A La Nina watch has been issued by the Bureau of Meteorology for the first time since February 2018, strengthening evidence that Australia may be heading out of drought.
"La Nina basically means, for Australia, an increased risk of rainfall — particularly in central, eastern and northern parts of the country," said Andrew Watkins, the head of long-range forecasting at the BOM.
A "watch" means the chance of La Nina forming this year is around 50 per cent — roughly double the average likelihood.
"La Nina is a cooling in the central and eastern tropical Pacific Ocean," Dr Watkins said.
"When that couples with the atmosphere, we start to see some global impacts, including those in Australia."
Along with the chance of increased rain, La Nina also increases the chance of flooding and cyclones in Australia.
"Historically, roughly half the La Nina events saw some degree of flooding," Dr Watkins said.
"We want to make sure people are aware that something could happen, so it's time to start thinking about what you might do in La Nina."
La Nina can be a drought-breaker
La Nina is crucial in ending droughts in Australia, according to new research co-authored by Andrew King, a climatologist from the University of Melbourne.
"La Nina seasons are just a lot wetter on average than either El Nino or neutral seasons," Dr King said.
El Nino seasons see cooler Pacific Ocean temperatures closer to Australia, leading to decreased chance of rain.
Dr King looked back through more than a century of climate data to show drought in Australia is strongly linked to an absence of the two main rain-bringing climate drivers: La Nina and a negative Indian Ocean Dipole (IOD), which is a La Nina-like condition in the Indian Ocean.
"Drought-breaking rainfall is considerably more likely to occur during a La Nina season than either an El Nino or ENSO-neutral season," Dr King said.
"With La Nina we tend to have a lot more moisture available and more rain bringing weather systems.
"It's just having a lack of those conditions associated often with La Nina that really helps droughts develop in Australia."
12 consecutive dry seasons
According to Dr King, Australia's last really wet winter was 2016 — the fourth wettest ever for the Murray Darling Basin.
That winter saw a strong negative IOD.
Since then, there have been no significant negative IOD or La Nina events, except for a weak La Nina over the summer of 2017-18.
At the same time, the Murray Darling Basin saw 12 consecutive seasons of below-average rainfall — the longest such period since 1900.
The basin experienced three dry winters, in 2017, 2018 and 2019 — the longest dry run since 1900.
"There haven't been many signals at all indicating above-average rainfall for anywhere in particular over Australia, at least for the last two years," the Bureau's Dr Watkins said.
"But this year? Yes."
Dr King said that "this Autumn was actually slightly wetter than normal in the Murray Darling Basin".
"And that broke the record-breaking run of dry seasons," he said.
According to the Bureau of Meteorology, a wetter than average first five months of 2020 has eased the severity of short-term deficiencies over much of eastern Australia and has provided a better start to the winter cropping season in many regions.
Many big droughts coincide with no La Nina or negative IOD
The more Dr King delved into Australia's weather records over the past 100 years, the more he found many major Australian droughts, including the Millennium and World War II droughts, coincided with several of the longer-lasting periods when La Nina and negative IOD events did not occur.
With this in mind, the climate scientist welcomes the increased chance of a return of La Nina.
"It's not a guarantee, but it's good news for sure," he said.
"It should mean it increases the likelihood of having wetter than normal conditions across large swathes of eastern Australia, which have been in drought."ABC