Page 70 - (DK) Ocean - The Definitive Visual Guide

68 CIRCULATION AND CLIMATE

El Niño and La Niña descending air

southeast
high pressure and
trade winds associated with
EL NIÑO AND LA NIÑA ARE LARGE climatic disturbances low-pressure system dry conditions
in western Pacific with
caused by abnormalities in the pattern of sea surface rising warm, moist
air and associated
temperature, ocean currents, and pressure systems. They heavy rainfall
are in the tropical Pacific Ocean. These disturbances have
important repercussions for weather throughout the Pacific
and beyond. Most scientists regard El Niño and La Niña as
extreme phases of a complex global weather phenomenon
called the El Niño–Southern Oscillation (ENSO).

El Niño Events

The Spanish term el niño means “the little boy” or “Christ child.” It originally pool of warm water
denoted a warm current that was occasionally noticed around Christmas off
Peru. Later it was restricted to unusually strong rises in temperature in the South
Equatorial
waters of the eastern Pacific, with a reduction in the upwelling of nutrient-rich Current upwelling of cold,
waters that normally occur there. It is now used to mean a much wider shift in nutrient-rich water
ocean and atmospheric conditions that affects the whole globe. El Niño events NORMAL PATTERN
typically last from 12 to 18 months and occur cyclically, although somewhat A low-pressure system in the western Pacific draws
unpredictably. On average, they occur about 30 times per century, with southeast trade southeast trade winds across from a high-pressure
intervals that are sometimes as short as three years and sometimes as long winds reverse or system over South America. These winds drive the
weaken
as 10 years. Their underlying cause is not understood. South Equatorial Current, which maintains a pool of
warm surface water in the western Pacific.
descending air and high
TEMPERATURE PATTERNS pressure brings warm,
These satellite-generated images dry weather low pressure and
rising warm, moist
of the Pacific compare surface air associated with
temperature patterns. Red and heavy rainfall
white indicate warm water; green
and blue denote cooler water.
warm water flows
eastward, accumulating
off South America

EL NIÑO PATTERN
During an El Niño event, the pressure
systems that normally develop in the
JANUARY 2011
(NORMAL) Pacific, and the southeast trade winds,
weaken or reverse. The pool of warm
surface water extends from the western
upwelling blocked by Pacific into the central and eastern Pacific.
warm water near surface
DECEMBER 2009
(EL NIÑO)
Effects of El Niño GIANT WAVES
During an El Niño event,
An El Niño event causes wetter-than-normal conditions, and floods, in storms become more
countries on the western side of South America, particularly Ecuador, frequent and violent in
Peru, and Bolivia. These conditions may also extend to the southeastern the central Pacific. These
storms can produce
United States. In other parts of the world, it causes drier conditions. gigantic waves, up to
Drought and forest fires become more common in the western Pacific, 33 ft (10 m) high, in
particularly in Indonesia and parts of Australia, but also in East Africa Hawaii, as here on the
and northern Brazil. The warmer waters in the eastern Pacific cause a island of Oahu.
reduction in the Peru Current and reduced upwelling near the coast of
South America. This reduces the level of nutrients
in the seawater, which has a negative impact on fish
INTRODUCTION sea ice around Antarctica. Japan, western Canada, CORAL BLEACHING
stocks. Other effects include a quieter Atlantic
hurricane season and an increase in the extent of
and the western US typically experience more
This small circular coral
storms and warmer weather than normal.
reef has suffered severe
bleaching (whitening).
width of rings
El Niño events are
EVIDENCE OF AN HISTORICAL EL NIÑO
directly related
often associated with
Increased tree growth can be linked to high
to amount of
bleaching caused by
rainfall that occurred during historic El Niño
growth
unusually high sea
events. One of the rings in this sample has
1746–47
surface temperatures.
been linked to an El Niño in 1746–47.
El Niño ring