Name
Class
CHAPTER 25
Date
Movements of the Ocean
1 Ocean Currents
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How do wind patterns, Earth’s rotation, and
continental barriers affect surface currents in
the ocean?
• What determines the direction of a surface current?
• How do differences in the density of ocean water
affect the flow of deep currents?
What Factors Affect Surface Currents?
The water in the ocean moves in currents. A current
is a large stream of water that flows through the ocean.
Scientists put ocean currents in two categories: surface
currents and deep currents. Surface currents move near
the surface of the ocean. The figure below shows Earth’s
major surface currents.
No
r
G
tG
re
e
No
rw
ay
Antartic Circum
en
Brazil
P er u
at
or
i
guela
South Equ
n
al
ia
East
Au
str
t Australian
Wes
60ºS
30ºN
B
pola
r
al
30ºS
lifornia
0ºN
io North Pacific
sh
ro
u
K North Equatorial
Equatorial
Countercurrent
Equatorial
South
ic
ant
A t l ea m
tr
fS
ul
No
rth Equatorial
orial 0ºN
uat
urrent
Eq unterc
Co Soutth
ua oria
l
Eq
30ºS
s
Ca
30ºN
th
lh
a
Alaska
Clarify Concepts Take turns
reading this section out
loud with a partner. Stop
to discuss ideas that seem
confusing.
Talk About It
60ºN
Canary
O
rador
y
o
hi
as
d
an
nl
Eas
Lab
60ºN
READING TOOLBOX
u
Ag
60ºS
Antartic Circumpolar
Discuss Study this map with
a partner. Together, identify
some of the patterns you see
in the movement of surface
currents.
LOOKING CLOSER
1. Identify Do most of the
currents in the Northern
Hemisphere move clockwise
or counterclockwise?
Warm currents
Cool currents
As you can see on the map above, some surface
currents are warm, and others are cold. Warm surface
currents are most common in areas near the equator.
Cold surface currents generally form near the poles.
If you study the map carefully, you will notice some
patterns in how the surface currents move. Global winds,
the locations of continents, and Earth’s rotation all affect
the directions of surface currents.
READING CHECK
2. Describe Where do most
cold surface currents form?
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Movements of the Ocean
Name
SECTION 1
Class
Date
Ocean Currents continued
WIND
READING CHECK
3. Identify What causes all
surface currents?
Wind causes all surface currents. Because wind is
moving air, it has kinetic energy. As wind moves across
the ocean’s surface, its energy is transferred to the water.
Therefore, the water at the surface begins to move.
Remember that wind forms because of differences in air
pressure. Air always moves from areas of high pressure to
areas of low pressure. The moving air is wind.
Global wind belts have the strongest effect on surface
currents. Recall that there are three main global wind
belts: trade winds, westerlies, and polar easterlies. The
map below shows where these wind belts occur.
POLAR EASTERLIES
LOOKING CLOSER
60ºN
4. Compare What is the
main difference between the
trade winds in the Northern
Hemisphere and the trade
winds in the Southern
Hemisphere?
WESTERLIES
30ºN
TRADE WINDS
0º
TRADE WINDS
30ºS
WESTERLIES
60ºS
POLAR EASTERLIES
Global wind belts affect the directions of surface currents.
LOCATIONS OF CONTINENTS
When a surface current meets a continent, the current
changes direction. For example, in the Pacific Ocean,
the North Pacific Current flows eastward. When it meets
North America, it turns and begins to flow south.
EARTH’S ROTATION
READING CHECK
5. Identify What is the main
factor that determines the
direction of surface current
flow?
As Earth spins on its axis, ocean currents and wind
belts curve. This curving is called the Coriolis effect.
The Coriolis effect causes surface currents to move in
huge circles. These circles of water are called gyres. In
the Northern Hemisphere, the gyres flow clockwise. In
the Southern Hemisphere, they flow counterclockwise.
The Coriolis effect is the main factor that determines the
direction in which a gyre flows.
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Movements of the Ocean
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SECTION 1
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Ocean Currents continued
What Are the Major Surface Currents?
Scientists often divide surface currents into four groups:
equatorial currents, Southern Hemisphere currents, North
Atlantic currents, and North Pacific currents.
EQUATORIAL AND SOUTHERN HEMISPHERE CURRENTS
The equator passes through the Atlantic, Pacific, and
Indian Oceans. Warm currents flow through each of these
oceans near the equator. The North Equatorial Current
and the South Equatorial Current both flow westward.
Between them is the weaker Equatorial Countercurrent,
which flows eastward.
There are three main gyres in the Southern Hemisphere.
These gyres are found in the Atlantic Ocean, the Pacific
Ocean, and the Indian Ocean. Like the other gyres in the
Southern Hemisphere, they flow counterclockwise.
The Antarctic Circumpolar Current is also found in
the Southern Hemisphere. This current flows eastward.
No continents strongly affect it. Therefore, it flows freely
and completely circles Antarctica.
READING CHECK
6. Identify Name three
equatorial surface currents.
NORTH ATLANTIC CURRENTS
The North Atlantic Gyre is the main gyre in the North
Atlantic Ocean. The figure below shows the four currents
that make up the North Atlantic Gyre.
Baffin Bay
ry Current
a
rw
No
Norwegian
Sea
nt
rre
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yC
North
Sea
EUROPE
Can
a
am T H
tre
lf S O R
Sargasso
Sea
NO
YR
RTH
AT L A N T I C G
a
E
Gu
Gulf of
Mexico
a
Atl
rth
No A N T I C G Y R
L
AT
Se
7. Explain The water in the
Sargasso Sea is very calm. It
does not have many strong
currents. What is the most
likely reason for this?
