August 20, 2003
El Niño's Surprising Steady Pacific Rains Can Affect World Weather
Scientists using data from a NASA satellite have found another piece
in the global climate puzzle created by El Niño. El Niño
events produce more of a steady rain in the middle of the Pacific Ocean.
This is important because whenever there is a change in the amount and
duration of rainfall over an area, such as the central Pacific, it affects
weather regionally and even worldwide.
The findings appeared in a paper authored by Courtney Schumacher and
Robert Houze, atmospheric scientists at the University of Washington,
Seattle, who used data from NASA's Tropical Rainfall Measuring Mission
(TRMM) satellite to look at rainfall over the Pacific during the 1997-1998
El Niño. The study was published in a recent issue of the American
Meteorological Society's Journal of Climate.
El Niño occurs when warm water shifts from the western to the
eastern Pacific Ocean and trade winds that usually blow from east to
west across the equator diminish. As a result, rainfall patterns around
the globe change during the life of these periodic climate events, and
in some areas create floods or droughts. By identifying the changes in
rainfall in one area of the globe, such as the central Pacific Ocean,
scientists continue to piece together the El Niño puzzle that
will help them improve rainfall forecasts around the world during the
life of El Niño.
Schumacher and Houze noticed an unusually steady rainfall over the central
Pacific, much like a gray day with light rain and drizzle. That is surprising
over the central Pacific, an area normally known for thunderstorms that
build up during the heat of the day from rapidly rising air or convection,
and then rain themselves out within an hour.
"Instead, the rainfall we saw is called 'stratiform rain,' which are
weaker rains that cover larger areas," Schumacher said. It's the type
of rain that makes people want to stay inside all day.
Seeing the difference between the convective and stratiform rain is
very important to climatologists. Rain type matters because when water
condenses and raindrops form, heat is released. That heat is a main driver
in the circulation or movement of the atmosphere. "Convective rain, like
from thunderstorms, releases heat lower in the atmosphere, while the
steady, light, stratiform rain releases heat higher in the atmosphere," Houze
said.
Heat released at different levels of the atmosphere affects the vertical
and horizontal movement of air. As a result, the heat from different
types of rain can alter weather patterns, such as the familiar jet stream
which impacts the United States. Weather prediction models benefit from
better knowledge of the changing jet stream patterns, because storms
like summer thunderstorms and winter snowstorms follow the path of the
jet stream.
The researchers noted that because increased stratiform rains warm the
upper levels of the atmosphere, that additional heat strengthens the
way air moves higher up and over a larger area than thunderstorm-generated
rainfall. The larger area of air affected can impact weather patterns
around the world.
Although scientists are still uncertain why such large amounts of stratiform
rain happen in the middle of the Pacific, especially during El Niño,
observations from the TRMM satellite have allowed them to recognize this
pattern. By identifying the type of rainfall, climatologists and meteorologists
can make better rainfall forecasts in other areas of the world during
the event.
"That's the beauty of NASA's TRMM satellite," said Jeff Halverson, TRMM
outreach scientist from NASA's Goddard Space Flight Center, Greenbelt,
Md. "TRMM can see what's happening in remote areas of the tropics that
can have affects on the rest of the world."
TRMM is a joint NASA/Japanese Space Agency mission to study tropical
rainfall and its implications for climate. Each day, the TRMM spacecraft
observes the Earth's equatorial and tropical regions.
This research was funded by NASA's Earth Science Enterprise (ESE), the
National Science Foundation and the National Oceanic and Atmospheric
Administration. NASA's ESE 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.
For more information and images on the Internet, visit: http://www.gsfc.nasa.gov/topstory/2003/0818elNino.html
For information about TRMM on the Internet, visit: http://www.trmm.gsfc.nasa.gov
For information about AMS on the Internet, visit: http://www.ametsoc.org/AMS/
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Contacts: Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
301/286-4044
Stephanie Kenitzer
American Meteorological Society
425/432-2192
Vince Stricherz
University of Washington
206/543-2580)
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![El Niño's Steady Pacific Rains](https://webarchive.library.unt.edu/eot2008/20080920053134im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030820/fig3_tn.jpg)
El Niño's Steady Pacific Rains
These composite images show that the largest amounts (red) of stratiform
rain, up to 60%, occur in the central Pacific during El Niño,
such as occurred in the 1998 event. 1999 and 2000 were not El Niño
years. CREDIT: University of Washington/NASA
![Graphing El Niño Rains](https://webarchive.library.unt.edu/eot2008/20080920053134im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030820/fig1_tn.jpg)
Graphing El Niño Rains
These graphs were created from the TRMM satellite's Precipitation Radar.
The top graph shows that almost 60% of the rains that fell around 135
degrees latitude around the equator (central Pacific Ocean) were stratiform,
so 40% were convective (thunderstorms). The bottom graph shows that almost
150 mm (5.9 inches) of rain fell in that area during an El Niño.
CREDIT: UWA/NASA
![A Heated Atmosphere From Rain](https://webarchive.library.unt.edu/eot2008/20080920053134im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030820/fig2_tn.jpg)
A Heated Atmosphere From Rain
These images show the upper level latent heat that is generated when
raindrops are created. The higher amounts of heat (yellow and red) occur
more in the central Pacific Ocean during an El Niño (1998) than
in the western Pacific during a La Niña (1999). CREDIT: UWA
![](https://webarchive.library.unt.edu/eot2008/20080920053134im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030820/a000256_pre_tn.jpg)
1997-1998 El Niño in 3-D
This animation shows El Niño's warmest sea surface temperatures
in red and rising sea surface heights in the Pacific between January
1997-July 1998 from NOAA AVHRR and TOPEX Poseidon. CREDIT: NASA SVS
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