Ocean Currents

155
Ocean currents affect the weather.

the east coast of the United States and then across the Atlantic
Ocean toward Europe. Because the current begins in a warm
part of the globe, close to the equator, it is still warm when it
reaches the east coast of the United States and then Europe.
The warm water also warms the climate in these areas. In fact,
if it werent for the Gulf Stream, much of Europegiven its
high latitudeswould have a climate more like Canadas.
Because all ocean currents in the northern hemisphere rotate
in a clockwise direction, water reaching the
beaches of California, unlike water reaching east
coast beaches, has just come from the north,
near Alaska, where it gets very cold.
Why do beach
Weather is not just affected by circulating air masses. It is

also affected by the oceans. The ocean is vast and deep,
warmed almost exclusively by the sun. And water is
continuously moving and mixing, due to a combination of
forces, including wind, the earths rotation, the gravitational
pull of the moon, temperature, and salt concentration.
These forces create several large, circular patterns of flow in
the worlds oceans, as illustrated in FIGURE 15-10.
Much of waters effect on weather stems from

its great capacity to absorb and hold heata
10,000 times greater capacity than air. This
means that temperatures fluctuate much more
communities have
Every two to seven years, a dramatic climate
in air than in water. At the beach, the air
milder weather
change driven by ocean currents, called
temperature can go from mild to very hot and
than inland
El Nio, causes a sustained surface temperature
back to mild over the course of a day, while the
communities (not change in the central Pacific Ocean (FIGURE
heat of the sun will have a tiny, almost
negligible effect on the water temperature.
as hot in summer, 15-11). It is blamed for flooding, droughts,
famine, and a variety of other extreme climate
Thus, during summers, much of the heat of
not as cold in
disruptions. We can more easily understand
the sun is absorbed and held by the ocean
winter)?
El Nio if we contrast it with the more
water in coastal towns, rather than causing hot
common climate pattern.
air temperatures. Conversely, during cold winter months,
heat from the water can reduce the coldness of the air.
In a typical year:
One of the strongest ocean currents is the Gulf Stream. It
travels north through the Caribbean, carrying warm water up
OCEAN CIRCULATION PATTERNS
There are several large, circular patterns of flowing water in the oceans due to a
combination of forces including wind, the earths rotation, the gravitational pull of
the moon, temperature, and salt concentration.
Beaches on the east coast of the United
States have warmer water than west
coast beaches at the same latitudes.
Warm ocean currents

Cold ocean currents
North
Atlantic
Ocean
North
Pacific
Ocean
South
Pacific
Ocean
South
Atlantic
Ocean
Pacific
Ocean
Indian
Ocean
1. Steady winds blow across the Pacific Ocean from South

America toward Southeast Asia and Australia.
2. These winds push the warm surface water away from
the coast of South America and raise the sea level in
Southeast Asia and Australia by about half a meter. The
warmth of the water heats the air above it, causing the
air to rise and produce great tropical rainstorms. (To
experience this effect, fill your bathtub with hot water
and feel the room get warmer and moister.)
3. Off the coast of South America, an upwelling of colder
water from deep in the ocean replaces the water that is
pushed west. The cold upwelling cools the air above it,
causing extremely dry weather.
4. The upwelling of deep water brings up rich nutrients
from below, enabling plankton to flourish and
nourishing huge populations of fish.
In an El Nio year:
1. The usual South AmericatoSoutheast Asia winds ease
just a bit. No one knows why this happens.
FIGURE 15-10 Ocean currents influence weather.
592
CHAPTER 15
Phelan Ch 15 pp 582-621.indd 592
2. Without wind pushing it, the warm water piling up in

