The Bureau of Meteorology (BOM) has held off declaring a La Niña event in its latest Climate Driver Update, despite other countries around the world recognising La Niña to be already in effect.
The Bureau's ENSO Outlook remains on La Niña alert, indicating at least a 70 per cent chance of La Niña reforming later this year.
"We're seeing a developing La Niña situation out there in the tropical Pacific, adding to the negative IOD (Indian Ocean Dipole) and the positive SAM (Southern Annular Mode) that's out there as well, keeping your outlooks quite wet," BOM head of long-range forecasts Andrew Watkins said.
He emphasised the need to go and look at the actual climate outlooks themselves.
"It's staying very wet for the eastern half of Australia, and even down into the northern and eastern parts of Tassie as well, " Dr Watkins said.
"We've already got wet soils, rivers, full dams as well.
"So we're really emphasising people keep in mind those potentially wet conditions over spring are going to keep the flood risk elevated."
La Niña overseas, but not in Australia
La Niña has already been declared by the United States, Japan and India, among other countries.
Australia has held off in part because of differing definitions of the phenomenon.
Most other nations require sea temperatures to be half a degree Celsius below average in the eastern tropical Pacific Ocean before a La Niña event is declared.
The BOM generally requires sea temperatures to be 0.8C below average before it declares an event.
The latest sea temperatures in the region are 0.7C below average.
But Dr Watkins says the BOM looks at more than just sea temperatures when calling a La Niña event.
"We focus on several things — winds, oceans and also the atmosphere as well, but also what the outlooks are saying," he said.
But there is another reason why Australia may have held off calling a La Niña longer than other countries.
Climate change has warmed the oceans where La Niña is measured by as much as 0.3C since the 1960s and Australia does not completely factor that in.
Other countries, including the US, factor the warmer average caused by climate change into their calculations.
Australia still uses 1961-1990 sea temperatures to define the average, meaning oceans have to be even more abnormally cool before the BOM declares a La Niña event.
"We still do use an older base period," Dr Watkins said.
"That was the official World Meteorological Organisation base period until fairly recently.
"We're certainly looking at at the moment about how we can make changes to update all our climate data sets to use a newer, more modern base period."
He said the BOM used expert assessments to factor climate change into La Niña events.
"We do discuss how the impacts of climate change may have made our values a little bit short of what the threshold is," he said.
Australian National University climatologist Professor Nerilie Abram said the way climate variability was monitored had been affected by climate change itself.
"We need to be thinking about that, adjusting for that, when we make declarations as to whether we're in an El Niño event or a La Niña event, because we've got that change in the background climate," she said.
Soggy summer driven by more than La Niña
A very strong negative Indian Ocean Dipole (IOD) event is adding to the risk of increased rain going into summer.
"This event looks to be shaping up to be one of the strongest negative IOD events on record, potentially rivalling 2016," Professor Abram said.
"That looks set to also be causing enhanced rainfall in Australia for the coming months — and that will be in addition to those impacts that we have from La Niña."
Triple La Niña events have only happened three times since 1950, according to the BOM.
The phenomenon was first recorded from 1954 to 1957, again from 1973 to 1976, and from 1998 to 2001.
The historic Maitland floods of 1955 and the Brisbane floods of 1974 occurred during multiple La Niña events.
During the most recent triple La Niña, from 1998 to 2001, there was widespread heavy rain over much of the country for the eight months from October 1999 to May 2000.
The calendar year 2000 was the second wettest year since 1900, with an Australia-wide average of 698mm. Only 1974 (760mm) was wetter.
The possibility of a La Niña should prompt those living in flood-prone areas to do their utmost to prepare, Natural Hazards Research Australia chief executive Andrew Gissing said.
"It's a really good idea to sit down with your family members or your employees and figure out what you're going to do, because the recent floods should be right at the forefront of people's minds," he said.
"For those that don't know whether or not they are in a floodplain, check out whether or not they do have a flood risk or not."
My Gissing said insurance losses from flooding this year have already exceeded $5 billion, the second-most expensive in the history of Australian national disasters.
Climate change fuelling La Niña rainfall
The recent devastating floods in Pakistan and the drought in Europe have been linked to La Niña, according to climatologists who say climate change is fuelling the extremes.
"Instead of having moderate rains spread out through the monsoon season, we now have long dry periods … with short spells of heavy rains," the Indian Institute of Tropical Meteorology's Roxy Mathew Koll said.
He said the India-Pakistan-Bangladesh region was witnessing a clear trend in rising heatwaves, floods, landslides, droughts and cyclones, which was affecting the food, water and energy security of the region.
"Unlike Australia, the population in these countries is quite large – close to two billion – and so is the vulnerability and exposure," Mr Koll said.
Professor Abram said Australians should expect bigger swings between wet and dry in the future as climate change intensifies the rainfall and droughts associated with natural cycles such as La Niña and the IOD.
"Even as our climate warms, those processes of swinging between extremes from one year to another are still going to be going on," she said.
"But one thing that we do think we can expect in the future, and we're probably already seeing some of this starting to play out, is that … the rainfall impacts associated with those swings will be more extreme than what we've experienced in the past."
