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http://www.jpl.nasa.gov

Alan Buis (818) 354-0474
Jet Propulsion Laboratory, Pasadena, Calif.

Gretchen Cook-Anderson (202) 358-0836
NASA Headquarters, Washington, D.C.

NEWS RELEASE: 2004-281 December 2, 2004

NASA Study Links Wind and Current Changes to Indian Ocean
Warming

A NASA study suggests changing winds and currents in the
Indian Ocean during the 1990s contributed to the observed
warming of the ocean during that period. The findings,
published in a recent issue of Geophysical Research Letters,
have potential implications for long-term regional climate
variability.

"Establishing this correlation provides an important missing
piece to the global ocean-warming puzzle and provides vital
information for regional governments and climate modelers,"
said Dr. Tong Lee, study author and researcher at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. "These findings from
satellite data also advance space exploration by increasing
understanding of how complex planetary system elements, such
as winds and currents, in our home planet interact to drive
climate change. Such technologies, which have been
demonstrated to be critical in understanding Earth's climate
system, may someday prove useful in studying climate systems
on other planets."

Lee's findings are based on sea level measurements from
NASA's Topex/Poseidon oceanographic satellite, sea-surface
temperature data from the National Oceanic and Atmospheric
Administration's Advanced Very High Resolution Radiometer
satellite, and wind data from the European Space Agency's
European Remote Sensing satellites. Collected between 1992
and 2000, the combined data reveal weakening of southeasterly
trade winds over the South Indian Ocean caused a major
circulation of this ocean to wane by nearly 70 percent of its
average strength.

The atmosphere heats the upper Indian Ocean. The circulation
of this ocean counteracts the atmospheric heating by
exporting warm surface water and importing colder subsurface
water.

The slowdown of this circulation tends to prevent warm
surface water from exiting and colder subsurface water from
entering the upper Indian Ocean, raising its average
temperature. During this period, the average sea-surface
temperature of the Indian Ocean increased by approximately
0.25 Celsius (0.45 Fahrenheit).

"This is a very important and intriguing element of climate
observations, suggesting convincingly that a major piece of
the world ocean has significantly changed its circulation
during the last decade," said Professor Jochem Marotzke,
director of the Max Planck Institute for Meteorology in
Germany. "While it is too early to say whether the underlying
cause is natural variability or human-induced global warming,
this result poses an interesting challenge to global climate
modelers all over the world," he added.

Multi-decadal warming of the Indian Ocean in the past has
affected the North Atlantic climate and was blamed for a
devastating drought along the southern edge of the Sahara
Desert in the 1970s and '80s. Understanding the cause of this
warming and predicting its future evolution are major
challenges to the climate community, as the ocean's warming
is tied into a much larger global cycle of events. This
research suggests the Indian Ocean is subject to the same
type of long-term ocean-circulation oscillations that drive
weather and climate patterns in the Pacific and Atlantic
Oceans.

"The waning wind and ocean currents of the Indian Ocean might
be a manifestation of decadal and longer climate variability.
This could have significant effects on the ocean's ability to
absorb atmospheric carbon dioxide and on the region's marine
food web," Lee said.

Like vegetation, the ocean is a natural carbon dioxide "sink"
that absorbs variable amounts of carbon dioxide from the
atmosphere, depending on winds, ocean currents and
temperatures. The weakened wind and ocean currents, along
with rising ocean temperatures, could hamper the Indian
Ocean's ability to absorb carbon dioxide. Ocean
phytoplankton, the base of the marine food web, relies on the
nourishment brought up by cooler, nutrient-rich subsurface
water to survive and reproduce. The slowed cycling of warm
and cold water could also bring fewer nutrients from the
depths of the ocean to the surface, resulting in a decrease
in the region's biological productivity.

To review the full text of Lee's study on the Internet,
visit:
http://www.agu.org/pubs/current/gl.shtml/

For information about Topex/Poseidon and its follow-on
satellites on the Internet, visit:
http://sealevel.jpl.nasa.gov/index.html

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