NATIONAL WEATHER SERVICE AUGUST 2006

What are El Niño, La Niña, and ENSO?

Near the end of each calendar year, ocean surface temperatures warm along the coasts of Ecuador and 

northern Peru. In the past, local residents referred to this annual warming as “El Niño,” meaning “The Child,” 

due to its appearance around the Christmas season. The appearance of El Niño signified the end of the 

fishing season and the arrival of the time for Peruvian fishermen to repair their nets and maintain their boats. 

Every two to seven years a much stronger warming appears along the west coast of South America, which 

lasts for several months and is often accompanied by heavy rainfall in the arid coastal regions of Ecuador 

and northern Peru. Over time the term El Niño began to be used in reference to these major warm episodes. 

During the 1960s, scientists began to link the abnormally warm waters along the west coast of South America 

with abnormally warm waters throughout the central and east-central equatorial Pacific. In addition, the warmer 

than average waters were shown to be closely related to 

a global atmospheric pressure oscillation known as the 

Southern Oscillation. 

The term El Niño now refers to the coupled oceanatmosphere phenomenon characterized by:

Abnormally warm sea surface temperatures from the 

date line (180W) east to the South American coast

Changes in the distribution of tropical rainfall from 

the eastern Indian Ocean east to the tropical Atlantic

Changes in sea level pressure throughout the global 

Tropics (low-index phase of the Southern Oscillation)

Large-scale atmospheric circulation changes in 

the Tropics and portions of the extratropics in both 

hemispheres. 

Other terms commonly used for the El Niño phenomenon 

include “Pacific warm episode” and “El Niño/ Southern 

Oscillation (ENSO) episode.”

In contrast to El Niño, La Niña is characterized by 

anomalously cool water in the central and eastcentral equatorial Pacific. It also results in changes in 

the intensity and distribution of rainfall in the Tropics 

and in changes in the patterns of sea level pressure 

(corresponding to the “high-index” phase of the Southern 

Oscillation) and atmospheric circulation that affect many 

areas worldwide. Another term commonly used for the La 

Niña phenomenon is “Pacific cold episode.”

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Maps showing latitudes, longitudes, and ocean 

temperatures (in degrees Celsius)

(NOAA/NCEP/CPC)
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The El Niño/ La Niña phenomena are the main sources 

of year-to-year variability in weather and climate for 

many areas of the world. El Niño and La Niña tend to 

alternate in an irregular cycle, which is often referred to 

as the ENSO cycle. The transition between El Niño and 

La Niña tends to be more rapid than the transition from 

La Niña to El Niño. El Niño episodes tend to:

Develop during the Northern Hemisphere spring 

season

Occur every 3-5 years

Usually last for 9-12 months. 

In contrast, La Niña may last 1-3 years; however, there 

is considerable event-to-event variability in the timing, 

intensity and evolution of both El Niño and La Niña. 

Periods when neither El Niño nor La Niña is present are 

referred to as ENSO-neutral.

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Why do we have ENSO?

ENSO appears to be a necessary mechanism 

for maintaining long-term global climate stability 

by transporting heat from the Tropics to the 

higher latitudes.

What are the Effects of 

El Niño and La Niña on the 

United States?

Effects of El Niño 

El Niño conditions influence wintertime 

atmospheric flow across the eastern 

North Pacific and North America. There is 

considerable event-to-event variability in the 

character of El Niño episodes and in some 

areas, impacts can vary substantially from 

Typical Winter Patterns during El Niño (NOAA/NCEP/CPC)
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one event to another. However, there are some 

sections of the United States where impacts are 

fairly consistent and predictable, especially when 

associated with strong El Niño episodes.

In general, El Niño results in increased precipitation 

across California and the southern tier of states, 

and decreased precipitation in the Pacific Northwest 

and in the Ohio and Tennessee Valleys. A milder 

than normal winter across the northern states and 

western Canada is also a common effect. In the 

eastern United States, El Niño episodes favor more 

coastal storms at the expense of Alberta Clippers 

(fast eastward-tracking storms across the northern 

states) in winter and early spring. During the warm 

season, El Niño influences hurricane development, 

resulting in more eastern Pacific hurricanes and 

fewer Atlantic hurricanes. 

Effects of La Niña

La Niña conditions tend to influence wintertime 

atmospheric flow across the eastern North Pacific 

and North America, as shown in the figure below. La 

Niña episodes display considerable event-to-event 

variability and the overall effects tend to be less 

predictable than those for El Niño. 

Seasonal precipitation impacts are generally 

opposite to those of El Niño. During La Niña winters, 

large portions of central North America experience 

increased storminess, and an increased frequency 

of significant cold-air outbreaks, while the southern 

states experience less storminess and precipitation. 

There also tends to be considerable month-to-month 

variations in temperature, rainfall, and storminess 

across central North America during the winter and 

spring seasons, in response to the more variable 

atmospheric circulation throughout the period. In the 

 Typical Winter Patterns during La Niña (NOAA/NCEP/CPC)
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eastern U.S., during the winter, there are generally 

fewer coastal storms and more Alberta Clippers (fast 

eastward-tracking storms across the northern states) 

than normal. In the summer and autumn, La Niña 

can influence hurricane development, often resulting 

in fewer eastern Pacific hurricanes and more Atlantic 

hurricanes.

How do Scientists Monitor 

and Predict ENSO

Collection of Data

NOAA operates a network of 70 stationary buoys in 

the equatorial Pacific, called the Tropical Atmosphere/

Ocean (TAO) array, which provides data about upperocean and sea surface conditions. Other important data 

come from satellites, radiosondes, and the high-density 

U.S. surface data network.

Map showing latitude, longitude, sea surface temperature in degrees 

Celsius, and the area known as Niño 3.4.

Where Can I Find More 

Information?

The NOAA Climate Prediction Center 

web site has myriad resources, including 

educational materials, information on 

current conditions and outlooks, and 

links to other informative sites: 

http://www.cpc.ncep.noaa.gov/products/

precip/CWlink/MJO/enso.shtml

NOAA has created one primary web site 

that allows access to many other 

resources: 

http://www.elnino.noaa.gov

Use of Indicators and Indices

One of the primary indicators of ENSO events, 

Sea Surface Temperature (SST), is closely 

monitored throughout the Pacific Ocean, 

including Niño 3.4, a region stretching along 

the equator from 170oW to 120oW longitude 

and from 5oN to 5oS latitude. An index for 

monitoring and assessing the oceanic state 

of ENSO, known as the Oceanic Niño Index 

(ONI), is based on this region and represents 

the sea surface temperature departure from a 

long-term average.

Use of Computer Models

Numerical models have been developed to 

study and predict ENSO events and their 

effects on weather patterns throughout the 

world. Though by no means perfect, under 

certain conditions, these models can indicate 

which climatic conditions are likely to prevail 

during the next season or two.