La Niña and monsoonal winds flood northern Australia

Blame the strong La Niña and a monsoonal low-pressure zone for the devastating floods in the Australian state of Queensland that have left towns and cities stranded this week.

In some parts of the state flood waters are now slowly receding, while other areas continue to wait for river levels to crest. The region's Fairbairn Dam remains over capacity, spewing 99,705 megalitres of water over its spillway daily.

At its peak, early on 31 December, the dam was at 176 per cent capacity, with water overflowing at a rate of 373,016 megalitres a day.

 

Boxing day deluge

Pradeep Singh of Australia's Bureau of Meteorology says heavy rains early last month saturated central Queensland and raised stream levels above average. But the final trigger was a deluge on 26 and 27 December, which broke river banks and flooded half of Queensland's 1.1 million square kilometres. Flooding now affects an area larger than Texas, or the size of France and Germany combined.

Jonathan Nott, a geoscientist specialising in extreme natural events at James Cook University in Cairns, Queensland, says pockets of heavy rain are common in the state. But this time the rain was unseasonably widespread and fell over the entire catchment area. "All of the tributaries that flow into the trunk stream, Fitzroy river, have been completely soaked," he says.

According to the Queensland government, some 200,000 people are affected by the floods. Nine flood-related deaths have been confirmed since November.

 

Niña and monsoon

Nott says the heavy flooding was caused by two largely unrelated weather phenomena.

The first is a strong La Niña event, an interaction between the atmosphere and the oceans: "The strongest we've had in several decades," he says. 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.

While no single event can be attributed to climate change, warming seawater "loads the dice" for more frequent extreme conditions such as this to occur, he says.