the west flows back toward South America. This water
warms the air, which rises and produces huge amounts
ECOSYSTEMS AND COMMUNITIES
9/19/11 4:53 PM
EL NIO
El Nio occurs every two to seven years and is blamed for flooding, droughts, famine, and a variety of other extreme climate disruptions.
USUAL CONDITIONS
EL NIO CONDITIONS
Steady winds blow westward across the Pacific Ocean from South
America toward Southeast Asia.
The usual South AmericatoSoutheast Asia winds ease just a bit.
These winds push warm surface water away from the coast of
South America, heating the air above it, which causes air to rise
and produce tropical rainstorms.
Without the push of the wind, warm water flows back toward South
America. The cooled water around Australia and the Philippines
cools the air and causes dry weather that can lead to droughts.
South
America
Pacific
Ocean
Australia
Australia
2
Wind
Wind
Sea level
Sea level
South
America
Pacific
Ocean
Sea level
Sea level
Off the coast of South America, colder water upwells from the
ocean depths. The cold water cools the air above, causing
extremely dry weather.
Without the warm surface water being blown west, colder water
from the ocean depths cannot upwell. The warm surface water
warms the air, resulting in rainfall.
Water from the depths of the ocean brings up rich nutrients,

enabling plankton to flourish and feed the huge populations of fish.
With no upwelling of cold water and nutrients, plankton levels drop

dramatically and fish stocks are wiped out.
Warm water does

not move away
Warm water
moves away
4
Cold water 3
upwells
Cold water 3
does not upwell
South America
Changes during an El Nio event start a chain reaction

of unusual weather around the globe: increased rainfall
in the U.S. Midwest, storms in coastal California, and
warmer weather in South Africa and Japan.
FIGURE 15-11 Domino effect. A reduction in the usual east-towest ocean breeze can cause a cascade of disastrous weather.
of rainfall on the coast of South America where it is

normally dry, often causing flooding.
3. Without the warm surface water being blown west,
the normal upwelling of cool, deep water with high
nutrient content cannot occur. The plankton levels
drop to a fraction of their normal levels, and the
typically rich fish stocks are wiped out.
4. Meanwhile, in the absence of warm water from the east,
Indonesia, the Philippines, and Australia experience
drastic climate changes. The cooler water cools the air
above and causes unusually dry weather, which can lead
to droughts and, in extreme years, famines.
mechanisms arent clear, in El Nio years, more rain falls in

the midwestern United States and storms form off the coast
of California, while South Africa, Japan, and Canada enjoy
warmer weather than usual. The counterpart of the El Nio
phenomenon is called La Nia and is characterized by the
reverse effects of El Nio. During La Nia periods, ocean
surface temperatures are lower than usual and the climate
effects are approximately opposite to El Nio effects.
TAKE-HOME MESSAGE 15 5
Oceans have global circulation patterns. Disruptions
in these patterns occur every few years and can cause
extreme climate disruptions around the world.
These predictable changes during an El Nio event can start

a global pattern of unusual weather. Although the
Phelan Ch 15 pp 582-621.indd 593
WHAT ARE
ECOSYSTEMS?
WEATHER
South America
ENERGY AND
CHEMICAL FLOW
SPECIES
INTERACTIONS
COMMUNITIES
5 93
9/19/11 4:53 PM

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155

Ocean currents affect the weather.

Weather is not just affected by circulating air masses. It is

Much of waters effect on weather stems from

Warm ocean currents

1. Steady winds blow across the Pacific Ocean from South

FIGURE 15-10 Ocean currents influence weather.

Phelan Ch 15 pp 582-621.indd 592

2. Without wind pushing it, the warm water piling up in

ECOSYSTEMS AND COMMUNITIES

The usual South AmericatoSoutheast Asia winds ease just a bit.

Water from the depths of the ocean brings up rich nutrients,

With no upwelling of cold water and nutrients, plankton levels drop

Warm water does

Changes during an El Nio event start a chain reaction

of rainfall on the coast of South America where it is

mechanisms arent clear, in El Nio years, more rain falls in

These predictable changes during an El Nio event can start

Phelan Ch 15 pp 582-621.indd 593

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