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A cool-water anomaly known as La Niña
occupied the tropical Pacific Ocean throughout 2007 and early 2008. In
April 2008, scientists at NASA’s Jet Propulsion Laboratory announced
that while the La Niña was weakening, the Pacific Decadal Oscillation—a
larger-scale, slower-cycling ocean pattern—had shifted to its cool
phase.
This image shows the sea surface temperature anomaly in the Pacific
Ocean from April 14–21, 2008. The anomaly compares the recent
temperatures measured by the Advanced Microwave Scanning Radiometer for
EOS (AMSR-E) on NASA’s Aqua
satellite with an average of data collected by the NOAA Pathfinder
satellites from 1985–1997. Places where the Pacific was cooler than
normal are blue, places where temperatures were average are white, and
places where the ocean was warmer than normal are red.
The cool water anomaly in the center of the image shows the
lingering effect of the year-old La Niña. However, the much broader
area of cooler-than-average water off the coast of North America from
Alaska (top center) to the equator is a classic feature of the cool
phase of the Pacific Decadal Oscillation (PDO). The cool waters wrap in
a horseshoe shape around a core of warmer-than-average water. (In the
warm phase, the pattern is reversed).
Unlike El Niño and La Niña, which may occur every 3 to 7 years and
last from 6 to 18 months, the PDO can remain in the same phase for 20
to 30 years. The shift in the PDO can have significant implications for
global climate, affecting Pacific and Atlantic hurricane activity,
droughts and flooding around the Pacific basin, the productivity of
marine ecosystems, and global land temperature patterns.
“This multi-year Pacific Decadal Oscillation ‘cool’ trend can intensify
La Niña or diminish El Niño impacts around the Pacific basin,” said
Bill Patzert, an oceanographer and climatologist at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. “The persistence of this
large-scale pattern [in 2008] tells us there is much more than an
isolated La Niña occurring in the Pacific Ocean.”
Natural, large-scale climate patterns like the PDO and El
Niño-La Niña are superimposed on global warming caused by increasing
concentrations of greenhouse gases and landscape changes like
deforestation. According to Josh Willis, JPL oceanographer and climate
scientist, “These natural climate phenomena can sometimes hide global
warming caused by human activities. Or they can have the opposite
effect of accentuating it.” Source
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