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What are El Niño and La Niña ?

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

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.”

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
  • 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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