|
NEW METHOD
LINKS RAINFALL PATTERNS TO DEVELOPING EL NIÑOS NASA
researchers have created a tool that can predict El Niño events months
before they occur, by linking variations in rainfall patterns over the Indian
Ocean with developing El Niños. Scott Curtis of the University of Maryland,
Baltimore County's (UMBC) Joint Center for Earth Systems Technology (JCET), and
Robert Adler of NASA's Goddard Space Flight Center in Greenbelt, Md., developed
an El Niño Prediction Index (PI) formula that uses satellite-based rainfall
data. Currently,
developing El Niño events are identified by the warming of sea surface
temperatures in the equatorial Pacific Ocean. However remote clues in the atmosphere
may precede the changes observed at the ocean surface. This new Prediction Index
looks for signs of El Niño development in the winter, providing six to
nine months lead-time. "This
statistical formula uses a time series of satellite-based data," Curtis said.
"We look for rapid changes (over 30-60 day time scales) in the difference
of precipitation between two specific geographic areas, namely off the coast of
Sumatra (Indonesia), south of the Equator, and in the central Indian Ocean, basically
on the Equator." If
this "see-saw" in precipitation is strong and the coast of Sumatra has
been wetter than normal and the central Indian Ocean drier than normal over the
past six months, then the formula yields a "high value," indicative
of a developing El Niño. They found high values that preceded the five
strongest El Niños of the study period, including the most recent 1997-1998
El Niño event. Curtis
and Adler checked satellite rainfall data on a weekly basis, and found that the
index surpassed the required threshold for shifts in rainfall in late January
and early February of this year. "We are therefore looking for the next El
Niño to begin sometime between July and October 2002, based on the 6-9
month lag observed in the first five cases," said Adler. "These
abrupt shifts in rainfall patterns are likely accompanied by strong westerly winds
that move from the Indian Ocean to the Pacific Ocean. If the winds reach the Pacific
Ocean and the ocean-atmosphere conditions are right, an El Niño will develop,"
said Curtis. The
National Oceanic and Atmospheric Administration's Climate Prediction Center (NOAA/CPC)
is the lead agency on El Niño forecasts. Vernon Kousky of NOAA's CPC said
the new prediction tool supports past research on events leading to El Niño.
"Their results are consistent with other studies that have focused on the
El Niño-Southern Oscillation (ENSO)-neutral period just prior to the development
of El Niños. Satellite estimates of precipitation are crucial for monitoring
and predicting climate variability on intra-seasonal and inter-annual time scales,"
he said. El
Niño events in the tropical Pacific Ocean have impacts on global weather
patterns, including increased rainfall in the eastern Pacific, and drought conditions
in Indonesia and Australia. Curtis
and Adler are also using satellite information to examine these global precipitation
patterns from 1979 to 2001 and how they evolve during El Niños. If the
2002-2003 event behaves like an "average" El Niño, then Curtis
and Adler expect it to last about a year, with the strongest effects occurring
next winter in various countries including the United States. Curtis
and Adler will use precipitation data from the NASA's Tropical Rainfall Measuring
Mission (TRMM) satellite to verify the results from their formula. TRMM, now in
its fifth year of operation, has led to greatly improved rainfall information
over the tropics and TRMM-based precipitation estimates will also be used to monitor
the expected changes related to the El Niño. The
Global Energy and Water Cycle Experiment (GEWEX) provided the 20-year rainfall
record that Curtis and Adler used to develop their Prediction Index. GEWEX is
an international cooperative effort that was initiated in 1988 by the World Climate
Research Program (WCRP) to observe and model the hydrologic cycle and energy fluxes
in the atmosphere, and at the land and ocean surface. Curtis
presents the 2002-2003 El Niño forecast paper at the Spring American Geophysical
Union meeting in Washington, D.C, at the Washington Convention Center in the H21B-04
session at 9:40 a.m., Tuesday, May 28, in Room WCC29. This
paper was funded by the TRMM science team and NASA's Atmospheric Dynamics and
Thermodynamics Program. Back
to Top | 
