A look at weather extremes and the big-picture climate connections.

Ocean Temperatures Show Possible Climate Change Connection to Australian Flooding

By Alyson Kenward

In spite of the snowstorm that rattled the South through the Northeast U.S. — not to mention the winter storm currently rolling through the Seattle area — the biggest news in weather this week actually comes all the way from Australia.

As you may have heard by now, the Australian state of Queensland has recently been inundated by flooding on a grand scale. This past week, the eastern coastline of the state was struck by torrential rains that have led to more severe flooding. Thousands of people have been evacuated around the state, at least 13 people have died, and in the city of Brisbane — with about two million people, it’s Australia’s third-largest city — nearly 20,000 homes are under flood risk after the Brisbane River, which runs through the center of the city, overflowed.

Floodwaters left cars and people stranded in East Toowoomba, Australia on January 10, 2011. (credit: Kingbob86/flickr)

Looking back over the past few months of precipitation records in Australia, which are compiled by the country’s Bureau of Meteorology, it’s clear that there has been a growing risk of flooding. Since before the beginning of their rainy season, which normally starts around September, the rain has been falling in unusual amounts. The period of October through December featured the heaviest rainfalls ever recorded over a three-month span.

December itself was also record-breaking, and much of Queensland recorded its heaviest rainfall on record for the month. With all that rain pummeling the state, it’s no surprise that towards the end of 2010 the rivers swelled, and flooding took broke out.

Then, less than two weeks into January — well before December's floodwaters had time to fully recede — the faucet was turned on again in Queensland. With more than 15 inches of rain falling during the past week in areas just north of Brisbane, flash flooding was inevitable. The worst of the flooding this week can be attributed to a nasty three-day weather pattern, but is there more to what’s going on in Australia than just a transient bout of bad weather?

Record Warm Ocean Temperatures

This year’s particularly rainy summer in Australia has been caused in large part by La Niña conditions in the Pacific Ocean. The same system that is characterized by cooler waters in the eastern tropical Pacific Ocean also brings warmer water to the northeastern coast of Australia. The combination of warm water and accompanying low atmospheric pressure during La Niña typically lead to an earlier and longer rainy season compared to average years.

While La Niña events aren’t exactly unusual, the strength of this year’s event appears to be unprecedented by at least one key metric.

But according to climate researcher David Karoly from the University of Melbourne, the strong La Niña isn’t solely responsible for this year’s heavy Australian rainfall. The sea surface temperatures around Australia, rather than just its northeastern region, also contribute to the rains in Queensland, he says.

“What gives very heavy rainfall is high Indian Ocean temperatures and La Niña in the Pacific,” Karoly explains. “This year we have both of those, and both are at record highs.”

December sea surface temperature departures from average, from 1900 to 2010. Credit: Australian Bureau of Meteorology.

The toasty temperatures in the Indian Ocean aren’t just a one or two year occurrence, however. Looking back over the past several years, there is a pronounced long-term warming trend in the waters near Australia. Karoly says, “This isn’t just climate variability. This is man-made climate change.”

Because Indian Ocean temperatures influence Australian climate, this means that as sea surface temperatures continue to warm, there will be an increased risk of these heavy rainfall events, says Karoly. As he explains, “we can’t say this individual event [in Queensland] is due to long term climate change, but we can say the overall global sea surface temperature increases are due to anthropogenic [man-made] forcings.”

While the high sea surface temperatures this year follow many years of ocean warming, this is not the first time the combination of a strong La Niña and above average Indian Ocean temperatures has caused heavy rains and flooding in Queensland. For some Brisbane locals, the rising waters today are a reminder of severe flooding that hit the city in 1974, an event known as the "Big Wet."   

Looking forward however, as global atmospheric and sea surface temperatures climb steadily higher, future La Niña years could bring similar conditions to both 1974 and 2010-11. Even in average years, many parts of northern Australia are bound to see warmer and wetter conditions than in the past. On the other hand, the southernmost regions of the vast country will experience a hotter a drier climate — something they’ve also had a taste of in 2010. Even though, as a whole, Australia just finished its third wettest year on record, the southwestern corner of the country actually experienced its driest year on record.

When thinking about the climate changes coming to Australia, Karoly says it brings to mind a famous poem, My Country, written by Dorothea Mackellar in 1905. In Mackellar’s description of why she likes her “sunburnt country”, she dotes on the land “of droughts and flooding rains.” Though the poem is dear to Karoly, he thinks the stanza could use a little rewording. “What I think now is that climate change is making Australia the land of more droughts and worse flooding rains.”  


View Queensland's big wet in a larger map

Australia’s ABC News Online produced this map showing the towns and cities currently affected by floods.

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