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El Niño in retreat, Pacific in transition
June 26, 1998
This image of the Pacific Ocean was produced using sea-surface
height measurements taken by the U.S.-French TOPEX/Poseidon
satellite. The image shows sea-surface height relative to normal
ocean conditions on June 14, 1998, and sea-surface height is an
indicator of the heat content of the ocean. This image shows that
the tropical Pacific has been switching from warm to cold during
the last few months. The purple area in the center of the image
is a pulse of cold water moving across the equator which the
satellite measures as a region of lower than normal sea level.
Scientists indicate that it appears that the central equatorial
Pacific ocean will stay colder than normal for some time to come
because sea level is about 18 centimeters (7 inches) below
normal, creating a deficit in the heat supply to the surface
waters. It is not certain yet, if this current cooling trend
(shown in purple) will eventually evolve into a long-lasting La
Niña situation. Remnants of the El Niño warm water pool, shown
here in red and white, are still lingering north of the equator
in the center of this image. The effects of El Niño can remain in
the climate system for a long time and could still impact weather
conditions around the world. The satellite's sea-surface height
measurements have provided scientists with a detailed view of the
1997-98 El Niño because the TOPEX/Poseidon satellite measures the
changing sea-surface height with unprecedented precision. In this
image, the white areas show the sea surface is between 14 and 32
centimeters (6 to 13 inches) above normal; in the red areas, it's
about 10 centimeters (4 inches) above normal. The green areas
indicate normal conditions. The purple areas are 14 to 18
centimeters (6 to 7 inches) below normal and the blue areas are 5
to 13 centimeters (2 to 5 inches) below normal. The El Niño
phenomenon is thought to be triggered when the steady westward
blowing trade winds weaken and even reverse direction. This
change in the winds allows a large mass of warm water (the red
and white area) that is normally located near Australia to move
eastward along the equator until it reaches the coast of South
America. The displacement of so much warm water affects
evaporation, where rain clouds form and, consequently, alters the
typical atmospheric jet stream patterns around the world. A La
Niña situation is essentially the opposite of an El Niño
condition, where the trade winds are stronger than normal and the
cold water that normally exists along the coast of South America
extends to the central equatorial Pacific. A La Niña situation
also changes global weather patterns, and is associated with less
moisture in the air resulting in less rain along the coasts of
North and South America. TOPEX/Poseidon will be able to track a
potentially developing La Niña with the same accuracy.
For more information, please visit the TOPEX/Poseidon project
web page at
http://topex-www.jpl.nasa.gov/
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