E
NORTH
AMERICA
a
nd
nt
rre
Cu
c
i
nt
N
The Sargasso Sea
is a large area of warm,
calm water in the center
of the North Atlantic Gyre.
r
do
bra nt
La urre e
C
S
or
vis
Darait rad
St Lab
Hudson
Bay
a
nl
ee
Gr
Critical Thinking
Caribbean Sea
AFRICA
North Equatorial Current
Warm currents
Cool currents
The North Atlantic Gyre is made up of the Gulf Stream, the North Atlantic Current,
the Canary Current, and the North Equatorial Current.
The Gulf Stream is a warm surface current that flows
northward along the east coast of North America. It
affects the climate of much of eastern North America.
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Movements of the Ocean
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Ocean Currents continued
NORTH PACIFIC CURRENTS
The pattern of currents in the North Pacific Ocean is
similar to the pattern in the North Atlantic Ocean. A large
gyre called the North Pacific Gyre is located in the North
Pacific Ocean. The North Pacific Gyre is made of four
currents: the Kuroshio Current, the North Pacific Current,
the California Current, and the North Equatorial Current.
What Factors Affect Deep Currents?
READING CHECK
8. Explain Why does the
water near the poles sink?
Critical Thinking
Deep currents are cold, dense currents that flow
far below the ocean’s surface. Deep currents form when
water near the poles sinks and flows beneath warmer,
shallower water. The ocean water near the poles sinks
because it is denser than the water in the rest of the
ocean. Two main factors affect the density of the ocean
water: salinity and temperature.
Much of the water near the poles is frozen in icebergs
and sea ice. Remember that when ocean water freezes,
the ice that forms does not contain salt. The salt remains
in the liquid water that is left behind when the ice forms.
Therefore, the unfrozen water has a high salinity. The
high salinity increases the water’s density.
Ocean water near the poles is very cold. The molecules
in cold water are closer together than the molecules in
warm water. Therefore, the cold water near the poles is
much denser than the warmer water in the rest of the
ocean.
The high density of the water near the poles makes
the water sink. The sinking water forms deep currents.
The figure below shows some of the deep currents in the
ocean.
Greenland
Antarctica
Warm tropical
surface water
9. Apply Concepts Which
current is probably denser—
North Atlantic Deep Water or
Antarctic Intermediate Water?
Explain your answer.
Antarctic
Intermediate Water
North Atlantic Deep Water
Antarctic Bottom Water
60ºN
30ºN
0º
30ºS
60ºS
The two main deep currents are Antarctic Bottom Water and North Atlantic Deep
Water.
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Movements of the Ocean
Name
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SECTION 1
Date
Ocean Currents continued
ANTARCTIC BOTTOM WATER
The water near Antarctica is very cold, and its salinity
is high. Therefore, the water is very dense. In fact, the
water near Antarctica is the densest in the world. This
dense, cold water sinks to the ocean bottom and forms a
deep current called the Antarctic Bottom Water.
NORTH ATLANTIC DEEP WATER
The water in the North Atlantic Ocean, south of
Greenland, is very cold. It also has a high salinity. This
cold, salty water forms a deep current that moves along
the east coast of the United States. It flows southward
below the Gulf Stream.
Near the equator, this deep current divides. Part of it
rises and begins to flow northward again. The other part
continues to flow south toward Antarctica. This part
of the North Atlantic Deep Water is less dense than the
Antarctic Bottom Water. Therefore, it flows above the
Antarctic Bottom Water.
What Are Turbidity Currents?
A turbidity current is a strong current caused by
an underwater landslide. Unlike surface and deep
currents, turbidity currents last for only a short time.
In addition, turbidity currents do not flow in specific,
predictable directions, like surface and deep currents
do. Instead, turbidity currents generally happen quickly
and unpredictably. The flowchart below shows how a
turbidity current forms.
READING CHECK
10. Identify Where is the
densest ocean water in the
world?
READING CHECK
11. Explain Why does the
North Atlantic Deep Water
flow above the Antarctic
Bottom Water?
Large amounts of sediment build up on the continental shelf
or continental slope.
The sediment suddenly breaks off and slides downhill.
The sediment mixes with the ocean water and makes it turbid, or cloudy.
It is denser than the water around it.
The turbid water flows quickly beneath the surrounding water
in a turbidity current.
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Movements of the Ocean
Name
Class
Date
Section 1 Review
SECTION VOCABULARY
Coriolis effect the curving of the path of a
moving object from an otherwise straight
path due to Earth’s rotation
current a horizontal movement of water in a
well-defined pattern, such as a river or stream
deep current a streamlike movement of ocean
water far below the surface
Gulf Stream the swift, deep, and warm Atlantic
current that flows along the eastern coast of
the United States toward the northeast
gyre a huge circle of moving ocean water found
above and below the equator
surface current a horizontal movement of ocean
water that is caused by wind and that occurs at
or near the ocean’s surface
1. Compare Describe the difference between how surface currents form and how
deep currents form.
2. Describe What happens when a surface current meets a continent?
3. Explain Describe two reasons why water near the poles is denser than water in
the rest of the ocean.
4. Compare What are two differences between turbidity currents and surface or
deep currents?
5. Identify What causes gyres to form?
6. Explain Why is the Antarctic Circumpolar Current able to flow all the way around
Antarctica without bending north or south?
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Holt McDougal Earth Science
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Movements of the Ocean