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The scale of the devastation caused by flood waters sweeping across vast sections of Queensland and northern NSW is mindboggling.

Some parts of the country have experienced more rain in the past month than they have over the past few years.

But in some ways this shouldn't have come as a surprise. Meteorologists were predicting the wet weather as far back as July 2010, when the early stages of La Niña appeared.

La Niña and El Niño are strong drivers of weather along the east coast of Australia. Understanding these phenomena is complex stuff - sea surface temperature, air pressure and winds all play a role. In simple terms, it's comes down to two pools of water in the South Pacific.

In 'normal' conditions, cold water from the eastern side of the South Pacific Ocean (near Chile) pushes warm water to the west. This warm water evaporates, fuelling the formation of clouds and the odd cyclone.

During an El Niño event, the cold water doesn't push 'hard' enough and the warm patch of water drifts towards the middle of the South Pacific. This results in less rain-bearing clouds forming east of Australia and is one of the reasons why El Niño is associated with drought.

The El Niño of 1997-98 was one of the strongest on record, while the more recent series of smaller but never-ending El Niños of last decade, resulted in large swathes of Australia experiencing drought conditions.

La Niña forms when the cold water in the South Pacific pushes the warm waters hard against the east coast of Australia.

Dr Andrew Watkins, manager of climate prediction services at the Bureau of Meteorology told ABC Science Online in July, "We tend to see wetter than normal conditions over much of the continent, including Tasmania. The wettest areas tend to be northern Australia and inland eastern Australia."

Cyclones are also more likely to cross the Australian coastline.

"The slightly warmer ocean conditions tend to favour tropical cyclone formation further west than normal and this brings them into our part of the world," said Watkins.
La Niña strengthens

Agata Imielska, a climatologist at the Bureau of Meteorology says there were indications that a La Niña event was forming as early as late-June.

For the next six months, the Bureau continued to put out updates showing that this La Niña was not going away quickly; in fact it was getting stronger.

"In September we put out an outlook for October to December which was favouring water conditions over most parts of mainland Australia. Our outlook issued in October, for November to January period, had quite strong odds favouring wetter conditions [over] northern and eastern parts of Australia, which included extensive areas of Queensland," says Imielska.

It may take several weeks of dry weather before the flood threat eases says Tony Weber, an Engineering and Environmental Consultants for BMT WBM and a member of the eWater Co-operative Research Centre.

"With the whole catchment now in a state of complete saturation, any subsequent rainfalls, even though they may not be record rainfalls, will likely result in extreme runoff events as we have seen in Toowoomba and the Lockyer Valley," he says.

"Until sufficient drying of the catchment occurs (likely to be in the order of several weeks with no further major rainfall), there will continue to be a very high risk of further major runoff events."

The latest update from the Bureau of Meteorology, released last week, predicts La Niña will persist well into March. This means the wet conditions aren't likely to let up in the short term, and neither is the flood threat.

There are been many comparisons made during the past 48 hours with the floods that struck Brisbane in 1974. That flood occurred smack in the middle of a series of La Niña events that stretched from 1973 through to 1976 - one of the wettest periods ever recorded across eastern Australia.

But in terms of strongest, you would have to go back a little further.

"If we take a longer perspective (July-December) then 1917 was stronger than 2010, but 2010 was still the second strongest in the historical record," writes Professor Neville Nicholls of Monash University in Melbourne and President of the Australian Meteorological and Oceanographic Society.

Whenever there are extreme weather events there's always the elephant in the room: climate change.

Imielska says recent increases to global temperature may have played a role in this La Niña event.

"In 2010, we had record warmest sea surface temperatures and that has implications upon weather. But understanding what kind of impact this might have on La Niña, and especially this current La Niña, would still take a bit of research," she says.

Nicholls agrees. "[This] La Niña is also associated with record warm sea surface temperatures around Australia and these would have contributed to the heavy rains," he writes. "The extent to which any of this (the floods, the warm oceans, or the very strong La Niña) is linked to global warming is unknown, because the requisite studies to test this have simply not been done yet."
An end in sight

The good news for rain-sodden Australia is that this La Niña will eventually run out of steam. According to Imielska, we should see some relief by April at the latest.

"Historically these events tend to decay over autumn."

But that's as far as the Bureau can predict. It won't be until June that they will know what next summer has in store.

"The question is what will happen beyond autumn, whether this particular event may return to La Niña conditions or transition into an El Niño. Or perhaps one of the options is that we might get neutral conditions."

Which begs the question - is it ever normal?

Imielska says, "It's definitely important to note that Australia is - it's a bit of cliché - a land of droughts and flooding rains. It is what we experience and what I suppose is considered normal for Australia."

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