Sydney was recently hit with its fourth major flood event in just two years. On the night of July 4 more than 100 rescues were made to save people from their inundated homes, while 50,000 people waited anxiously on evacuation alert.
Severe floods are not unheard of in NSW, which has recorded destructive rains, especially around the Hawkesbury River, since records began. But a recent report by the Bureau of Meteorology (BOM) found that while climate change was not directly responsible for this latest spate of floods, it may have made these disasters worse – a warmer atmosphere can hold 7% more moisture per 1°C of warming, making storm clouds more destructive.
But what complicates the picture even more is the as-yet poorly understood relationship between these isolated weather events, Earth’s broader climate oscillations (such as La Niña), and climate change.
What are Earth’s climate oscillations?
“Ultimately, all weather systems are driven by the energy the Earth receives from the Sun, with equatorial regions receiving more sunlight than polar regions,” says Dr Joel Hirschi, associate head of marine systems modelling at the National Oceanography Centre, UK.
“This leads to a temperature difference between high and low latitudes, and our weather is an expression of the energy exchanges in the atmosphere and the ocean acting to reduce this temperature gradient.
“The characteristic wind patterns associated with our weather – westerlies, trade winds – are due to the rotation of the Earth around its axis, which causes air masses moving meridionally [along the meridian] to be zonally deflected because of the Coriolis force.”
The Coriolis force is an effect on fluids moving in a rotating system: the force pushes fluids perpendicular to the direction of motion, tending to deflect a fluid to the right in the northern hemisphere, or to the left in the southern hemisphere. It’s this effect that forms tropical cyclones, though the jury’s out on whether it causes the water in your sink to empty in opposite directions depending on your hemisphere.
All these competing forces, however, are not in constant balance: sometimes, often at regular intervals, the natural cycles of Earth’s climate, at various points around the globe, are tipped over the edge – and they react with violence. These are known as oscillations.
La Niña, thought by many to be a driving force behind Australia’s recent floods, is part of one such oscillation, known as the El Niño Southern Oscillation (ENSO).
“The El Niño-Southern Oscillation is a climate mode of variability, because it varies on timescales of years between its two opposing phases, El Niño and La Niña,” explains Dr Andrew King, a senior lecturer at the ARC Centre of Excellence for Climate Extremes at the University of Melbourne. In general, that timescale hovers between three and eight years, according to the BOM.
The ENSO cycle is actually a deviance in a much broader system known as the Walker Circulation, which works like this: in an average year, trade winds blow east to west across the Pacific, following a pressure gradient between the dominant weather systems in the east and west. This movement of the trade winds brings warm water and warm, wet air across to the western Pacific. Higher up in the atmosphere, winds blow west to east, carrying the air back to begin the cycle again.
So where do La Niña and El Niño fit in?
“El Niño corresponds with a weaker Walker Circulation, La Niña with a stronger Walker Circulation,” explains Dr Agus Santoso, a senior research associate at the Climate Change Research Centre at UNSW Sydney.
In an El Niño year, when the Walker Circulation is weakened, the prevailing trade winds aren’t as strong, so the warmer waters of the western Pacific leak back to the east, creating warmer, moister and stormier conditions along the coast of South America and the southern US states. Australia, on the other hand, tends to be at risk of drought in an El Niño year, because less moisture is reaching its shores.
In a La Niña year, on the other hand, the Walker Circulation is strengthened, resulting in even stronger trade winds shunting east to west across the Pacific. In this case, Australia’s east coast experiences higher-than-average rainfall and storms. Meanwhile the west coast of the Americas, particularly around the tropics, dries up.
It’s not just Australia and South America that are vulnerable to the whims of the ENSO cycle. The US, East Africa and South Asia are all susceptible to weather extremes when La Niña or El Niño come out to play.
The North Atlantic Oscillation (NAO) describes the dynamic between a low-pressure system over Iceland and a high-pressure system over the Azores, off the coast of Portugal. The NAO controls the strength and direction of winds moving west to east into Europe.
Roughly every five years, this system alters: a larger pressure difference between the two points, known as a high-index year, leads to stronger winds, cooler summers and mild, wet winters in northern Europe. A high index year can also bring damaging winter floods in Europe.
A smaller pressure differential between the two points, known as a low-index year, brings suppressed winds and cold, dry winters in northern Europe, and storms and increased rainfall in southern Europe and North Africa.
Another is the Indian Ocean Dipole (IOD), which operates in much the same way as the ENSO cycle. A positive phase sees higher rainfall in the western Indian Ocean and drier, cooler air and water in the east. Because of this, a positive IOD phase can cause severe droughts in Indonesia and Australia – the IOD was in its positive phase during the devastating 2019-20 Australian bushfire season.
