Ocean temperature made Queensland floods worse: study

Nicky Phillips 

The Age, May 16, 2012

Abnormally high ocean temperatures off the coast of northern Australia contributed to the extreme rainfall that flooded three-quarters of Queensland over the summer of 2010-11, scientists report.

A Sydney researcher, Jason Evans, ran a series of climate models and found above average sea surface temperatures throughout December 2010 increased the amount of rainfall across the state by 25 per cent on average.

While the study did not look at the cause of ocean warming in the region, a physical oceanographer, Matthew England, said climate change could not be excluded as a possible driver of this extreme rainfall event.

Advertisement: Story continues below

Between December 23 and 28 many places experienced up to 400 millimetres of rain in a few days. "That [means] 100 millimetres of rain was attributable to sea surface temperatures," said Dr Evans, a future fellow at the University of NSW's Climate Change Research Centre.

While the flooding occurred during one of the strongest La Nina events on record it was insufficient to produce the extreme rainfall recorded, he said.

The effect of the high sea surface temperatures coupled with the impact of a La Nina, both of which are associated with above average rainfall over eastern Australia, plus tropical cyclone Tasha, combined to create an extreme weather event, he said.

The resulting floods stretched across 1.3 million square kilometres all the way to Brisbane, caused billions of dollars in damage and killed 35 people.

Matthew England, who was not involved in the study, said ocean temperatures off northern Australia were the highest on record at the time of the Queensland floods.

"While the La Nina event played a big role in this record ocean warmth, so too did the long-term warming trend over the past 50 years," Professor England, the co-director of the UNSW Climate Change Research Centre, said.

To measure the extent high sea surface temperature contributed to the rainfall, Dr Evans used a regional climate model to compare the effect of the 2010 December sea surface temperatures with the sea surface temperatures from previous La Nina events.

"All the simulations produced high rainfall, but only the simulations with the high sea surface temperatures [from December 2010] were able to produce extreme precipitation," said Dr Evans, whose findings are published in the journal Geophysical Research Letters.

Warmer sea surface temperatures increase the amount of moisture transferred from the ocean to the atmosphere.

"If you've got the right winds they carry this moisture to land, and [because] land is hotter than the ocean during the day it will cause convection and rain," Dr Evans said.

While both simulations underestimated the amount of rain that fell in Queensland in December 2010, the model that used the higher sea surface temperatures came closest to the rainfall recorded in the region.

"The model doesn't replicate the observations perfectly but it clearly shows what we saw, which was from Cairns to south-east QLD all [regions] received abnormally high precipitation," he said.

If increases in sea surface temperatures can be attributed to global warming, the probability of La Nina events producing extreme rainfall in the future would also rise, he said.