Coverage of wild weather is fast eclipsing Covid-19 as the news du jour, with a flurry of warnings being issued across the country in recent weeks. As Alice Springs contends with the aftermath of its biggest downpour in 20 years, and New South Wales, Queensland and South Australia brace for more wild weather, the Bureau of Meteorology (BOM) says there is a 70 per cent chance of La Niña forming, and being officially declared, in the coming months – which could make our collective vision of a hot vaxx summer a little soggier than we hoped for.

What to expect
BOM says that from November to January we can anticipate rainfall above the median for most of the country’s eastern states and in parts of eastern WA – and November threatens to be particularly sodden.

This summer, parts of southern Australia are three to four times more likely than usual to experience rain in the top 20 per cent of historical downpours (so don’t forget to clean your gutters). In true Melbourne style, parades will likely be rained upon.

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While La Niña hasn’t yet been declared, Andrew King – a senior lecturer in climate science at Melbourne university – says, “During La Niña events we tend to see a shift in weather patterns, with more low-pressure systems that bring rainfall and more tropical moisture over most of Australia. This means over northern and eastern areas of the continent we’re more likely to see events like the current heavy rains affecting much of the east of the continent.”

As the waters of the tropical Pacific continue to cool, the chance of a La Niña event being confirmed increases – but even if those thresholds aren’t met, BOM is still predicting a few months of pesky precipitation.

Not only is it likely to be wet, wet, wet, but temperatures across the country may also be affected. For eastern NSW and southern Queensland, it’s looking like daytime temperatures will be lower than average, while Melbourne and the far south-east can look forward to some warmer than usual days. That goes for most of Western Australia and parts of the north-east coast as well. King says the effects of La Niña are negligible in Perth and little use in predicting summer weather.

The net result of all of this is likely to be unusually sultry conditions in typically temperate Melbourne, and more wet and chilly summer days in Sydney and Brisbane between now and the end of January.

Is this for sure?
Climate models around the world have upgraded their La Niña forecasts, but recent oceanic and atmospheric observations haven’t yet reached the levels necessary to officially declare formation of the event. But the patterns being noted in the Pacific, BOM says, are likely to cause above-average rainfall for most of our summer, even if technical thresholds aren’t met

What exactly is La Niña anyway?
Both La Niña and her more famous brother, El Niño, are part of a natural-forming weather cycle called El Niño-Southern Oscillation. It originates in the equatorial Pacific Ocean but can have wide-ranging effects on weather around the globe. Those effects can be mild or severe, or not occur at all. La Niña usually means increased rainfall across much of Australia, plus warmer overnight temperatures and cooler days. It can also mean greater risk of cyclones and an earlier start to the monsoon season.

Scientists don’t really know what triggers the phenomena, but essentially their formation depends on the surface temperature of water in the Pacific, the atmospheric pressure above it, and how the trade winds push both around. It’s said that El Niño was named by South American fishermen in the 17th century who occasionally noticed warmer waters on the east coast around the end of the year. They named the phenomenon “the boy” – as in Jesus – because it showed up at Christmas time. La Niña, (“the girl”), was a term coined much later to describe the opposite conditions, after Norwegian meteorologist Jacob Bjerknes convincingly linked the oscillating pressure patterns in the Pacific with global weather outcomes in the 1960s.

Hang on, didn’t we have a La Niña event last year?
Yes, last summer was terrible, and yes we have La Niña to thank for it. It’s fairly unusual to have La Niña form two years in a row (the last time it happened was in 2010). On average both El Niño and La Niña occur every two to seven years – but we have no way of knowing when either will appear.

Is it getting worse?
According to BOM, as global warming continues to impact Australia’s climate, recent decades have seen a trend towards high-intensity wet weather events regardless of La Niña.

Is summer cancelled?
King reassures us that “in general, the effects of La Niña are clearer away from the coast, so for major cities the effects are a bit weaker”. The other thing to keep in mind, he says, “is that we have other influences on our weather in Australia, so not all La Niña events follow this pattern. La Niña loads the dice towards having generally wetter conditions across eastern Australia, but it doesn’t guarantee a very wet summer.”