Drought and floods: what's coming next?

The devastating floods in Queensland may be subsiding, but the weather pattern that caused it is not over - and Sydney could be in the firing line.

Sydney's last major floods were in 1986 and 1988, said the Bureau of Meteorology. In the Liverpool region in 1986, water levels for the Georges River reached the major flood level of 4½ metres while in the North Richmond area in 1986 and 1988, the Hawkesbury River peaked at 12.8 and 14.4 metres, high above the 11-metre major flood level.

In 1986 in Sydney alone, 327.6 millimetres of rain fell in just 24 hours, with the western suburbs experiencing totals of more than 250 millimetres.

That was in between two periods of El Ninos, but the phenomenon that is causing the current devastation is La Nina - or "little girl" in Spanish - during which the Pacific cools, causing more clouds over Australia, PNG and Indonesia.

The phenomenon is set to hit Australia hard for at least two more months before it dissipates, and at an intensity that has rarely been seen in recent climate history, forecasters say.

"It's the strongest La Nina episode since 1974, when Brisbane flooded the last time," said Clem Davis, a former weather bureau meteorologist and visiting fellow at Fenner School of Environment and Society, Australian National University.

"It is probably within the top five events over ... 130 years of records."

His view is echoed by US space agency NASA, which said last week that "this latest event appears to be one of the strongest ones over this time period".

The naturally occurring weather phenomenon usually lasts for about a year and shifts what normally occurs over the Pacific Islands towards eastern Australia, bringing with it more rain.

It is expected to last until early autumn this year and, with moisture in the atmosphere remaining high, above average rainfall will continue to trend and further flooding cannot be ruled out, weatherzone.com.au meteorologist Brett Dutschke said.

"While the sea temperature just off east coast remains high, there's potential for similar flooding in NSW and in Sydney. But it'll be less likely the further south you go, because the moisture is generally less the further south you go."

In Queensland, the weather pattern could see the remnants of a cyclone dump heavy rain on to the flood-affected regions, he said.

La Nina and its opposite phenomenon, El Nino or "little boy", are often said to alternate, with an average of five to six years between each pattern.

In theory, that means when this season of La Nina is over, Australia will get a reprieve from the big wet for a few years.

But hydroclimatologist Associate Professor Stewart Franks from the University of Newcastle believes the phenomena occur in clusters and we could be in for a series of La Ninas that could continue to drench the eastern regions of Australia for years to come.

"That's actually misleading," he said about the estimates about the frequency of La Nina and El Nino.

"Our research has shown that they tend to cluster. For instance, in the first half of the 20th century, we had very frequent El Nino events, with very few La Nina. For eastern Australia, that was a time of drought.

"From 1945 to 1975, the global climate experienced again a natural change - a shift towards a dominance of La Nina. Nearly every other year was La Nina, and that period was associated with devastating floods across Australia."

By the mid-1970s, El Nino became dominant again, with drought in the early '80s, '90s and then a decade of drought in the Murray-Darling Basin, he said.

And in the past few years, two very strong La Nina have taken place, starting in 2007 and 2008 where a series of storms hit the east coast.

"What this indicates to my mind at least is if one looks at the history of Australia's climate variability, it could well be that these La Nina events we've had recently that have wrought such devastation are perhaps the first sign that we might actually have to expect more frequent La Ninas and more frequent flooding just like we did between 1945 and 1975."

Climate change has also been mooted as one of the reasons for the severity of the floods, but Associate Professor Franks and other climate scientists have been quick to point out that researchers have yet to discover any link between it and the two weather patterns.

"You've got natural variability and you've got what global warming may be impacting," said Mr Davies, noting that recent climate data in Australia go back only to 1876.

"The records aren't long enough, so it's hard to see which is impacting on the other. [That's why] researchers are looking at paleoclimate records to see what the cycles may have been in the past."

If there is any fundamental change, it is the intensification of climate patterns, said environmental scientist Chris Cocklin of Queensland's James Cook University.

"You've got to be very careful about saying that ... the intensity of La Nina ... is a product of climate change. But more intense weather patterns is certainly one of the strong predictions of climate science.

"If you look at one of the significant predictions throughout many parts of Australia - it's that rainfall will become more intensified. So all of that will add up to patterns that we have got to get used to."

Weatherzone.com.au is a Fairfax Media company.

Follow Environment on Twitter