274 CHAPTER 11 The Ocean and Fisheries
The Global Ocean
LEARNING OBJECTIVES
continent, it is deflected northward, where the wester-
lies begin to influence it. As a result, the current flows
eastward in the mid-latitudes until it reaches the landmass
of Europe. Here some water is deflected toward the pole
and some toward the equator. The water flowing toward
the equator comes under the influence of trade winds
again, producing the circular gyre. Although surface–
ocean currents and winds tend to move in the same di-
rection, there are many variations to this general rule.
The Coriolis effect influences the paths of ocean cur-
rents just as it does the winds (see Figure 8.5). Earth’s ro-
tation from west to east causes surface ocean currents to
swerve to the right in the Northern Hemisphere, helping
establish the circular, clockwise pattern of water currents.
In the Southern Hemisphere, ocean currents swerve to
the left, thereby moving in a circular, counterclockwise
pattern.Vertical Mixing of Ocean Water Variations in
the density (mass per unit volume) of seawater—caused
by wind–driven temperature differences between water
layers—affect deep-ocean currents. Cold, salty water is
denser than warmer, less salty water. (The density of wa-
ter increases with decreasing temperature down to 4°C.)
Through the ocean meridional circulation, colder, salty
ocean water sinks and flows under warmer, less salty wa-
ter, generating currents far below the surface. Deep-ocean
currents often travel in different directions and at differ-
ent speeds than do surface currents, in part because the
Coriolis effect is more pronounced at greater depths.
Figure 11.1b shows the present global circulation of shal-
low and deep currents—the ocean conveyor belt—that
transfers heat and salt, moving cold, salty deep-sea water
from higher to lower latitudes, where it warms up. Note
that the Atlantic Ocean gets its cold deep water from
the Arctic Ocean, whereas the Pacific Ocean and Indian
Ocean get theirs from the water surrounding Antarctica.
The ocean conveyer belt affects regional and pos-
sibly global climate. As the Gulf Stream and North
Atlantic Drift push into the North Atlantic, they deliver
an immense amount of heat from the tropics to Europe
( Figure 11.1c). As this shallow current transfers its
heat to the atmosphere, the water becomes denser and- Describe the global ocean and its significance
to life on Earth. - Discuss the roles of winds and the Coriolis
effect in producing global water flow patterns,
including gyres. - Define El Niño–Southern Oscillation (ENSO)
and La Niña and describe some of their effects.
T
he ocean is a vast wilderness, much of it
unknown. It teems with life—from warm-
blooded mammals such as whales to soft-
bodied invertebrates such as jellyfish. The
ocean is essential to Earth’s hydrologic cycle, which pro-
vides us with water. It affects cycles of matter on land, in-
fluences our climate and weather, and provides foods
that enable millions of people to survive. The ocean
dominates Earth, and its condition determines the future
of life on our planet. If the ocean dies, then we do as well.
Yet we lack a full understanding of many oceanic
process—there remains much for us to discover.
The global ocean is a huge body of salt water that sur-
rounds the continents and covers almost three-fourths of
Earth’s surface. It is a single, continuous body of water, but
geographers divide it into four sections separated by the
continents: the Pacific, Atlantic, Indian, and Arctic oceans.
The Pacific is the largest: It covers one-third of Earth’s sur-
face and contains more than half of Earth’s water.
Patterns of Circulation in the Ocean
The persistent prevailing winds blowing over the ocean
produce currents, mass movements of surface–ocean
water (Figure 11.1a). The prevailing winds generate
gyres, circular ocean currents. In the North Atlantic
Ocean, the tropical trade winds
tend to blow toward the west,
whereas the westerlies in the mid-
latitudes blow toward the east.
This helps establish a clockwise
gyre in the North Atlantic. That
is, the trade winds produce the westward North Atlan-
tic Equatorial Current in the tropical North Atlantic
Ocean. When this current reaches the North American
gyres Large, circular
ocean current
systems that often
encompass an entire
ocean basin.