This year's La Niña is unusual in that "it extends for a great distance across the equatorial Pacific basin," with cold water stretching west almost to Indonesia, Kirtman said. "What that does is pushes or shifts where the rainfall distribution is, so it rains even harder than it normally would over Australia."
According to the Australian Bureau of Meteorology, last month marked the wettest December on record for Queensland and for eastern Australia as a whole.
"La Niña doesn't necessarily predict flooding," said Michelle L'Heureux, a meteorologist at the U.S. National Oceanic and Atmospheric Administration's Climate Prediction Center in Maryland. "What we're seeing there is exceptional."
La Niña on steroids
But Kirtman said scientists aren't sure how climate change is affecting El Niño and La Niña, which comprise a naturally occurring weather cycle known as the El Niño-Southern Oscillation (ENSO). What is clear is that the Earth is warming, and a warmer atmosphere can hold more moisture, which means rainfall events are predicted to intensify, on a general basis, as the planet heats up.
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., told Reuters that waters off northern Australia are more than 1.5 degrees Celsius warmer than they were before 1970 -- and a third of that warming can be attributed to climate change.
"The extra water vapor fuels the monsoon and thus alters the winds and the monsoon itself and so this likely increases the rainfall further," he said.
Kirtman said there is also ongoing discussion about whether climate change is changing the behavior of the El Niño Southern Oscillation.
The Intergovernmental Panel on Climate Change said in 2007 that computer model projections did not indicate the weather cycle's strength or frequency would alter during the 21st century. But some scientists, Kirtman among them, now suggest that ENSO may shift as climate change intensifies.
"One theory out there is that we're going to El Niños and La Niñas that are centered further out toward the western Pacific," he said. "What that means, if it turns out to be true, is that Australia will get a stronger response with El Niños and La Niñas than it has gotten in the past."
Kirtman said there are "hints" the shift is occurring, and the current La Niña "would be consistent with this notion that things are shifting towards the west," but major questions remain.
"One is whether it's a real thing -- do we have enough [historical weather] data to detect it?" he said.
"The second is, even if there are these spatial changes ... how confident are we in the evidence that this is changing with the climate? It's clearly a hot topic, with no consensus at the moment."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500