Page 324 - Earth's Climate Past and Future

300 PART V • Historical and Future Climate Change

BOX 16–2 CLIMATE INTERACTIONS AND FEEDBACKS
El Niño and ENSO

he El Niño circulation pattern interrupts the normal this reason, Peruvian fishermen named this phenomenon
Tcirculation of the Pacific Ocean at irregular intervals “El Niño,” or “the boy child.”
ranging from 2 to 7 years. During the normal years that El Niño events are part of a larger-scale circulation
occur between El Niños, surface temperatures along the spanning the entire tropical Pacific. In the 1920s the
coasts of Peru and Ecuador and in the eastern equatorial atmospheric scientist Gilbert Walker found matching
Pacific are near 18°C (50°F) in winter—far cooler than changes in atmospheric pressure between the western
typical tropical temperatures (25°C, 77°F) and in fact the Pacific (northern Australia and Indonesia) and the south-
coolest tropical surface water on Earth. central Pacific island of Tahiti. High pressures over Australia
The lower non–El Niño temperatures result from correlate with low pressures in the south-central Pacific
upwelling driven by strong winds in southern hemisphere and vice versa. Low atmospheric pressures are associated
winter (August). The winds from the south drive warm with rising air motion and rainfall, while high surface
surface waters westward away from the coast of South pressure is associated with sinking motion and dry con-
America, and cooler water wells up from below. The winds ditions (see the companion Web site, pp. 15–18). The
then turn to the west near the equator and drive warm opposing pressure trends through time across the tropi-
surface water toward the southwest, causing cool water to cal Pacific are part of an enormous circulation cell called
well up near the equator. Upwelling water brings nutrients the Southern Oscillation. Sinking and rising motions
to the surface, supplying food to an ecosystem ranging occur at opposite times over northern Australia and
from plant plankton to fish (anchovies and tuna), sea Indonesia in the west and across the south-central Pacific
birds, and marine mammals (seals and sea lions). Non–El in the east.
Niño years are also dry along the coast of South America El Niño and the Southern Oscillation are linked. El
because cool, upwelling waters are a poor source of water Niño years, with warm ocean temperatures and heavy
vapor for the atmosphere. As a result, the coastal deserts rains in Peru, are times of high pressure and drought over
of Peru and Chile are normally among the driest regions northern Australia and of low pressures and high rainfall in
on Earth. the south-central Pacific. Non–El Niño years, with cool
El Niño years change all this. During El Niño winters, ocean temperatures near South America, are times of low
strong southerly winds fail to blow in the eastern and pressure and increased rainfall in northern Australia and of
tropical Pacific, upwelling does not occur, and the surface higher pressures and reduced rainfall in the south-central
waters along the South American coast warm by 2°–5°C. Pacific. This linked circulation is known as ENSO (El
Without upwelling, the plankton populations crash, and Niño–Southern Oscillation).
most fish die or move away. Without fish, sea birds on The physical link between these two systems occurs in
tropical islands cannot feed their young, and they aban- the lower atmosphere and the upper ocean. Strong trade
don their nests to fly elsewhere in search of food. In severe winds that cause upwelling in the eastern Pacific during
El Niño years, a significant fraction of the year’s popula- non–El Niño years also drive warm surface water westward
tion of young sea birds and mammals dies. Ocean warm- across the tropical Pacific. Warm water piles up in the
ing near the coastal South American deserts produces a western Pacific at a height several tens of centimeters
large source of moisture, and rain falls in cloudbursts that above the level of the eastern Pacific and forms a natural
produce flash floods in regions with little or no natural source of moisture for evaporation and precipitation in
vegetation cover to absorb the water. The warm rains also northern Australia and Indonesia. Some of the rising air
favor the breeding and spread among humans of tropical flows eastward at high elevations and sinks in the east-
diseases such as malaria and cholera. central Pacific, contributing to the normally cooler and
Even though the El Niño circulation pattern reaches its drier conditions near South America.
height during southern hemisphere winter in August, the During El Niño years, without strong trade winds push-
first hint of unusual warming of the surface ocean is often ing water westward, some of the pool of warm water in the
detected during the previous summer, near Christmas. For western Pacific flows back eastward and becomes a source