From: nasanews
Date: Dec 7 20:04 UTC
Subject: Topex Detects Climate Change
TOPEX/POSEIDON BUILDS CRITICAL DATA BASE ON CLIMATE CHANGE
Scientists, using data from the U.S.-French oceanography satellite
TOPEX/Poseidon, say they appear to have detected a rise in the average
global sea level over the past two years. If scientists detect a
global, long-term rise in sea level, they will be able to test theories
about the effects of climate change.
"A rise in global mean sea level is an important indicator of global
change, because it can be caused by thermal expansion of the oceans and
melting of glaciers and the polar ice caps," said Dr. R. Steven Nerem
of NASA's Goddard Space Flight Center, Greenbelt, MD. "Therefore, if a
long-term rise in global mean sea level were detected, this would
provide further evidence to support the global warming predicted by
some climate models due to an increase in the 'greenhouse' gases."
This and several other findings were announced today at the American
Geophysical Union's fall meeting in San Francisco. Other topics at the
meeting related to TOPEX/Poseidon include an experiment to study
phenomena that can disrupt offshore drilling and the study of
deep-ocean tides.
The insights from TOPEX/Poseidon add to data collected from tide gauges
over the last century which suggest that average sea level has been
rising at a rate of about .04 to .08 inches (1 to 2 millimeters) per
year, roughly equivalent to the rate expected from global warming,
Nerem said. "The data (from Dec. 1992 to Sept. 1994) show a rise in
average sea level of about .12 inches (3 millimeters) per year, which
is in reasonable agreement with the tide gauge results," Nerem
explained. However, tide gauge measurements can be affected by the
movement of land masses and are too sparsely distributed to provide
global coverage.
TOPEX/Poseidon was launched on Aug. 10, 1992, to study how long-term
ocean circulation affects climate change. The satellite measures the
sea-surface height along a groundtrack that repeats once every 10
days. The satellite also measures absolute sea level relative to the
center-of-mass of the Earth. The measurement data is unaffected by
land movements.
Nerem cautioned that the results are preliminary and could change as a
longer time series is collected and as the measurement errors are
better understood.
"It should also be noted that since the sea level rise is only measured
over two years, it could represent a short-term variation unrelated to
the long-term signal expected from global warming. Nevertheless,
TOPEX/Poseidon appears to be providing corroborating evidence that
global sea level is indeed rising," Nerem said.
TOPEX/Poseidon data are helping scientists and a U.S. oil company in
the Gulf of Mexico study potentially dangerous ocean phenomena that can
disrupt offshore oil drilling.
"Oil industry operations in deep water can be adversely affected by
high ocean currents," said Dr. George Born of the University of
Colorado's Colorado Center for Astrodynamics Research (CCAR). "In the
Gulf, eddies spun off from the Loop Current have caused serious
operational problems, including delays of several weeks, lost and
damaged equipment and millions of dollars in unproductive expense."
The Loop Current is a strong ocean current that circulates around the
Gulf. Roughly once a year, a large eddy is spun off from the Loop
Current and moves west through the Gulf.
"An eddy can be thought of as a hurricane in the water," Born said.
"The Gulf of Mexico has some of the largest eddies in the world,
ranging up to 180 to 250 miles (300 to 400 kilometers) in diameter."
In late summer 1994, a large eddy, called Eddy Yucatan, broke off from
the Loop Current. Oceanographers from Texaco, Inc. used TOPEX/Poseidon
data to augment their drifting buoy data and track this eddy. As of
late October, Eddy Yucatan was located approximately 93 miles (150
kilometers) southeast of the Texaco drilling site where it appeared to
be moving away from the site.
"However, the predictive models available for the Gulf are still under
development and it is impossible to predict the future path of the eddy
with great certainty," Born said. "Work is currently underway at CCAR
to use TOPEX/Poseidon data to produce a more reliable forecast. This
work ultimately will benefit not only offshore oil drilling, but many
oceanographic activities in the Gulf of Mexico."
In another application, the precision of the TOPEX/Poseidon ocean
measurements has enabled scientists to calculate global tides across
all the open oceans, an important step toward monitoring global ocean
circulation from space and understanding the complexities of global
climate change.
"Using the first year of data we can now predict deep ocean tides
everywhere over the areas covered by the satellite with an accuracy of
0.8 to 1.1 inches (2 to 3 centimeters)," said Dr. Christian Le Provost,
of the Institut de Mecanique de Grenoble in France.
"It should also be noted that since the sea level rise is only measured
over two years, it could represent a short-term variation unrelated to
the long-term signal expected from global warming. Nevertheless,
TOPEX/Poseidon appears to be providing corroborating evidence that
global sea level is indeed rising," Nerem said.
TOPEX/Poseidon data are helping scientists and a U.S. oil company in
the Gulf of Mexico study potentially dangerous ocean phenomena that can
disrupt offshore oil drilling.
"Oil industry operations in deep water can be adversely affected by
high ocean currents," said Dr. George Born of the University of
Colorado's Colorado Center for Astrodynamics Research (CCAR). "In the
Gulf, eddies spun off from the Loop Current have caused serious
operational problems, including delays of several weeks, lost and
damaged equipment and millions of dollars in unproductive expense."
The Loop Current is a strong ocean current that circulates around the
Gulf. Roughly once a year, a large eddy is spun off from the Loop
Current and moves west through the Gulf.
"An eddy can be thought of as a hurricane in the water," Born said.
"The Gulf of Mexico has some of the largest eddies in the world,
ranging up to 180 to 250 miles (300 to 400 kilometers) in diameter."
In late summer 1994, a large eddy, called Eddy Yucatan, broke off from
the Loop Current. Oceanographers from Texaco, Inc. used TOPEX/Poseidon
data to augment their drifting buoy data and track this eddy. As of
late October, Eddy Yucatan was located approximately 93 miles (150
kilometers) southeast of the Texaco drilling site where it appeared to
be moving away from the site.
"However, the predictive models available for the Gulf are still under
development and it is impossible to predict the future path of the eddy
with great certainty," Born said. "Work is currently underway at CCAR
to use TOPEX/Poseidon data to produce a more reliable forecast. This
work ultimately will benefit not only offshore oil drilling, but many
oceanographic activities in the Gulf of Mexico."
In another application, the precision of the TOPEX/Poseidon ocean
measurements has enabled scientists to calculate global tides across
all the open oceans, an important step toward monitoring global ocean
circulation from space and understanding the complexities of global
climate change.
"Using the first year of data we can now predict deep ocean tides
everywhere over the areas covered by the satellite with an accuracy of
0.8 to 1.1 inches (2 to 3 centimeters)," said Dr. Christian Le Provost,
of the Institut de Mecanique de Grenoble in France.
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(_) Computer Laboratory - Department of Communication
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