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In normal years, easterlies blow clear across the equatorial
Pacific transporting sea surface water westward. Off
the west coast of South America, an upwelling of deep
ocean waters occurs to replace surface waters being
transported westward away from the continent. The
cold upwelling currents keep sea surface temperatures
cool in the eastern Pacific. The air above stays too
cool and dense to rise high enough to form rain clouds,
so cool, dry conditions typical of high pressure systems
prevail. The monsoon rains are confined to the western
Pacific where sea surface and air temperatures are
warm, and a low pressure system is maintained.
When winds shift during an El Niño event, water is
no longer being transported away from the west coast
of South America. Upwelling slows or stops, allowing
sea surface and air temperatures to rise in the eastern
Pacific. Atmospheric pressure becomes anomalously
low in this region. The air becomes buoyant enough
to rise and form large rain clouds, and the monsoon
rain zone migrates east, usually ending up in the
central Pacific. This migration directly affects rainfall
in the region, bringing drought to the western Pacific,
and flood to the east; but the significant shift in
tall, massive rain clouds can also disrupt upper atmospheric
circulation. Dense, tropical rain clouds distort the
air flow aloft, producing waves thousands of miles
long in the horizontal direction. These waves determine
the positions of monsoons and jet streams. The shifting
position of the tropical rain clouds during El Niño
affects these waves, distorting jet streams and rain
belts and bringing drought, flood and unseasonable
weather to far reaches of the planet.
The effects El Niño has on global climate are quite
widespread and concern every citizen of Earth. The
Microwave
Limb Sounder (MLS)
instrument on the UARS
satellite
has, in the past, been making El Niño related
measurements. The MLS
instrument that will be present on EOS Aura 1 is an
updated version of the UARS
instrument.
Due to the age of UARS
the spacecraft is presently unable to make complete
El Niño measurements in a year that appears will
have a strong El Niño event. Continuous measurement
of global El Niño effects will be a vital role
the Aura platform will play in future years. In addition,
due to the differences in the Aura orbit, Aura will
be able to provide pole-to-pole coverage every day,
whereas UARS
data has monthly gaps which caused the loss of data
during critical periods.
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