NOAA National Weather Service OCTOBER 2007 

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. 

In contrast to El Niño, La Niña is characterized by 

anomalously cool water in the central and east-central 

equatorial Pacific. Both El Niño and La Niña result in 

changes in the intensity and distribution of rainfall in 

the Tropics and in changes in the patterns of sea level 

pressure and atmospheric circulation that affect many 

areas worldwide. 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. 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.

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.

Maps showing latitudes, longitudes, and ocean 

temperatures (in degrees Celsius)

(NOAA/NCEP/CPC)
NOAA National Weather Service Page 2

Where Can I Find More Inf

Effects of El Niño on the U.S.

El Niño impacts can vary substantially from 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 on the U.S.

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-tomonth variations in temperature, rainfall, and 

storminess across central North America during 

the winter and spring seasons. In the 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.

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 Persistent extended Pacific Jet Stream 

& amplified storm track

L

 Low Pressure

Typical Winter Patterns during El Niño

(NOAA/NCEP/CPC)