SHAKE AND BLOW


by Staff Writers
Boulder CO (SPX) Aug 26, 2013


Heavy rains transformed Australia’s landscape, as show in these two NASA satellite images of floodplains in southwestern Queensland. The first image was taken on September 26, 2009. By the time of the second image, on March 26, 2011, so much rain had been driven over Australia instead of falling on the ocean that global sea levels temporarily dropped. (Image taken with the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite.)

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 Nina, which cooled tropical surface waters in the eastern Pacific and suppressed rainfall there while enhancing it over other portions of the tropical Pacific, Africa, South America, and Australia.

But an analysis of the historical record showed that past La Nina events only rarely accompanied such a pronounced drop in sea level.

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