The Pacific Ocean, the largest and deepest of the world’s
oceans, suffers periodic mood swings that have a dramatic impact
on our weather. These mood swings include a climate phenomenon
known as Pacific Decadal Oscillation, or PDO. It’s an El Nino-
like shift in the ocean’s temperature that scientists once
thought cycled every 15 to 20 years.
However, there’s new NASA research that now shows there may
be a second, much longer, PDO cycle that lasts about 70 years.
Dr. Yi Chao, an oceanographer at NASA’s Jet Propulsion
Laboratory, Pasadena, Calif., and colleagues Drs. Michael Ghil
and James McWilliams of the University of California, Los
Angeles, have found evidence of the PDO’s two-part structure in a
study of the past 92-year record of sea-surface temperatures in
the North and South Pacific. The results of their study appears
in the August 1, 2000, issue of Geophysical Research Letters.
If El Nino is a brief sonata, then the Pacific Decadal
Oscillation is a much larger symphony. “The El Nino is well
defined,” said Chao. “We know when it is born, can see its rise
and fall and measure its strength. We can forecast its
consequences. But the PDO is larger, longer and more difficult to
visualize. An explanation might be that it isn’t just one thing –
– it’s potentially two big events going on.”
In their study, Chao and his colleagues found large-scale
temperature oscillations taking place in the Pacific basin
approximately every 15 to 20 years. “While we are only talking
about a one-to-two degree centigrade difference in sea-surface
temperature, we are talking about a huge area. This temperature
difference has a big impact on the climate of North America.”
The change in location of cold and warm water in the Pacific
alters the path of the jet stream, the conveyor belt for storms
across the continent. Chao’s study supports and expands the
previous studies by University of Washington researchers that
gave a name to the phenomenon only five years ago.
However, in addition to this regular and relatively short
fluctuation in the Pacific basin’s temperature, Chao also has
found evidence of another temperature shift that appears to take
place on a much longer time scale, about 70 years. At the
beginning of this century, sea-surface temperatures seem to
gently drop to a low in the 1930s, gradually rise again until the
1970s, and then begin a similarly paced decline to the present.
“While we were only able to see one cycle in our data, tree-ring
records, which go back 200 to 300 years, and fishery data, show a
similar time-scale shift,” added Chao.
In Chao’s analysis of the past century’s sea-surface
temperatures, the PDO also reveals striking symmetry between the
northern and southern Pacific. In its “cool” phase, the PDO is a
giant horseshoe-shaped arc of warmer-than-normal water off the
coast of Japan, enclosing a wedge of cooler-than-normal water
near the equator. In Chao’s study, this same approximately 20-
year cycle appears around 1976, 1957, 1941 and 1924.
“What’s striking is that the PDO pattern is similar in both
the North and South Pacific and covers a huge area from the
Aleutian Islands to the South Pacific,” said Chao. “No computer
models developed so far have been able to reproduce this
symmetric pattern across the equator. This symmetry is a key to
understanding what creates the PDO.”
“Looking into the future,” Chao continued, “we are now
analyzing the temperature below the sea surface. The goal is to
get a three-dimensional picture of the PDO that might help us
reach the ultimate goal, a realistic computer model linking the
ocean and the atmosphere that will help us predict Earth’s
climate.”
This research is supported by NASA’s Earth Science
Enterprise, Washington, D.C., which is dedicated to studying how
human-induced and natural changes affect our global environment.
More information about the Pacific Decadal Oscillation is
available on the Internet at:
http://topex-www.jpl.nasa.gov/discover/PDO.html and
http://tao.atmos.washington.edu/pdo/
JPL is a division of the California Institute of Technology
in Pasadena.
