When will eastern Australia’s wet weather end and will we get a La Niña ‘three-peat’?

The Bureau of Meteorology now says the La Niña event in the Pacific will not break down until late autumn, or almost two months later than originally forecast.

And there is a chance that another La Niña will emerge by next summer. That’s not the kind of outlook flood-hit or rain-soaked regions of eastern Australia want to hear.

So, what’s going on with this La Niña and when will the rainy weather finally end?

Current forecasts

The swings between La Niñas and El Niños in the Pacific, with neutral spells in between, are the biggest drivers of short-term global climate variability.

During the La Niña phase – which we’re in now – there’s a cooling of the ocean surface in the central and eastern tropical Pacific. The normal easterly blowing winds along the equator are stronger than usual, pushing rainfall patterns into eastern and northern Australia (and away from California).

The latest model runs used by the Bureau of Meteorology point to La Niña lasting through May, and possibly into winter.

Andrew Watkins, the head of the bureau’s long-range prediction services, says the original forecasts were for the La Niña to start breaking down in the late summer and be gone by early to mid-autumn.

“Now we’re saying late autumn,” Watkins says. “It certainly has had an injection of life over the last couple of weeks from those stronger trade winds.”

Staying relatively wet in the east

The Pacific is not the only influence on the weather for an inland continent like Australia. An extended La Niña, though, does tilt the odds in favour of above-average rainfall for eastern and northern parts of the country.

The bureau’s latest outlook, updated on Thursday, reveals there is an increased chance of unusually high rainfall for the April-June period – in the top 20% of historical records, no less – across the northern tropics and most of the rest of Queensland, and small areas of western and coastal New South Wales. In fact, 1.5 to 3.5 times the usual chance.

Now many of those areas do not typically get a lot of rain during this time of year so it may not take a lot to be above normal.

Still, as Watkins says, for areas that are sodden or have lately been flooded, above-average rain could be bad news.

“Even if the rainfall might not be as extreme as what we’ve seen more recently, the landscape is now very wet, the rivers high, and the dams full,” he says. “So even relatively small amounts of rain are something to be cautious of.”

The big floods from late February and early March in south-eastern Queensland and coastal NSW left a damage bill in the billions. Insured claims have already reached $2.22bn as of Thursday, and counting, the Insurance Council of Australia says.

La Niña periods also tend to have more cyclones than in neutral years, so an extended phase might see a bit more tropical storm activity before the season is done.

Could the back-to-back La Niñas become a ‘three-peat’?

This summer’s La Niña was the second in a row, and both have combined to break the severe and prolonged drought over most of eastern Australia.

And so, we should see the back of it soon, and look forward to drier times, at least where it has been wet. (Western Australia and western Tasmania wouldn’t mind some of the rain sent their way.)

Or could we get three La Niñas in a row?

Watkins notes we have had La Niñas in three consecutive years before, such as in 1954-57 (with some very big floods in the Murray River), 1973-76 (some very muddy football seasons) and 1998-2001.

“At the moment it doesn’t look like there’s a strong chance of there being a triple La Niña event,” Watkins says. “But you can’t rule it out.”

‘Predictability gaps’

Ruling it in or out just now is actually not so easy.

As it happens, we are coming to the so-called “autumn predictability gap” (or spring gap, if you’re based in the northern hemisphere).

It’s the time of the year when the Pacific and other regions enter a reset mode with temperature differences across the ocean basins relatively small.

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“Basically it means that there’s not a strong push one way or another,” Watkins says. “Small influences have the greatest effect at that time of year because the [sea-surface temperature] gradient isn’t particularly strong.”

Most models the are not pointing to a triple-header La Niña, although one US model does.

The probability of a La Niña reforming after the current one breaks down ranges between 44% to 47% during the second half of the year, according to the Climate Prediction Center/International Research Institute model.

Other regions to watch

The Pacific does call many of the weather shots, at least for eastern and northern Australia from spring into summer. But it is not the only influence and sometimes it is also not the dominant one.

The Southern Ocean, for instance, influences how far north or south storm tracks move.

A positive phase of the so-called Southern Annular Mode, as we have seen during much of the past summer, means drier conditions in southern Australia. Tasmania’s summer was the driest since 1980-81.

Come winter and into spring, the Indian Ocean can be the main driver of how much rainfall finds its way across the continent and soaks – or parches – south-eastern Australia.

The bureau stresses the five models it uses to predict changes in the Indian Ocean Dipole – which tracks the temperature gradient across the tropical regions – have low accuracy beyond autumn.

For the limited skill they have, though, the models currently indicate the IOD will be in its negative phase by next winter.

Should that transpire, we might well see above-average rainfall patterns extending well into 2022.

For some, then, the rainy season may have a ways to go yet.