Essentials of Ecology

SUPPLEMENT 8 S49

the warmer waters of the western Pacifi c to
move toward the coast of South America,
which suppresses the normal upwellings of
cold, nutrient-rich water (Figure 4, right). The
decrease in nutrients reduces primary productiv-
ity and causes a sharp decline in the populations
of some fi sh species.
A strong ENSO can alter the weather of
at least two-thirds of the globe (Figure 5)—
especially in lands along the Pacifi c and Indian
Oceans. Scientists do not know for sure the
causes of an ENSO, but they know how to de-
tect its formation and track its progress.
La Niña, the reverse of El Niño, cools some
coastal surface waters, and brings back upwell-
ings. Typically, La Niña means more Atlantic
Ocean hurricanes, colder winters in Canada and
the northeastern United States, and warmer and
drier winters in the southeastern and southwest-
ern United States. It also usually leads to wetter
winters in the Pacifi c Northwest, torrential rains
in Southeast Asia, lower wheat yields in Argen-
tina, and more wildfi res in Florida. Figure 6 uses
satellite data to show changes in the locations
of masses of warm and cold water in the Pacifi c
Ocean during an El Niño (top) and a La Niña
(bottom).

Drought

Unusually high rainfall

Unusually warm periods

El Niño

Figure 5 Typical global weather effects of an El Niño–
Southern Oscillation. During the 1996–1998 ENSO, huge
waves battered the coast in the U.S. state of California and
torrential rains caused widespread flooding and mudslides.
In Peru, floods and mudslides killed hundreds of people,
left about 250,000 people homeless, and ruined harvests.
Drought in Brazil, Indonesia, and Australia led to mas-
sive wildfires in tinder-dry forests. India and parts of Africa
also experienced severe drought. A catastrophic ice storm
hit Canada and the northeastern United States, but the
southeastern United States had fewer hurricanes. (Data
from United Nations Food and Agriculture Organization)

Data and Map Analysis

1. How might an ENSO affect the weather where you live or
   go to school?


2. Why do you think the area to the west of El Niño suffers
   drought?

November 10, 1997

February 27, 1999

Cool Warm

Warm water of El Niño

Cool water of La Niña

Figure 6 Locations

of flowing masses of

warm and cold water

in the Pacific Ocean

during an El Niño

(top) and a La Niña

(bottom). (Data from

Jet Propulsion Lab,

NASA)