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ENCYCLOPEDIC ENTRY
El Niño
El Niño-Southern Oscillation (ENSO)
El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern equatorial Pacific Ocean. Trade winds and
atmosphere are also impacted by El Niño.
Image courtesy NASA
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Vocabulary
El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical
Paci c Ocean. El Nino is the “warm phase” of a larger phenomenon called the El Nino-Southern Oscillation
(ENSO). La Nina, the “cool phase” of ENSO, is a pattern that describes the unusual cooling of the region’s
surface waters. El Niño and La Niña are considered the ocean part of ENSO, while the Southern Oscillation is
its atmospheric changes.
El Niño has an impact on ocean temperatures, the speed and strength of ocean currents, the health of coastal
sheries, and local weather from Australia to South America and beyond. El Niño events occur irregularly at
two- to seven-year intervals. However, El Niño is not a regular cycle, or predictable in the sense that ocean
tides are.
El Niño was recognized by shers off the coast of Peru as the appearance of unusually warm water. We have
no real record of what indigenous Peruvians called the phenomenon, but Spanish immigrants called it El Niño,
meaning “the little boy” in Spanish. When capitalized, El Niño means the Christ Child, and was used because
the phenomenon often arrived around Christmas. El Niño soon came to describe irregular and intense climate
changes rather than just the warming of coastal surface waters. Led by the work of Sir Gilbert Walker in the 1930s, climatologists determined that El Niño occurs
simultaneously with the Southern Oscillation. The Southern Oscillation is a change in air pressure over the
tropical Paci c Ocean. When coastal waters become warmer in the eastern tropical Paci c (El Niño), the
atmospheric pressure above the ocean decreases. Climatologists de ne these linked phenomena as El NiñoSouthern Oscillation (ENSO). Today, most scientists use the terms El Niño and ENSO interchangeably.
Scientists use the Oceanic Nino Index (ONI) to measure deviations from normal sea surface temperatures. El
Niño events are indicated by sea surface temperature increases of more than 0.9° Fahrenheit for at least ve
successive three-month seasons. The intensity of El Niño events varies from weak temperature increases
(about 4–5° F) with only moderate local effects on weather and climate to very strong increases (14–18° F)
associated with worldwide climatic changes. Upwelling
In order to understand the development of El Niño, it’s important to be familiar with non-El Niño conditions in
the Paci c Ocean. Normally, strong trade winds blow westward across the tropical Paci c, the region of the
Paci c Ocean located between the Tropic of Cancer and the Tropic of Capricorn. These winds push warm
surface water towards the western Paci c, where it borders Asia and Australia. Due to the warm trade winds, the sea surface is normally about .5 meter (1.5 feet) higher and 45° F warmer in
Indonesia than Ecuador. The westward movement of warmer waters causes cooler waters to rise up towards
the surface on the coasts of Ecuador, Peru, and Chile. This process is known as upwelling. Upwelling elevates cold, nutrient-rich water to the euphotic zone, the upper layer of the ocean. Nutrients in the
cold water include nitrates and phosphates. Tiny organisms called phytoplankton use them for
photosynthesis, the process that creates chemical energy from sunlight. Other organisms, such as clams, eat
the plankton, while predators like sh or marine mammals prey on clams.
Upwelling provides food for a wide variety of marine life, including most major sheries. Fishing is one of the
primary industries of Peru, Ecuador, and Chile. Some of the sheries include anchovy, sardine, mackerel,
shrimp, tuna, and hake. The upwelling process also in uences global climate. The warm ocean temperature in the western Paci c
contributes to increased rainfall around the islands of Indonesia and New Guinea. The air in uenced by the
cool eastern Paci c, along the coast of South America, remains relatively dry.
El Niño Events
El Niño events are de ned by their wide-ranging teleconnections. Teleconnections are large-scale, long-lasting
climate anomalies or patterns that are related to each other and can affect much of the globe.
