How an ocean went into hiding in Australia.


A chunk of the oceans took a wrong turn in 2011. Instead of going from sky to rain, into rivers, and then back into the oceans in the usual water cycle, it got stuck in Australia, caught up in record-breaking floods and rivers that run backwards into the continent. So much water got lost down under that global sea levels fell and stayed low for more than a year.

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John Fasullo of the National Center for Atmospheric Research in Boulder, Colorado, noticed something amiss while looking at trends in global sea levels. They had been rising steadily by about 3 millimetres every year, but in the second half of 2010, they suddenly plummeted. By early 2011, they had dropped by 7 mm, the biggest drop since satellite measurements began in 1992.

The reversal lasted until late 2011. Not many people noticed: at the time, the world was preoccupied with massive floods in Australia. Fasullo wondered whether the two events might be linked. If more water than usual was evaporating from the oceans and falling on Australia, that might help to explain both the floods and sea level drop.

Satellite data showed that more water was stored on land in 2011 than in previous years, most of it in Australia, South America and South-East Asia (Geophysical Research Letters, doi.org/nhk). An early explanation was a strong La Niña, which funnelled warm, moist air towards Australia. But La Niña events happen every few years and regularly make it rain in Australia, says Fasullo. “Why don’t we see massive sea level drops after all La Niñas?”

Triple whammy

Re-examining weather records revealed two other factors. The Indian Ocean was much warmer in the east than in the west, pushing yet more warm, moist air towards Australia. This had not happened for 20 years. At the same time, a band of winds circling Antarctica shifted to the south, boosting the effect still further (Geophysical Research Letters, doi.org/ngx).

The rare combination of events led to unusually heavy Australian rainfall, says Fasullo. But why did the water stay out of the oceans for so long? Extra rainfall on land should get washed back out to sea by rivers within a few months.

It turns out that Australia has an uncanny ability to trap water for long periods. River channels are sparse in the west, so rainwater tends to sit in the sandy soil. And in the east, many of the channels run into a low-lying desert basin at the centre of the continent rather than out to sea. With heavy rains, the basin fills up to become Kati Thanda-Lake Eyre; fish eggs lying dormant in the soil hatch, and an ecosystem briefly comes to life. “It’s an instant inland sea,” says Fasullo.

The unusual weather and geology offer a convincing explanation for the sudden drop in sea levels, says John Church of CSIRO Marine and Atmospheric Research in Hobart, Tasmania. But his colleague Xuebin Zhangsays questions remain. He calculates that the extra water on land accounts for about 3.75 mm of the 7 mm fall in sea levels.

Similarly heavy rain fell on Australia in 1973-74, but sea-level records from then are not detailed enough to draw conclusions, and the three climate systems might not align again for decades.

Source: newscientist.com

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