As Sun Tzu said, you should know your enemy – and there aren’t many more formidable foes than El Niño, the sort-of random climate phenomenon that can devastate everything from natural wonders like marine biodiversity to made-up human concepts like “the economy”.
Unfortunately, our knowledge of this weather pattern has always been spotty at best: we can’t predict it very far in advance, because it doesn’t follow a set schedule; we don’t know exactly what causes it; and every time it happens, it’s different from before. In short, the entire planet is kind of at the mercy of this “little boy.”
But wait! It gets worse – because according to a couple of papers from researchers out of Innsbruck, Austria, this year, we may actually know even less than we thought.
What is El Niño?
You’ve likely heard of El Niño before, but understanding the interactions that underlie it – and, therefore, how our knowledge has changed thanks to these new studies – is a little more complex.
For one thing, it’s only half of the picture, meteorologically speaking. “El Niño and La Niña are two phases of the naturally occurring climate phenomenon called the El Niño–Southern Oscillation (ENSO),” explains Imperial College London’s Grantham Institute.
“[The ENSO] leads to the most dramatic year-to-year variation of Earth’s climate,” it continues. “El Niño is characterised by warmer global temperatures, while La Niña years are typically cooler.”
When and why either of these weather patterns occur is kind of a mystery, even today. We know El Niño is more frequent than La Niña, generally speaking, and both tend to last for nine to 12 months on average – but neither event happens on a regular schedule. The best we can say is that they turn up every two to seven years or so.
“During normal conditions in the Pacific Ocean, trade winds blow west along the equator, taking warm water from South America towards Asia,” explains the National Ocean and Atmospheric Administration (NOAA) factsheet on the phenomenon. “To replace that warm water, cold water rises from the depths – a process called upwelling.”
But “El Niño and La Niña are two opposing climate patterns that break these normal conditions,” the agency continues. During El Niño, the Pacific winds weaken, reducing that upwelling of cold water in the East and pushing warmer tides towards the west coast of the Americas; sea surface temperatures can rise by up to 4°C (7°F) across the Pacific, and atmospheric circulation patterns can be affected on a global scale.
“El Niño causes many changes in weather patterns across the globe,” said Auroop Ganguly, co-director of Northeastern's Global Resilience Institute, in a statement this October.
“It has been called the ‘seesaw’ effect,” he added: the phenomenon often brings more frequent and intense storms over the west coasts of North and South America, while causing droughts in Africa and South Asia.
What drives El Niño?
We may not know precisely what sets off an El Niño event, but scientists have had suspicions for a while as to what may govern the patterns of these boisterous weather phenomena. One of those ideas – the so-called “bipolar seesaw mechanism” – is pretty well-accepted; the other – a link with the sun’s magnetic cycle – is less so. Guess which one the new research supports?
“Our findings… [challenge] the bipolar seesaw hypothesis,” notes one of the new papers, published in The Innovation Geoscience. “While the bipolar seesaw hypothesis is well-supported for the Atlantic sector, its relevance for worldwide millennial-scale climate change remains uncertain.”
The question that the hypothesis is aimed at answering is the cause of climate change on the millennium scale – something that has been “a long-standing challenge in paleoclimate science,” the researchers note. Put as simply as possible, it suggests that it’s changes in the Atlantic Meridional Overturning Circulation, or AMOC – a large system of ocean currents that carry warm water from the tropics into the North Atlantic – which govern climatic shifts in the Southern hemisphere.
“This concept postulates that an AMOC collapse would block northward heat flow, with heat left to accumulate in the Southern Hemisphere,” explain the researchers. “As AMOC stabilizes, northward flow would resume, causing cooling in the Southern Hemisphere.”
But if the hypothesis is correct, the team write, then we should expect climate records from the Pacific to line up with those from the Atlantic. And it turns out, they’re not.
How do they know? Luckily, we have a very good record of the climate history of the Atlantic, in the form of Greenland ice cores. They’re “one of the best tools to reconstruct the climate prior to the instrumental era,” according to Liz Thomas, head of the ice cores team at British Antarctic Survey, providing information on everything from the particular makeup of the atmosphere throughout the millennia to evidence of massive ancient solar storms.