During an El Niño event, westward-blowing trade winds weaken along the Equator. These changes in air
pressure and wind speed cause warm surface water to move eastward along the Equator, from the western
Paci c to the coast of northern South America. These warm surface waters deepen the thermocline, the level of ocean depth that separates warm surface
water from the colder water below. During an El Niño event, the thermocline can dip as far as 152 meters (500
feet). This thick layer of warm water does not allow normal upwelling to occur. Without an upwelling of nutrient-rich
cold water, the euphotic zone of the eastern Paci c can no longer support its normally productive coastal
ecosystem. Fish populations die or migrate. El Niño has a devastating impact on Ecuadorian and Peruvian
economies.
El Niño also produces widespread and sometimes severe changes in the climate. Convection above warmer
surface waters bring increased precipitation. Rainfall increases drastically in Ecuador and northern Peru,
contributing to coastal ooding and erosion. Rains and oods may destroy homes, schools, hospitals, and
businesses. They also limit transportation and destroy crops. As El Niño brings rain to South America, it brings droughts to Indonesia and Australia. These droughts
threaten the region’s water supplies, as reservoirs dry and rivers carry less water. Agriculture, which depends
on water for irrigation, is threatened. Stronger El Niño events also disrupt global atmospheric circulation. Global atmospheric circulation is the
large-scale movement of air that helps distribute thermal energy (heat) across the surface of the Earth. The
eastward movement of oceanic and atmospheric heat sources cause unusually severe winter weather at the
higher latitudes of North and South America. Regions as far north as the U.S. states of California and
Washington may experience longer, colder winters because of El Niño.
El Niño events of 1982-83 and 1997-98 were the most intense of the 20th century. During the 1982-83 event,
sea surface temperatures in the eastern tropical Paci c were 9-18° F above normal. These strong
temperature increases caused severe climatic changes: Australia experienced harsh drought conditions;
typhoons occurred in Tahiti; and record rainfall and ooding hit central Chile. The west coast of North
America was unusually stormy during the winter season, and sh catches were dramatically reduced from
Chile to Alaska.
The El Niño event of 1997-98 was the rst El Niño event to be scienti cally monitored from beginning to end.
The 1997-98 event produced drought conditions in Indonesia, Malaysia, and the Philippines. Peru experienced
very heavy rains and severe ooding. In the United States, increased winter rainfall hit California, while the
Midwest experienced record-breaking warm temperatures during a period known as “the year without a
winter.”
El Niño-related disruption of global atmospheric circulation extends beyond Paci c Rim nations. Strong El
Niño events contribute to weaker monsoons in India Southeast Asia. ENSO has even contributed to increased
rainfall during the rainy season in sub-Saharan Africa.
Diseases thrive in communities devastated by natural hazards such as ood or drought. El Niño-related
ooding is associated with increases in cholera, dengue, and malaria in some parts of the world, while
drought can lead to wild res that produce respiratory problems.
'Flavors' of El Niño
Variations of El Niño are referred to as “ avors.” The transition period of an El Niño event, for instance, is
called a "Trans Niño." Trans Niño events occur at the onset and closing of an El Niño event. Trans Niño events
often include increased tornado activity in the American Midwest.
Another " avor" of El Niño is the El Niño Modoki, or Modoki Niño. Modoki is a Japanese word meaning
"similar, but different." Modoki Niño, also called the Central Paci c Niño, is characterized by changes in seasurface temperatures in the central, not eastern, Paci c. Some Modoki Niño events are distinct from
traditional El Niño events, such as increased hurricane activity in the Atlantic and Gulf of Mexico. Many
meteorologists are critical of the Modoki Niño, calling for more climate models to study the proposed
phenomenon.
Monitoring El Niño Scientists, governments, and non-governmental organizations (NGOs) collect data about El Niño using a
number of technologies. The National Oceanic and Atmospheric Administration (NOAA), for
instance, operates a network of scienti c buoys. These buoys measure ocean and air temperatures, currents,
winds, and humidity. The buoys are located at about 70 locations in the southern Paci c Ocean, from the
Galapagos Islands to Australia. These buoys transmit data daily to researchers and forecasters around the world. Using data from the buoys,
along with visual imagery they receive from satellite imagery, scientists are able to more accurately predict El
Niño and visualize its development and impact around the globe.