Global Sea Level Rise Dampened by Australia Floods, NCAR Research
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PRWeb | August
19, 2013
When enough raindrops fall over land instead of the ocean,
they begin to add up. New research led by NCAR shows that three
atmospheric patterns drove so much precipitation over Australia in
2010 and 2011 that the world’s ocean levels dropped measurably.
Boulder, CO (PRWEB) August 19, 2013
When enough raindrops fall over land instead of the ocean, they begin
to add up.
New research led by the National Center for Atmospheric Research
(NCAR) shows that when three atmospheric patterns came together over the
Indian and Pacific oceans, they drove so much precipitation over
Australia in 2010 and 2011 that the world’s ocean levels dropped
measurably. Unlike other continents, the soils and topography of
Australia prevent almost all of its precipitation from running off into
the ocean.
The 2010-11 event temporarily halted a long-term trend of rising sea
levels caused by higher temperatures and melting ice sheets.
Now that the atmospheric patterns have snapped back and more rain is
falling over tropical oceans, the seas are rising again. In fact, with
Australia in a major drought, they are rising faster than before.
“It’s a beautiful illustration of how complicated our climate system
is,” says NCAR scientist John Fasullo, the lead author of the study.
“The smallest continent in the world can affect sea level worldwide. Its
influence is so strong that it can temporarily overcome the background
trend of rising sea levels that we see with climate change.”
The study, with co-authors from NASA’s Jet Propulsion Laboratory and
the University of Colorado at Boulder, will be published next month in
Geophysical Research Letters. It was funded by the National Science
Foundation, which is NCAR’s sponsor, and by NASA.
Consistent rising, interrupted
As the climate warms, the world’s oceans have been rising in recent
decades by just more than 3 millimeters (0.1 inches) annually. This is
partly because the heat causes water to expand, and partly because
runoff from retreating glaciers and ice sheets is making its way into
the oceans.
But for an 18-month period beginning in 2010, the oceans mysteriously
dropped by about 7 millimeters (about 0.3 inches), more than offsetting
the annual rise.
Fasullo and his co-authors published research last year demonstrating
that the reason had to do with the increased rainfall over tropical
continents. They also showed that the drop coincided with the
atmospheric oscillation known as La Niña, which cooled tropical surface
waters in the eastern Pacific and suppressed rainfall there while
enhancing it over portions of the tropical Pacific, Africa, South
America, and Australia.
But an analysis of the historical record showed that past La Niña
events only rarely accompanied such a pronounced drop in sea level.
Using a combination of satellite instruments and other tools, the new
study finds that the picture in 2010–11 was uniquely complex. A rare
combination of two other semi-cyclic climate modes came together to
drive such large amounts of rain over Australia that the continent, on
average, received almost one foot (300 millimeters) of rain more than
average.
The initial effects of La Niña were to cool surface waters in the
eastern Pacific Ocean and push moisture to the west. A climate pattern
known as the Southern Annular Mode then coaxed the moisture into
Australia’s interior, causing widespread flooding across the continent.
Later in the event, high levels of moisture from the Indian Ocean driven
by the Indian Ocean Dipole collided with La Niña-borne moisture in the
Pacific and pushed even more moisture into the continent’s interior.
Together, these influences spurred one of the wettest periods in
Australia’s recorded history.
Australia’s vast interior, called the Outback, is ringed by coastal
mountains and often quite dry. Because of the low-lying nature of the
continent’s eastern interior and the lack of river runoff in its western
dry environment, most of the heavy rainfall of 2010–11 remained inland
rather than flowing into the oceans. While some of it evaporated in the
desert sun, much of it sank into the dry, granular soil of the Western
Plateau or filled the Lake Eyre basin in the east.
“No other continent has this combination of atmospheric set-up and
topography,” Fasullo says. “Only in Australia could the atmosphere carry
such heavy tropical rains to such a large area, only to have those rains
fail to make their way to the ocean.”
Measuring the difference
To conduct the research, the scientists turned to three cutting-edge
observing instrument systems:
- NASA’s Gravity Recovery and Climate Experiment (GRACE)
satellites, which make detailed measurements of Earth’s gravity
field. The satellites enable scientists to monitor changes in the
mass of continents.
- The Argo global array of 3,000 free-drifting floats that
measure the temperature and salinity of the upper 6,000 feet of the
world’s oceans.
- Satellite-based altimeters that are continuously calibrated
against a network of tide gauges. Scientists subtract seasonal and
other variations to closely estimate global sea level changes.
Using these instruments, the researchers found that the mass in
Australia and, to a lesser extent, South America began to increase in
2010 as the continents experienced heavy and persistent rain. At the
same time, sea levels began to measurably drop.
Since 2011, when the atmospheric patterns shifted out of their
unusual combination, sea levels have been rising at a faster pace of
about 10 millimeters (0.4 inches) per year.
Scientists are uncertain how often the three atmospheric events come
together to cause such heavy rains over Australia. Fasullo believes
there may have been a similar event in 1973-74, which was another time
of record flooding in that continent. But modern observing instruments
did not exist then, making it impossible to determine what took place in
the atmosphere and whether it affected sea level rise.
“Luckily we’ve got great observations now,” Fasullo says. “We need to
maintain these observing platforms to understand what is a complicated
climate system.”
The University Corporation for Atmospheric Research (UCAR) operates
NCAR under primary sponsorship by the National Science Foundation.
Opinions, findings, conclusions, or recommendations expressed in this
release do not necessarily reflect the views of the National Science
Foundation.
-----Scientific contact-----
John Fasullo, NCAR scientist
303-497-1712
fasullo(at)ucar(dot)edu
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