Australia floods briefly halted sea level rise

The following is a news release from the National Science Foundation.

Threeatmospheric patterns came together above the Indian and Pacific Oceans in 2010and 2011.  When they did, they drove so much precipitation over Australiathat the world's ocean levels dropped measurably.

Unlikeother continents, the soils and topography of Australia prevent almost all itsprecipitation from flowing into the ocean.

The2010-11 event temporarily halted a long-term trend of rising sea levels causedby higher temperatures and melting ice sheets, according to a team of researchersat the National Center for Atmospheric Research (NCAR) in Boulder, Colo., andother institutions.

Now thatthe atmosphere's circulation has returned to its previous patterns,the seas are again rising.

Theseresults will appear next month in the journal Geophysical Research Letters, published bythe American Geophysical Union.

Australian floods seen from space from NASA Earth Observatory.

Flooding in Queensland
Image acquired January 4, 2011

Flooding in Queensland
Image acquired December 14, 2010

Co-authorsof the paper are affiliated with NASA's Jet Propulsion Laboratory and theUniversity of Colorado at Boulder.

Theresearch was funded by the National Science Foundation (NSF), which sponsorsNCAR, and by NASA.

"Thescientists conclude that the Outback region in Australia played a crucial rolein trapping a large amount of rainfall when widespread floods occurred over thecontinent," says Anjuli Bamzai, program director in NSF's Division ofAtmospheric and Geospace Sciences, which funded the research.

"It'sa beautiful illustration of how complicated our climate system is," saysNCAR scientist John Fasullo, lead scientist on the project.

"Thesmallest continent in the world can affect sea level worldwide.  Itsinfluence is so strong that it can temporarily overcome the background trend ofrising sea levels we see with climate change."

As theclimate warms, the world's oceans have been rising in recent decades by just overthree millimeters annually.

This ispartly because heat causes water to expand, and partly because runoff fromretreating glaciers and ice sheets is making its way into the oceans.

But for an18-month period beginning in 2010, the oceans mysteriously dropped by aboutseven millimeters, more than offsetting the annual rise.

Fasulloand co-authors published research results last year demonstrating that thereason was related to the increased rainfall over tropical continents.

They alsoshowed that the drop coincided with the atmospheric oscillation known as LaNiña, which cooled tropical surface waters in the eastern Pacific andsuppressed rainfall there--while enhancing it over portions of the tropicalPacific, Africa, South America and Australia.

However,an analysis of the historical record showed that past La Niña events onlyrarely accompanied such a pronounced drop in sea level.

Using acombination of satellite instruments and other tools, the new study finds thatthe picture in 2010-11 was uniquely complex.

Inaddition to La Nina, a rare combination of two other semi-cyclic climate modescame together.  They drove such large amounts of rain over Australia thatthe continent received almost one foot (300 millimeters) of rain more thanaverage.

The initialeffects of La Niña were to cool surface waters in the eastern Pacific Ocean andpush moisture to the west.

A climatepattern known as the Southern Annular Mode then coaxed the moisture intoAustralia's interior, causing widespread flooding across the continent.

Later inthe event, high levels of moisture from the Indian Ocean driven by what's knownas the Indian Ocean Dipole collided with La Niña-borne moisture in thePacific, pushing even more moisture into the continent's interior.

Theseinfluences spurred one of the wettest periods in Australia's recorded history.

Australia'svast interior, called the Outback, is ringed by coastal mountains and is oftenquite dry.

Because ofthe low-lying nature of the continent's eastern interior, and the lack of riverrunoff in its western dry environment, most of the heavy rainfall of 2010-11remained inland rather than flowing to the oceans.

While someof it evaporated in the desert sun, much of it sank into the dry, granular soilof the Western Plateau or filled the Lake Eyre basin in the east.

"Noother continent has this combination of atmospheric set-up andtopography," Fasullo says.  "Only in Australia could theatmosphere carry such heavy tropical rains to such a large area, only to havethose rains fail to make their way to the ocean."

Forexample, the Great Basin in the southwestern United States could trap watermuch like Australia--but atmospheric patterns don't transport such a largeamount of moisture from the ocean to that arid region.

To conductthe research, the scientists turned to three observing instrument systems:

· NASA'sGravity Recovery and Climate Experiment satellites, which make detailedmeasurements of Earth's gravity field.  The satellites enable scientiststo monitor changes in the mass of continents.

· The Argoglobal array of 3,000 free-drifting floats that measure the temperature andsalinity of the upper 6,000 feet of the world's oceans.

·Satellite-based altimeters that are continuously calibrated against a networkof tide gauges. Scientists subtract seasonal and other variations to closelyestimate global sea level changes.

Usingthese instruments, the researchers found that the land mass in Australia and,to a lesser extent, South America began to increase in 2010 as the continentsexperienced heavy and persistent rain.

At thesame time, sea levels began to drop.

Since2011, when the atmospheric patterns shifted out of their unusual combination,sea levels have been rising at a faster pace of about 10 millimeters per year.

Scientistsare uncertain how often the three atmospheric events come together to causesuch heavy rains over Australia.

Fasullobelieves there may have been a similar event in 1973-74, which was another timeof record flooding.

But modernobserving instruments did not exist then, making it impossible to determinewhat took place in the atmosphere and whether it affected sea level rise.

"Luckily we'vegot great observations now," Fasullo says. "We need to maintain theseobserving platforms to understand our climate system."