Explainer: what is La Niña?

The Bureau of Meteorology has officially declared a La Niña event is happening this summer. So what is a La Niña event, and how do we know it’s happening?

What is La Niña?

La Niña, and its counterpart El Niño, are large-scale weather events that happen in the Pacific Ocean.

El Niño was originally named by Peruvian fishermen when they noticed the warmer currents around their coast. It means ‘little boy’ in Spanish, and La Niña means ‘little girl’.

There is normally a large body of warm water in the Western Pacific, to the north-east of New Guinea. Sea-surface temperatures there are some of the highest in the world, and – as a result – a lot of water evaporates in that area and leads to precipitation around the Western Pacific.

In a La Niña season, this body of water becomes even warmer, and the waters around northeastern Australia heat up more than usual. This means that more water is evaporating and turning into precipitation around eastern Australia.

By contrast, the eastern side of the Pacific sees cooler water than usual in a La Niña season, meaning that there are drier conditions on the coasts of the Americas.

“The trade winds are stronger during a La Niña,” says Dr Nina Ridder, a researcher at the Centre of Excellence  for Climate Extremes at the University of New South Wales. Trade winds are winds that usually blow east-to-west around the equator.

“What is happening is that the temperature difference between east and west over the Pacific increases, so we get stronger winds.”

El Niño is when the opposite event happens: water by the Americas heats up, and Western Pacific water cools down. This causes drier conditions in Australia, and wetter on the other side of the Pacific.

How do we know it’s happening?

The Bureau of Meteorology has suspected a La Niña was on the way this summer, with a watch for it declared in September and an alert in October. How can meteorologists and climatologists tell what’s happening?

“We base them on sea surface temperature, so the temperature of the surface ocean in the Pacific and the equatorial Pacific,” says Ridder.

“There are specific regions that we observe and that we measure, and if these regions show higher than normal temperatures, then it’s La Niña, and if it’s lower than normal temperatures it’s a El Niño.”

These measurements are mostly taken through satellite data, but sometimes buoys on the ocean are used to make conclusions as well.

The BOM uses seven different modelling systems to make its La Niña predictions, one of which was developed in-house, with the other six coming from other countries.

This is the second La Niña summer in a row: there was also a La Niña event from September 2020 – February 2021. This La Niña is expected to continue until at least the end of January 2022.

How will it affect different parts of Australia?

Eastern Australia can expect more rain and windy conditions over summer, and an increased risk of floods. It’s also likely that there will be more tropical cyclones than usual in northeastern Australia.

Because there will probably be more rain, there’s likely to be fewer bushfires than a typical summer.

“It does also mean we get cooler temperatures for Australia, particularly in parts of eastern Australia in the south, overall,” says Dr Andrew Watkins, head of operational climate services at the BOM.

“But we can still get heat waves. And the heat waves we get over summer tend to be longer. Even if they’re not as extreme they do they tend to be longer and more humid.”

In Western Australia, La Niña has a much smaller effect.

“The El Niño Southern Oscillation (ENSO) does not have as large an influence on weather in southwest Western Australia as compared to Eastern Australia,” says Dr Jatin Kala, a senior lecturer in Atmospheric Science at Murdoch University.

“Other modes of natural climate variability can have an effect on rainfall in southwest WA, but it’s a mixed bag.”

He adds that La Niña can affect oceanic currents around Western Australia – particularly “the Leeuwin current, which flows south along the WA coast from Indonesia, bringing warmer waters further south”. This has caused marine heatwaves in the past and could again.

How strange is this?

Not very. La Niñas frequently come in pairs. In fact, “back-to-back La Niña events are not unusual, with around half of all past events returning for a second year”, according to Watkins. Doubling up doesn’t necessarily mean that they’re going to be stronger or weaker, though.

This La Niña is not expected to be particularly strong, and could even be weaker than the 2020-21 event.

However, because things are already wet, it’s possible that floods will be more intense.

“Getting a La Niña after having a very wet spring will increase the flood risk,” says Ridder.

“Not only do we get more precipitation, but […] the soil moisture is saturated, so the soil can’t take up more water. So if we now add water on top of that, flooding becomes a lot more likely.”

Is climate change affecting it?

La Niña and El Niño are natural phenomena that have cycles every 3–7 years on average. But climate change is certainly affecting their patterns and frequency.

There’s not clear consensus on whether we can expect more or fewer La Niñas as the Earth warms, with one recent large-scale review stating that the events will get more intense, but other smaller models contradicting that.

La Niña and El Niño are both only part of the weather story in Australia – with other oceanic events like the Indian Ocean Dipole, the Southern Annular Mode and the Madden-Julian Oscillation all having different effects on the weather too.

As the Earth heats up, it can be difficult to tell how these events are all going to interact with each other – but things are definitely going to change.