Overview of the ENSO System
ENSO Basics
What are El Niño and La Niña?
The term El Niño
was first coined more than 100 years ago to describe
the unusually warm waters that would occasionally form
along the coast of Ecuador and Peru. This phenomenon typically
occurred late in the calendar year near Christmas, hence
the name El Niño (spanish for "the boy child", referring
to the Christ child). Today the term El Niño is used
to refer to a much broader scale phenomenon associated with unusually
warm water that occasionally forms across much of the tropical
eastern and central Pacific. The time between successive
El Niño events is irregular but they typically tend to recur
every 3 to 7 years.
La Niña is the counterpart to El Niño and is
characterized by cooler than normal SSTs across much of the equatorial
eastern and central Pacific. A La Niña event
often, but not always, follows an El Niño and
vice versa. Once developed, both El Niño and La
Niña events tend to last for roughly a year although occasionally
they may persist for 18 months or more. El Niño and
La Niña are both a normal part of the earth's climate
and there is recorded evidence of their having occurred for
hundreds of years.
Figure 1 below shows examples of the typical
extent of the warming and cooling in the equatorial
Pacific during developed El Niño and La Niña
events.
Departure of sea surface temperature
from the long-term average for an El Niño during
December 1991. Yellow shading indicates warmer than
average temperatures. Units are degrees. Celsius and
contours are drawn at 0.5 degrees C intervals.
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Figure 1(a). El Niño
conditions
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Departure of sea surface temperature
from the long-term average for an La Niña during
December 1988. Blue shading indicates colder than average
temperatures. Units are degrees. Celsius and contours
are drawn at 0.5 degrees C intervals.
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Figure 1(b). Typical La Niña
conditions
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Although El Niño and
La Niña events are characterized by warmer
or cooler than average sea surface temperatures in the tropical
Pacific, they are also associated with changes in wind,
pressure, and rainfall patterns. In the tropics where El Niño
and La Niña form, rainfall tends to occur over areas having
the warmest sea surface temperature. The
Figure 2
below shows a schematic view
of the links between sea-surface temperatures and tropical
rainfall:
- Normal conditions (top-most figure below). The warmest
water is found in the western Pacific, as is the greatest
rainfall. Winds near the ocean surface travel from east
to west across the Pacific (these winds are called easterlies
).
- El Niño conditions (lower-left figure). The
easterlies weaken, warmer than average sea surface temperatures
cover the central and eastern tropical Pacific, and the region
of heaviest rainfall moves eastward as well.
- La Niña conditions (lower-right figure).
Could be thought of as an enhancement of normal conditions.
During these events, the easterlies strengthen, colder than
average ocean water extends westward to the central Pacific,
and the warmer than average sea-surface temperatures in
the western Pacific are accompanied by heavier than usual rainfall.
Figure 2. Schematic view of sea
surface temperature and tropical rainfall in the the
equatorial Pacific Ocean during normal, El Niño, and
La Niña conditions.
The sea-surface temperature
is shaded: blue-cold and orange-warm. The dark arrows
indicate the direction of air movement in the atmosphere:
upward arrows are associated with clouds and rainfall and downward-pointing
arrows are associated with a general lack of rainfall.
Figures courtesy of NOAA, Climate
Prediction Center
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What is ENSO?
While the tropical ocean affects the atmosphere above it, so too
does the atmosphere influence the ocean below it. In fact,
the interaction of the atmosphere and ocean is an
essential part of El Niño and La Niña
events (the term coupled system is often used
to describe the mutual interaction between the ocean and atmosphere).
During an El Niño, sea level pressure tends to be
lower in the eastern Pacific and higher in the western Pacific
while the opposite tends to occur during a La Niña.
This see-saw in atmospheric pressure between the eastern and western
tropical Pacific is called the Southern
Oscillation, often abbreviated as simply the SO .
A standard measure of the Southern Oscillation is the
difference in sea level pressure between Tahiti and Darwin, Australia
(see figure below). Since El Niño and the Southern Oscillation
are related, the two terms are often combined into a single phrase,
the El Niño-Southern Oscillation, or
ENSO for short. Often the term ENSO
Warm Phase is used to describe El Niño and ENSO
Cold Phase to describe La Niña.
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Why do we care about El Niño and La Niña?
Once developed, El Niño and La Niña events typically
persist for about a year and so the shifted rainfall patterns
associated with them typically persist for several seasons as
well. This can have a significant impact on people living in areas
of the tropical Pacific since the usual precipitation patterns can
be greatly disrupted by either excessively wet or dry conditions.
In addition, the shifting of tropical rainfall patterns during El
Niño and La Niña not only affects the tropical Pacific
region but areas away from the tropical Pacific as well. This includes
many tropical locations as well as some regions outside the tropics
in both the Northern and Southern Hemispheres. For information about why
this happens, see the ENSO and Climate
section.
Seasonal climate forecasts made possible
The persistence of tropical sea surface temperature (and rainfall)
patterns (such as those associated El Niño and La Niña)
plays a fundamental role in making seasonal (3-month) climate
forecasts possible. In the absence of El Niño and La
Niña, seasonal climate forecasts are still possible because
unusually warm or cold sea surface temperatures in other parts
of the tropics can still occur. For more details on seasonal climate
forecasts, see
Forecasting Climate
.
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