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La Nina is here – it’s going to bring a big summer weather change

After years of one of our main climate drivers doing – well, not a whole lot really – suddenly the El Nino Southern Oscillation (ENSO) has woken from its slumber. And that could have a big effect on weather for months or even years to come.Today, the Bureau of Meteorology (BOM) officially declared the ENSO…

After years of one of our main climate drivers doing – well, not a whole lot really – suddenly the El Nino Southern Oscillation (ENSO) has woken from its slumber.

And that could have a big effect on weather for months or even years to come.

Today, the Bureau of Meteorology (BOM) officially declared the ENSO cooling phase, better known as a La Nina, was active.

The last La Nina event was declared in 2010.

It comes several weeks after the US also declared a La Nina was present during August that was likely to persist all through the northern winter.

What gear the ENSO is in – La Nina, neutral, or El Nino – can mean the difference between devastating droughts and much needed rains.

“The (ENSO) has in the most part been in a neutral state since early 2016 although we have seen brief periods of La Nina and El Nino, but not for long enough to be classified as an event,” Sky News Weather senior meteorologist Mr Saunders told news.com.au.

A La Nina generally means above average rain for northern and eastern Australia for the second half of the year, often spreading far inland. More cyclones are also likely with Yasi appearing in a La Nina year.

It’s often cooler but heatwaves can be muggier due to the moisture.

However, Western Australia is likely to be impacted far less.

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ENTERING LA NINA CONDITIONS

The ENSO is a measure of sea surface temperatures and winds in the Pacific Ocean. It’s a climate driver that impacts not just us, but also the Asia Pacific and the Americas as well.

A La Nina or El Nino is called when the sea surface temperatures in a defined chunk of the Pacific Ocean – called Nino 3.4 and located 4000km east of Australia – differ markedly from the average. The colder the water gets the closer a La Nina comes.

The US declared La Nina earlier than Australia, but it has a different threshold to meet. However, Australia’s tougher milestone of temperatures 0.8C below normal has now been met.

RELATED: Spring weather could be wetter, cooler

RELATED: Nino 3.4 – Pacific Ocean patch that could change Australia’s weather

A La Nina sees cooler waters from the depths of the ocean hauled up to the surface in the central and eastern equatorial Pacific region. Stronger winds from east to west then push warmer seas closer to Australia. That aids in the creation of more clouds and so moisture and windier conditions for the continent.

It’s the reverse for the Americas where moisture is sucked away from the continent which is one of the reasons behind the US’ terrible recent fires.

A neutral ENSO phase, the one we’ve just left, can lead to average conditions but also leaves the continent susceptible to the vagaries of other climate drivers.

Last year saw one of the strongest Indian Ocean Dipoles (IOD) – similar to ENSO but on the other side of Australia – on record and that had a big role to play in the horror bushfire season.

“It was so dry because there was a positive IOD. But now that’s been reversed so we have a negative IOD and we have a developing La Nina,” Mr Saunders told news.com.au last month.

“It’s the polar opposite of what we had last year.”

THE EFFECT OF 2020’S LA NINA

One thing to note is that this could be a weak La Nina.

“It is only just passing thresholds so presently is not strong,” said Mr Saunders.

“A La Nina will normally peak in late spring and early summer then weaken by autumn although many La Nina are back to back events which last for two years.”

He reiterated that the Indian Ocean also has a big role to play.

“The (IOD) is currently neutral but the index is on the negative side, which is the wet side. So it’s currently helping the La Nina produce rain across Australia but to a lesser extent than was the case in 2010.”

The most notable effect of La Nina is that this spring weather is likely to be very different to 2019.

New South Wales, which was 100 per cent in drought in January, is now only 35 per cent in drought conditions due to the already wetter weather.

Across much of the south east of Australia expect more rainfall than average.

That’s good news for dam levels and for farmers; but if there’s too much rain that could lead to flooding which will destroy crops.

It should be wetter in the tropical north east too but that could lead to more cyclones.

There could also be more tropical cyclones around the Kimberley and Top End.

More rain means more clouds which in turn should lead to cooler days, as the sun is blotted out, but also warmer night as daytime heat is trapped. Heatwaves are still a possibility. They might be less intense but could be muggier.

Much of Western Australia, aside from the extreme south west and north west, can count itself out of any unprecedented precipitation.

Most long term forecasts suggest much of the moisture will come to a screeching halt somewhere in the middle of the Nullarbor, depriving areas west of extra rain.

Around Broome and Port Hedland and there’s a very good chance it will be drier than usual for October onwards.

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