But finding a record of the same history in the Pacific has proven a little more tricky – which is why, instead of ice, the researchers looked to cave deposits known as speleothems for their information.
“We report an independently dated high-latitude speleothem proxy record from Alaska, which provides valuable insights into the North Pacific climate,” the paper reports. “Our findings reveal that this speleothem record is not in-sync with the Greenland ice-core record… [but] aligns with the tropical Pacific [record].”
So, if it’s not a planetary playground apparatus that’s to blame for the millennia of ENSO cycles, then what is it?
The Walker switch
It’s here that we find the Austrian team’s first big result: the existence of what they’ve termed the “Walker switch.”
Named for the Walker circulation – ENSO’s “atmospheric buddy,” according to meteorologist Tom Di Liberto – the proposed Walker switch mechanism puts the blame for ENSO events on two separate, yet intertwined, phenomena. The first is the so-called “ocean thermostat” mechanism: it “infers that if there is heating over the entire tropics, then the Pacific will warm more in the west than the east because strong upwelling and surface divergence in the east moves some of the heat poleward,” the researchers explain.
“Therefore, the east-west temperature gradient will strengthen, causing easterly winds to intensify, further enhancing the zonal temperature gradient,” they write. “This process leads to a La Niña-like mean state in response to increased solar forcing.”
But there were times, the team found, when the climate record didn’t quite match up with what they would expect, if that was all that was going on. Instead, they suggest, this thermostatic relationship might be weakened once a certain threshold gets passed: too much radiation from the sun, they posit, and the surface temperatures even out across the ocean enough for the Walker circulation to become more influential than the thermostat mechanism.
“The ‘Walker switch’ concept helps us better explain the complex interplay of factors that have shaped climate dynamics” in the equatorial Pacific and northern latitudes, said Paul Wilcox, a researcher in the Department of Geology at the University of Innsbruck and co-author of both studies, in a statement.
Of course, it’s only a hypothesis. “We acknowledge that this conceptual mechanism is currently difficult to fully prove,” the authors admit.
“Nevertheless,” they say, “based on the existing evidence, it offers a feasible solution to several climate enigmas.”
Radiation from the sun? Is that what you were talking about before?
Not exactly. See, all that Walker switch stuff was about explaining El Niño on a millennia-long scale – but when it comes to shorter-term patterns, there’s something altogether more sci-fi going on.
A few years ago, a team of scientists from the University of Maryland and the National Center for Atmospheric Research made a controversial suggestion: that ENSO patterns were linked to the sun’s magnetic cycle. While the exact mechanism was – and still is – hazy, their evidence did seem to show that a switch between El Niño and La Niña tends to be aligned with what’s known as “terminator events” in the solar cycle (it sounds worse than it is – it’s basically just the Sun’s New Year).
“We are not the first scientists to study how solar variability may drive changes to the Earth system,” said Bob Leamon, an Associate Research Scientist at the University of Maryland and co-author of the paper that proposed the link back in 2021. “But we are the first to apply the 22-year solar clock. The result – five consecutive terminators lining up with a switch in the El Niño oscillation – is not likely to be a coincidence.”
At first, many other climate scientists were skeptical. “I wouldn’t go so far as to call the results of this work a ‘conclusion’ per se,” space weather physicist Tamitha Skov told Washington Post at the time; “rather something akin to a steppingstone in a new direction.”
But the second paper from the team at Innsbruck, which was published in the journal Geophysical Research Letters in October, seems to support the hypothesis – at least, to a point. By analyzing speleothems in southeastern Alaska, the researchers were able to deduce a record of the influence of solar radiation on the local climate – and, indeed, they write, “ENSO was significantly influenced by solar irradiance over the past ∼3,500 years.”
But very, very recently – only about 50 years ago – that relationship started to break down. And the reason? You guessed it: our old friend climate change.
“ENSO [is] now being dominated by anthropogenic forcing,” the authors write. “This implies a new ENSO mean state that will need to be incorporated into future climate projections.”
So what’s the takeaway here?
Combined, the two papers have a pretty simple message: we don’t know as much as we thought we did.
But we’re getting there. And with these new hypotheses to work with, perhaps we’re one step closer to unraveling the mechanisms behind the “little boy” with the power to devastate the planet.