22 August 2013
Oz rains soak up sea level rise
by Will Parker
Researchers at the National Center for Atmospheric Research (NCAR) report 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.
The new research, published in Geophysical Research Letters, notes that the continent of Australia gained significant mass over the period, largely because unlike other continents, the soils and topography of Australia prevent almost all of its precipitation from running off into the ocean.
According to the researchers, the 2010-11 event temporarily halted a long-term trend of rising sea levels caused by higher temperatures and melting ice sheets. But now that the atmospheric patterns have snapped back and more rain is falling over tropical oceans, the seas are rising again.
"It's a beautiful illustration of how complicated our climate system is," says NCAR scientist John Fasullo, 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."
Fasullo explained that as the climate warms, the world's oceans have been rising in recent decades by around 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.
Using NASA's Gravity Recovery and Climate Experiment satellites, the scientists found that the mass of Australia began to increase in 2010 as the continent experienced heavy and persistent rain. At the same time, sea levels began to measurably drop.
Fasullo says a rare combination of La Niña and two other semi-cyclic climate modes came together to deliver 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," explained Fasullo. "Together, these influences spurred one of the wettest periods in Australia's recorded history."
The report notes that the low-lying nature of the continent's eastern interior and the lack of river runoff in its western dry environment are also important factors. "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."
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.
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