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What is an El
Niño?
El Niño is characterized by unusually warm ocean
temperatures in the Equatorial Pacific, as opposed
to La Niña, which characterized by unusually
cold ocean temperatures in the Equatorial
Pacific . El Niño is an oscillation of the ocean-
atmosphere system in the tropical Pacific having
important consequences for weather around the
globe .
Among these consequences are increased rainfall
across the southern tier of the US and in Peru,
which has caused destructive flooding, and drought
in the West Pacific, sometimes associated with
devastating brush fires in Australia. Observations
of conditions in the tropical Pacific are considered
essential for the prediction of short term (a few
months to 1 year) climate variations.
To provide necessary data, NOAA operates a
network of buoys which measure temperature,
currents and winds in the equatorial band. These
buoys daily transmit data which are available to
researchers and forecasters around the world in
real time.
NOTE: Until 1997, the 1982-1983 El Niño, was the
largest El Niño of the twentieth century. These two
important El Niño events are used for illustrations
in this web page.
In normal, non-El Niño conditions (top panel of
schematic diagram), the trade winds blow towards
the west across the tropical Pac:ific. These winds
pile up warm surface water in the west Pacific, so
that the sea surface is about 1/2 meter higher at
Indonesia than at Ecuador.
The sea surface temperature is about 8 degrees C
higher in the west, with cool temperatures off
South America, due to an upwelling of cold water
from deeper levels. This cold water is nutrient-
rich, supporting high levels of primary productivity,
diverse marine ecosystems, and major fisheries.
Rainfall is found in rising air over the warmest
water, and the east Pacific is relatively dry. The
observations at 110 W (left diagram of 110 W
conditions) show that the cool water (below about
17 degrees C, the black band in these plots) is
within 50m of the surface.
During El Niño (bottom panel of the schematic
diagram), the trade winds relax in the central and
western Pacific leading to a depression of the
thermocline in the eastern Pacific, and an
elevation of the thermocline in the west. The
observations at 110W show, for example, that
during 1982-1983, the 17-degree isotherm dropped
to about 150m depth. This reduced the efficiency
of upwelling to cool the suface and cut off the
supply of nutrient rich thermocline water to the
euphotic zone. The result was a rise in sea surface
temperature and a drastic decline in primary
productivity, the latter of which adversely
affected higher trophic levels of the food chain,
including commercial fisheries in this region. The
weakening of easterly tradewinds during El Niño is
evident in this figure as well. Rainfall follows the
warm water eastward, with associated flooding in
Peru and drought in Indonesia and Australia. The
eastward displacement of the atmospheric heat
source overlaying the warmest water results in
large changes in the global atmospheric circulation,
which in turn force changes in weather in regions
far removed from the tropical Pacific.
Recognizing El Niño
El Niño can be seen in Sea Surface Temperature in
the Equatorial Pacific Ocean
El Niño can be seen in measurements of the
sea surface temperature, such as those shown
above, which were made from the TAO Array
of moored buoys. In December 1993, the sea
surface temperatures and the winds were
near normal, with warm water in the Western
Pacfic Ocean (in red on the top panel of
December 1993 plot), and cool water, called
the "cold tongue" in the Eastern Pacific Ocean
(in green on the top panel of the December
1993 plot). The winds in the Western Pacific
are very weak (see the arrows pointing in the
direction the wind is blowing towards), and
the winds in the Eastern Pacific are blowing
towards the west (towards Indonesia). The
bottom panel of the December 1993 plot shows
anomalies, the way the sea surface
temperature and wind differs from a normal
December. In this plot, the anomalies are very
small (yellow/green), indicating a normal
December. December 1997 was near the peak
of a strong El Niño year. In December 1997,
the warm water (red in the top panel of the
December 1997 plot) has spread from the
western Pacific Ocean towards the east (in
the direction of South America), the "cold
tongue" (green color in the top panel of the
December 1997 plot) has weakened, and the
winds in the western Pacific, usually weak,
are blowing strongly towards the east,
pushing the warm water eastward. The
anomalies show clearly that the water in the
center of Pacific Ocean is much warmer (red)
than in a normal December.
December 1998 was a strong La Niña (cold)
event. The cold tongue (blue) is cooler than
usual by about 3° Centigrade. The cold La
Niña events sometimes (but not always) follow
El Niño events.
Animation of El Niño
Animation of physical processes allow scientists to
better understand El Niño
If you have an MPEG animation viewer, and
sufficient memory, you can view an animation of El
Niño which shows the changes in monthly sea
surface temperature in the tropical Pacific Ocean.
The animation is about 1 Megabyte in size. As you
view this animation, you will see the warm water
spreading from the western Pacific to the eastern
Pacific during 1997. The bottom panel in the
animation, labeled anomalies, shows how much the
sea surface temperature for each month is
different from the long term average for that
month. The red color in the anomalies plot indicates
that the temperature of the water is much
warmer than is normal for that month. Blue color
indicates that the water is much cooler than is
normal for that month
Historical El Niños
Recent and historical El Niños can be seen in
Pacific Sea Surface Temperature representations
In the left hand panel, you see the sea surface
temperature at the Equator in the Pacific Ocean
(Indonesia is towards the left, South America is
towards the right). Time is increasing downwards
from 1986 at the top of the plot, to the present,
at the bottom of the plot. The first thing to note
is the blue "scallops" on the right of the plot, in
the eastern Pacific. These indicate the cool water
typically observed in the Eastern Pacific (called
the "cold tongue"). Cold tongue temperatures vary
seasonally, being warmest in the northern
hemisphere springtime and coolest in the northern
hemisphere fall. The red color on the left is the
warm pool of water typically observed in the
western Pacific Ocean. El Niño is an exaggeration
of the usual seasonal cycle. During the El Niño in
1986-1987, you can see the warm water (red)
penetrating eastward in the Spring of 1987. There
is another El Niño in 1991-1992, and you can see
the warm water penetrating towards the east in
the northern hemisphere spring of 1992. The El
Niño in 1997-1998 is a very strong El Niño. El Niño
years are easier to see in the anomalies on the
right hand panel. The anomalies show how much the
sea surface temperature is different from the
usual value for each month. Water temperatures
significantly warmer than the norm are shown in
red, and water temperatures cooler than the norm
are shown in blue.
In the right-hand plot of sea surface temperature
anomalies, it is very easy to see El Niños, with
water warmer than usual (red) in the eastern
Pacific, during in 1986-1987, 1991-1992, 1993,
1994 and 1997-1998. Notice the very cool water
(blue), in the Eastern Pacific, in 1988-1989. This is
a strong La Niña , which occurs after some (but not
all) El Niño years. 1995-1996 was a weaker La Niña
year. It is unusual for El Niños to occur in such
rapid succession, as has been the case during
1990-1994.