour (1,440 cm
divided by 6 hours). If you have walked
down a beach with a steep cliff alongside
(which is common there), make sure you
watch the tides. If you walk for about an
hour and then notice that the tide is
coming in, the water will be over your
head before you get back to where you
started!
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 24 hours or based on their height.
Tides based on Frequency
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.
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, the moon
and the 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 (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 gravitational force is limited and
the tidal ranges are less than their average
heights.
When the earth is closest to the sun
(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 (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
2024-25
MOVEMENTS OF OCEAN WATER 111
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 under way.
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 namely :
(i) primary forces that initiate the movement of
water; (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. Heating
by solar energy causes the 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. 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. Gravity tends to pull
the water down the 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. These produce large circular currents
in all the ocean basins.
Differences in water density affect vertical
mobility of ocean currents. Water with high
salinity  is denser than water with low salinity
and in the same way cold water is denser than
warm water. Denser water tends to sink, while
relatively lighter water tends to rise. Cold-water
ocean currents occur when the cold water at
the poles sinks and slowly moves towards the
equator. Warm-water currents travel out from
the equator along the surface, flowing towards
the poles to replace the sinking cold water.
Types of Ocean Currents
The ocean currents may be classified based on
their depth as surface currents and deep water
currents : (i) surface currents constitute about
10 per cent of all the water in the ocean, these
waters are the upper 400 m of the ocean;
(ii) deep water currents make up the other 90
per  cent of the ocean water. These waters move
around the ocean basins due to variations in
the density and gravity. Deep waters sink into
the deep ocean basins at high latitudes, where
the temperatures are cold enough to cause the
density to increase.
Ocean currents can also be classified
based on temperature : as cold currents and
warm currents: (i) 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; (ii) 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.
Major Ocean Currents
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. The air circulation over the
oceans in the middle latitudes is mainly
anticyclonic (more pronounced in the southern
hemisphere than in the northern hemisphere).
The oceanic circulation pattern also
corresponds with the same. At higher latitudes,
Characteristics of Ocean Currents
Currents are referred to by their “drift”.
Usually, the currents are strongest near
the surface and may attain speeds over
five knots. At depths, currents are
generally slow with speeds less than 0.5
knots. We refer to the speed of a current
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.
2024-25
Page 5


MOVEMENTS OF OCEAN WATER
T
he 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
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. Ocean currents
are the continuous flow of huge amount of
water in a definite direction while the waves
are the horizontal motion of water. W ater 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
in 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 the
crests of the waves downward.  The falling water
pushes the former troughs upward, and the
Figure13.1 : Motion of waves and water molecules
CHAPTER
2024-25
MOVEMENTS OF OCEAN WATER 109
wave moves to a new position (Figure 13.1).
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.
Characteristics of Waves
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.
Wave amplitude : It is one-half of the wave
height.
Wave period : It is merely 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.
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.
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.
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
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 13.2).
The ‘tide-generating’ force is the difference
between these two forces; i.e. 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. On the
surface of the earth, the horizontal tide
generating forces are more important than the
vertical forces in generating the tidal bulges.
Figure13.2 : Relation between gravitational
forces and tides
The tidal bulges on wide continental
shelves, have greater height. When tidal bulges
hit 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. When the tide is channelled
between islands or into bays and estuaries
they are called tidal currents.
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 110
Tides of Bay of Fundy, Canada
The highest tides in the world occur in
the Bay of Fundy in Nova Scotia, Canada.
The tidal bulge is 15 - 16 m. Because
there are two high tides and two low tides
every day (roughly a 24 hour period); then
a tide must come in within about a six
hour period. As a rough estimate, the tide
rises about 240 cm an hour (1,440 cm
divided by 6 hours). If you have walked
down a beach with a steep cliff alongside
(which is common there), make sure you
watch the tides. If you walk for about an
hour and then notice that the tide is
coming in, the water will be over your
head before you get back to where you
started!
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 24 hours or based on their height.
Tides based on Frequency
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.
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, the moon
and the 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 (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 gravitational force is limited and
the tidal ranges are less than their average
heights.
When the earth is closest to the sun
(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 (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
2024-25
MOVEMENTS OF OCEAN WATER 111
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 under way.
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 namely :
(i) primary forces that initiate the movement of
water; (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. Heating
by solar energy causes the 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. 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. Gravity tends to pull
the water down the 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. These produce large circular currents
in all the ocean basins.
Differences in water density affect vertical
mobility of ocean currents. Water with high
salinity  is denser than water with low salinity
and in the same way cold water is denser than
warm water. Denser water tends to sink, while
relatively lighter water tends to rise. Cold-water
ocean currents occur when the cold water at
the poles sinks and slowly moves towards the
equator. Warm-water currents travel out from
the equator along the surface, flowing towards
the poles to replace the sinking cold water.
Types of Ocean Currents
The ocean currents may be classified based on
their depth as surface currents and deep water
currents : (i) surface currents constitute about
10 per cent of all the water in the ocean, these
waters are the upper 400 m of the ocean;
(ii) deep water currents make up the other 90
per  cent of the ocean water. These waters move
around the ocean basins due to variations in
the density and gravity. Deep waters sink into
the deep ocean basins at high latitudes, where
the temperatures are cold enough to cause the
density to increase.
Ocean currents can also be classified
based on temperature : as cold currents and
warm currents: (i) 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; (ii) 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.
Major Ocean Currents
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. The air circulation over the
oceans in the middle latitudes is mainly
anticyclonic (more pronounced in the southern
hemisphere than in the northern hemisphere).
The oceanic circulation pattern also
corresponds with the same. At higher latitudes,
Characteristics of Ocean Currents
Currents are referred to by their “drift”.
Usually, the currents are strongest near
the surface and may attain speeds over
five knots. At depths, currents are
generally slow with speeds less than 0.5
knots. We refer to the speed of a current
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.
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 112
where the wind flow is mostly cyclonic, the
oceanic circulation follows this pattern. In
regions of pronounced monsoonal flow, the
monsoon winds influence the current
movements. Due to the coriolis force, the warm
currents from low latitudes tend to move to the
right in the northern hemisphere and to their
left in the southern hemisphere.
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. The major currents
in the different oceans are shown in Figure13.3.
Prepare a list of currents which are
found in Pacific, Atlantic and Indian
Oceans.
How is the movement of currents
influenced by prevailing winds? Give
some examples from Figure13.3.
Effects of Ocean Currents
Ocean currents have a number of direct and
indirect influences on human activities. 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 a
narrow diurnal and annual ranges. There is
fog, but generally the areas are arid. 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. 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. 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.
Fig.13.3 : Major currents in the Pacific, Atlantic and Indian oceans
2024-25