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Why does warm water near Peru mean we need to crank up the air conditioner?
You might hear talk of El Niño and La Niña when people discuss the weather – particularly in the summertime.
So what exactly are they – and why do they have such a big influence over our weather? And is there anything else that affects Australian weather? Cosmos explains.
What are El Niño and La Niña?
El Niño and La Niña, also called El Niño Southern Oscillation, or ENSO, is a pattern that appears in the water and air around the Pacific Ocean.
The western Pacific Ocean has a large body of warm water — often the warmest in the world — to the north of Australia.
The “Indo-Pacific Warm Pool” as it’s known, has surface temperatures that are always above 28°C and is sometimes called the ‘heat engine’ of the globe. Because warm water evaporates best, the area of the warm pool also features giant convective clouds which can reach altitudes up to 15 km and generate much latent heat.
Over on the eastern side of the Pacific, the water is usually a bit cooler, because of the way the Earth rotates, trade winds blow warm air and water to the west, and leave the eastern side to suck in cool air from the north and south poles.
Sometimes, the trade winds get more intense, and the temperature difference between east and west gets even more extreme. When there is more warm, damp air over eastern Australia – the continent gets more rain, floods and cyclones. Over in the Americas, it’s drier than usual. This is what’s called a La Niña year.
In other years, the trade winds break down and the warm water in the west is able to flow back to the east. Then, that warm water makes it rain over the Americas, and cold water dries out eastern Australia. This is an El Niño year, and it brings droughts, heatwaves and bushfires to eastern Australia.
Both El Niño and La Niña are natural phenomena. Some years, the Pacific cycle is neutral, some years it intensifies and becomes a La Niña pattern, and some years it reverses for an El Niño.
Generally, both El Niño and La Niña appear over winter and spring, and dissipate back to neutral by the next autumn. But they can persist. In 2022, La Niña hung around until June, leading to record-breaking rains and floods.
How do we know when it’s happening?
Meteorologists look at sea surface temperature to see what the Pacific is up to.
They use satellite data, and some buoys floating in the ocean, to monitor temperature in key regions. If the spots near Australia are warmer than normal — La Niña — and if they’re cooler than normal — El Niño.
That’s also how they get the “strength” of the El Niño or La Niña event: the more extreme the temperatures, the stronger the event.
To predict whether an El Niño or La Niña is on the way, meteorologists feed all of this temperature data into computer models. The Australian Bureau of Meteorology (BOM) uses seven different modelling systems to make its ENSO predictions, one of which they developed themselves.
These predictions come with different levels of certainty. Autumn is the hardest time for weather modellers, because there’s a lot of variable things going on in the atmosphere. By late winter, things will have settled into more of a pattern, and we can expect a final verdict on ENSO each spring.
Why is it called El Niño?
The name itself comes from 17th century Peru. Around this time, Peruvian fisherman noticed that some years, peaking in December, fish would be unusually easy to catch.
The good fishing always came at the same time as certain ocean currents and weather patterns off the coast of South America, which made the fish clump together and easy to net in groups.
They called years when this happened “El Niño de Navidad”, meaning “The Christmas Boy”: the theory was that baby Jesus had given them a Christmas treat of good fishing. This got shortened to El Niño: Spanish for ‘little boy’.
When the counterpart to El Niño was observed, it was named La Niña: ‘little girl’.
What else decides what our weather is doing?
ENSO isn’t the only thing that drives Australia’s weather. In fact, in Western Australia, El Niño and La Niña have very little influence at all – and there are other big weather patterns that play a role across the whole continent.
Over in the Indian Ocean, on the other side of Australia, the Indian Ocean Dipole (IOD) holds sway.
The IOD is sort of like a smaller ENSO. In what’s called a negative phase IOD, there’s warm water near northwest Australia, which drives moist air and precipitation across all of Australia. When it’s in a negative phase, that warm water moves away from Australia and the continent gets drier.
The wet negative IODs often, but not always, come at the same time as La Niñas, and the dry positive phase often happens at the same time as El Niño – meaning the IOD tends to exacerbate whatever ENSO is doing. But this relationship isn’t set in stone, and the IOD can also reduce the effects of ENSO if it’s in a different phase.
Down in the Southern Ocean, the Southern Annular Mode (SAM) plays a role. This is a belt of winds, storms and cold fronts which circle Antarctica. It also has a rainy phase, a neutral phase and a dry phase, but they have different effects depending on where and when you are in Australia, and the phases last for a few weeks rather than a year.
In a positive SAM, the belt of wind shrinks. In summer, this draws moist air from the tropics further down over eastern Australia, making it wetter. In winter, that tropical precipitation doesn’t make it as far south, meaning that northeast Australia stays wet, but southern Australia is drier than usual.
In a negative SAM, the belt of wind strays further north. This makes it drier across most of southeastern Australia in summer. In winter, the storms get far enough north that they hit the bottom of the continent, making it wetter than usual in southern Australia – but the dry air above that makes northern Australia drier.
Finally, the Madden-Julian Oscillation (MJO) can influence tropical weather from week to week in summer. It’s a pulse of wet, stormy weather which travels from west to east across the tropics every month or two, forming over Africa and travelling to the Pacific, where it dissipates. When it’s over Australia, it brings more rain.
What will climate change do to ENSO?
Climate change is generally making it hotter and drier in southern Australia, and hotter and wetter in northern Australia. But it’s hard to unpick exactly what it’s going to do to ENSO, or any of these other climate patterns. Some models suggest there will be fewer neutral years and more El Niños and La Niñas; other models find the opposite.
What is clear is that even the cooler phases of the cycles are warm. La Niña has an overall cooling effect, for instance, but 2022 was still one of the hottest yearson record.