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JMA Index Showing El Niño
and La Niña Years
El Niño is a climate
phenomenon that results from an intimate connection between
ocean and atmosphere. During an El Niño event, slackening
trade winds result in unusually warm sea surface temperatures
in the eastern and central Pacific. These warm temperatures
trigger droughts in Australia, a shift in storm patterns in
North America, and a host of other climatic anomalies around
the world. El Niño is also know as the warm phase of
the El Niño/Southern Oscillation, or ENSO. La Niña
is the cold phase of ENSO.
When El Niño is on the way, air pressures rise over
northern Australia and fall over the central Pacific—changes
indicating the weakening of the westward trade winds. Westward
flowing currents that carry warm surface waters in the Pacific
slow down. This leads to a rise in surface sea temperatures
in the Pacific all along the equator from the west coast of
South America to the international date line.
The ENSO Index gives the changing sea surface temperatures
from year to year. As you can see, the shift between neutral
temperatures, unusually warm temperatures (El Niño
years, shown in red), and unusually cold temperatures (La
Niña years, shown in blue) is periodic but not predictably
so. One goal of ENSO data collection is to find an underlying
pattern that can predict El Niño events—and the
weather changes they can bring—far in advance.
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JMA
Index Showing El Niño and La Niña
Years - Shown
here are sea surface temperature (SST) measurements
taken over the last few decades in a zone
of the equatorial Pacific Ocean. The vertical
scale on the right shows the temperature
anomaly —
the difference
between the measured temperature and the
expected temperature. During El Niño
events, the water temperatures in this area
are warmer than usual, shown here in red
as “warm phase.” Source: Center
for Ocean-Atmospheric Prediction Studies,
Florida State University(COAPS)
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The many factors that affect sea
surface temperature make interpretation of ENSO
Index data difficult. Beyond daily weather, variable
currents and irregular oceanic changes create
“noise” that make it hard to identify
and explain larger trends. Over geological time
scales, even changes in the earth’s orbit
may affect ENSO patterns. The seemingly endless
influences behind sea surface temperature have
made it difficult to develop fully realistic computer
models of the ENSO phenomenon.
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glossary terms