Some oscillations operate on much shorter or longer timescales, or interact with other climate cycles. The Madden Julian Oscillation (MJO), for example, is characterised by the enhancement or suppression of tropical rainfall as it travels eastward over the Indian and Pacific Oceans. The MJO operates on timescales of between 30 and 60 days, like a travelling pulse of rainfall. And the MJO actually affects the ENSO: it can contribute to the speed of development and the intensity of El Niño or La Niña.
Are climate oscillations responsible for Sydney’s floods?
The most recent floods to hit Sydney happened because of the destructive meeting of two weather phenomena: an “atmospheric river” of water vapour snaking its way south from the tropics, which collided with a classic Aussie east-coast low – a low-pressure system that sits above the water off Australia’s eastern margins and can cause severe rains and heavy wind.
But the severity of the floods, linked to the massive moisture content of both the rains themselves and Sydney’s soils, is doubtless affected by the double La Niña experienced on the continent – and the evidence is growing that these events will occur more frequently, and more intensely, in future.
Though the ENSO cycle has been ramping up over the last 50 years, according to the IPCC’s sixth assessment report (AR6), there’s no unequivocal evidence, yet, that these changes are a direct result of climate change. In fact, palaeo-climatic evidence shows that ENSO has been through successive phases of varying intensity throughout the Holocene, the roughly 11,000-year-period since the last Ice Age.
“Our models have deficiencies when simulating ENSO,” says King. Nonetheless, “there is a suggestion that El Niño and La Niña events will become stronger as the planet continues to warm.”
“We’re not yet certain whether changes in the observed ENSO cycle since 1900 are due to climate change, because of the relatively short observational record,” agrees Santoso. “But there have been more occurrences of strong El Niño and La Niña events in 1960 to present, compared with 1900 to 1960. In addition, palaeo-records suggest that the present ENSO cycle is stronger than in the preindustrial era.”
What would a worsening ENSO cycle mean for the world?
“Should the ENSO cycle strengthen under climate change, it means regions that are affected [by the cycle] should prepare for more intense and frequent drought (and associated risks such as forest fires), and floods,” says Santoso. That means Australia, Indonesia, parts of South Asia, Africa, the US and South America could all face heightened polarities of drought and flood.
And Santoso notes that as the planet warms, even if El Niño and La Niña themselves did not change in amplitude, they would still be more impactful because of the higher moisture-holding capacity of warmer air.
Much like the ENSO cycle, there is still a lack of clarity about how much climate change is affecting Earth’s other oscillations.
“I think that the jury is out on whether many weather systems have changed to a degree that we can confidently detect,” says Dr Judah Cohen, a climatologist at the Massachusetts Institute of Technology (MIT), US.
There is evidence, however, that at least some of Earth’s oscillations are tipping their balance as the climate warms.
And research suggests the balance of the NAO index has been shifting to positive over the last few decades, which some scientists have linked to climate change.
If the NAO tips its balance to more positive index years, the nature of European winters will change – while milder and wetter, Europeans could face a heightened flood risk.
There’s reason for caution here, though.
“Observational data from the last 120 years suggests that the phase and amplitude of the NAO can shift notably from decade to decade in a way which doesn’t seem to be related to climate change,” says Dr Kristian Strommen, a research fellow in climate science at the University of Oxford, UK. “This makes it very challenging to tell if more recent changes to the NAO are due to climate change or are just another example of such decadal variability.”
How will the oscillations affect one another?
Given Earth is a dynamic system, a spinning globe enveloped by a churning atmosphere, it’s reasonable to assume that these cycles might interact with one another if they change in future. So, can we predict how that might happen?
“There are multiple cases of ‘teleconnections’ in the climate system,” says Hirschi. “We already know that the ENSO phenomenon can have impacts on the North Atlantic region, and changes in the tropics can readily be communicated to other parts of the globe by fast atmospheric wave processes as well as by much slower ocean processes.”
“Associated with the weakening of the Walker Circulation, an El Niño is often accompanied by a positive phase of the Indian Ocean Dipole, while La Niña is accompanied by a negative IOD,” adds Santoso. “And warming of the tropical Atlantic tends to promote La Niña conditions in the Pacific.”
However, “there are still many unanswered questions regarding such teleconnections, how they work and if or how they may change in the future,” cautions Hirschi.
According to Strommen, whose research focuses on better predicting the NAO, there are so many factors at play that it’s difficult to know how each of these oscillations will interact with one another, and how much of the observable change is anthropogenic.
“The main problem here is that there is a so-called ‘tug of war’ happening,” he says. “Several big players, like ENSO, the stratosphere, and Arctic sea ice, all appear to be tugging the future winter NAO in different directions, and our climate models don’t agree on who is going to win out.”
Ultimately, elucidating these connections will be critical if scientists hope to predict extreme weather in a changing climate.
“A better understanding of teleconnections and of the underlying mechanisms is key to improving seasonal and longer predictions,” says Hirschi. “A lot more research is needed to bring out the interplay between the different weather systems and their separate and joint response to climate change.”