Could a fourth La Niña follow Sydney’s record-breaking rainfall?

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Sydney’s Observatory Hill has recorded the city’s rainfall since 1858 but shortly after 1pm on Thursday – on another day of torrential rain – a record was washed away.

The wettest year ever for Sydney had been 1950, when 2,194 mm fell. But with almost three months still to go, that record sank.

“It’s not like we’ve just scraped in,” says meteorologist Tom Saunders, the ABC’s weather forecaster for Sydney. “The record has been obliterated. I’ve never seen anything like it.”

So why has Sydney been so wet this year? Has the climate crisis made things worse? And could a fourth La Niña be on the cards?

How wet has it been?

As well as breaking its annual record this year, the rain gauge at Observatory Hill also delivered the wettest March and July on record.

Sydney airport also broke its annual record on Thursday and by the end of September weather stations at Randwick, Bankstown and Lucas Heights had already beaten their annual records.

The rain in Sydney has been persistent, punctuated by torrential downpours that have pushed up river heights and flooded homes, suburbs and put roads and bridges underwater.

There has been rain at Observatory Hill on about 160 days so far this year. Spring, usually the driest season, has been very wet.

Why all the rain?

Three “climate drivers” have combined at different times of the year to increase the chances of rain. But the climate crisis may also have played a role.

Bookending the beginning and end of this year is La Niña – where strengthening winds across the equatorial Pacific push warmer ocean water to the continent’s north-east.

That warmer water increases the chance of rain over northern and eastern parts in spring and summer.

Twice this year Australia has been in a negative phase of the Indian Ocean Dipole (IOD) that sees warmer waters to the continent’s north-west, making more moisture available to fall as rain.

Dr Andrea Taschetto, a Sydney-based climate scientist at the University of New South Wales, says individual weather systems act on top of the IOD and La Niña.

“La Niña sets the scene, but it comes down to the weather systems like the low pressures that can have a lot of power in them to bring heavy rain,” she said.

A third phenomenon, known as the Southern Annular Mode (SAM), relates to the position of the winds to the south of the continent. In a positive phase, the winds are farther south.

Saunders says SAM has been key for Sydney, as it tends to reduce the westerly winds and increase winds coming from the east “and they are the ones with all the moisture”.

But what about the climate crisis?

Guardian Australia spoke to several climate scientists who all said a warmer atmosphere, and warmer oceans, can lead to higher rainfall.

But they also said understanding specific events will require detailed modelling studies.

For example, a study of the flooding rains in Queensland during the 2010 and 2011 La Niña found human-caused ocean warming had increased the chance of extreme rainfall.

But statistically there is always a random chance of records being broken.

“We know the background warming of the ocean and the atmosphere has increased the moisture that can bring rain,” Taschetto said. “There are projections showing that in the future there will be an increased frequency of extreme La Niña and El Niño events.”

Some Australian scientists think global heating will bring more La Niña-like conditions in the future.

Could there be a fourth La Niña?

According to BoM, this summer is only the fourth time since 1900 the country has seen three La Niñas in a row, the others being 1954–57, 1973–76, and 1998–2001.

Current predictions are for the current La Niña to end by February next year. Four La Niña events in a row has never happened before.

All the experts Guardian Australia spoke to said it was too early to be able to forecast what might happen across the Pacific next year, but all thought the chances of another La Niña were very low.

Between February and May, models hit a “predictability barrier” because of the changes in ocean temperatures. After that, forecasters get more confident about what is to come.

“[A fourth La Niña in a row] has never happened and I’d say it’s very unlikely,” Saunders said. “But you can’t rule it out entirely.”

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Graham Readfearn

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