SCIENCE

La Nina Brings Flooding to Australia

January 5, 2011

If you think everything is bigger in Texas, then you've never been to Australia, where an area as large as that U.S. state is now under water. And flooding across this section of the state of Queensland, north of Brisbane, could last for weeks. The culprit? La Niña.

La Niña is the cold water counterpart of the more familiar El Niño, which brings plenty of winter rain to the southern half of the United States. During a La Niña event, the eastern equatorial Pacific is colder than normal and that band of cool water extends farther to the west. That change in sea surface temperature brings changes in atmospheric pressure, rainfall and atmospheric circulation. During a La Niña event, the weather patterns are almost opposite of an El Niño. In the U.S., that means that it's usually drier in the southwest and colder and wetter in the northwest. But in Australia, which experiences more drought and accompanying wildfires with El Niño, La Niña brings rain to the northeast during December, January and February.

But Queensland is suffering from more than just La Niña, says New Scientist. There are two factors at play here:

The first is a strong La Niña event, an interaction between the atmosphere and the oceans: "The strongest we've had in several decades," . In 2002, an El Niño cooled Australia's eastern coastal waters, resulting in severe droughts and wildfires. The La Niña, on the contrary, is now trapping warm water at the coast, fuelling cloud development and rainfall, says Nott.

Plus, there is a low-pressure zone over Queensland, he says. Monsoonal troughs are pressure troughs that seasonally enter the southern hemisphere, dragging moisture from north-westerly monsoonal winds into the region. This encourages heavy rain, too. "With the La Niña and monsoonal trough, conditions were absolutely set for these floods to occur," says Nott.

El Niño and La Niña events occur about once every three to five years, with El Niño lasting about 9 to 12 months and La Niña one to three years (so it may be wet in Queensland for some time). This natural cycle has been going on for thousands, perhaps millions, of years. However, there is evidence that climate change may be making the El Niño events more intense and more frequent.