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Pacific Dictates Droughts and Drenchings
January 23, 2004
The cooler and drier conditions in Southern California
over the last few years appear to be a direct result of a long-term
ocean pattern known as the Pacific Decadal Oscillation
(PDO), according to research presented at the 2004 meeting of
the American Meteorological Society.
The study, by Steve LaDochy, associate professor of geography
at California State University-Los Angeles; Bill Patzert,
research oceanographer at NASA's Jet Propulsion Laboratory in
Pasadena, Calif.; and others, suggests Pacific oceanic and
atmospheric measurements can be used to forecast seasonal West
Coast temperatures and precipitation up to a year in advance,
from Seattle to San Diego.
An important climate controller, the PDO is a basin-wide
oceanic pattern similar to El Niño and La Niña but much larger.
The PDO lasts many decades rather than just a few months like
El Niño and La Niña. The climatic fingerprints of the PDO are
most visible in the North Pacific and North America, with
secondary influences coming from the tropics. The long-term
nature of the PDO makes it useful for forecasting, as its
effects persist for so long.
Since mid-1992, NASA has been able to provide space-based,
synoptic views of the entire Pacific Ocean's shifts in heat
content with the Topex/Poseidon mission and its follow-up
mission, Jason (which began in 2001). Before these satellites
were available, monitoring oceanic climate signals in near-real
time was virtually impossible.
The remarkable data and images can tag and monitor the shifts
in short-term climate events, like El Niño and La Niña, and
long-term events such as the PDO. These data provide a 13-year
continuous, complete time-series of two major El Niños and two
La Niñas, and have made it possible to detect a major phase
shift of the PDO. Patzert and LaDochy show these data, combined
with longer-term studies of land-based data, provide a powerful
set of forecasting tools.
The PDO shifted to a negative, cool phase, leading to wetter
conditions in the U.S. Pacific Northwest, and drier than normal
conditions in Central and Southern California this decade.
Since the last 1997-1998 El Niño, the Los Angeles area had only
79 percent of its normal rainfall, Patzert said. Lake Mead, the
great fresh-water reservoir in southeast Nevada, is at less
than 50 percent of normal capacity. Also, huge West Coast fires
over the past few years have been greatly exacerbated by PDO-
induced drought, Patzert added.
"These shifts in the PDO are long-term tendencies, which
actually have a bigger economic impact than El Niño," said
Patzert. "People talk about floods from El Niño, but what
really has a harsh and costly impact is a five-year drought."
"A full cycle of the PDO (cool to warm and back to cool) runs
about 50 years," said LaDochy. "Over the next several years
there is going to be a tendency toward dry and colder
temperatures in the southern U.S. West Coast. It is very
difficult to forecast day-to-day here on the West Coast, but we
can say with some confidence that over the next five years,
we'd better start saving water."
The researchers used over 50 years of U.S. climatic
information, and Pacific atmospheric and oceanic data from the
National Oceanic and Atmospheric Administration National
Centers for Environmental Prediction. By comparing data sets,
they saw strong correlations between Pacific climate patterns,
temperatures and precipitation trends on the West Coast. They
then were able to develop "hindcasts" to explain temperature
and precipitation variability for West Coast regions. These
decadal cycles also will be useful for explaining future
regional climate variability.
NASA's Earth Science Enterprise is dedicated to understanding
the Earth as an integrated system and applying Earth System
Science to improve prediction of climate, weather and natural
hazards using the unique vantage point of space.
More on the PDO.
For more information and images about the research on the
Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/0116westcoast.html
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