ENSO and global warming: what’s the connection?

It has been a big week for the El Niño Southern Oscillation, or ENSO.

First, US researchers linked Australian wildfires of 2019-20 causally to the triple La Niñas of 2020-22 events that led to the widespread flooding of Australia’s east coast, with many regional areas the hardest hit.

Then this week, CSIRO climate researchers answered the big question on everyone’s lips: Was ENSO being affected by global warming, and if so, by how much?

Their research revealed that the intensity of ENSO fluctuations had risen since 1960 and we could expect more dramatic La Niña and El Niño events as the climate warms.

Both findings have major implications for Australia’s climate future and for other jurisdictions within ENSO’s pan-Pacific field of influence.

Did wildfires really spark three La Niñas?

The Australian wildfires of 2019-20 were severe, throwing smoky particulates skyward at a scale to rival a major volcanic eruption, and in enormous plumes that could clearly be seen from space.

The effects on our regional climate were undoubtedly profound.

A US research study published this month in the journal Science Advances suggests the fires were responsible for triggering the triple La Niña that followed.

The research findings, and their unstated implication – that ENSO may somehow be quietly going off the rails under climate change – caused quite a stir in climate circles.

But can we point the finger solely at the fires?

Climatically speaking, the Pacific region is complex, and a myriad of natural climatic and oceanic forces is always at play.

Unsurprisingly, experts suggest exercising due caution when it comes to blame.

“It’s definitely a line of evidence to suggest why the triple La Nina occurred,” says University of Melbourne climatologist Dr Andrew King. “But it’s not the end of the story, by any means.”

The research also underscored the importance of including realistic wildfire emissions in seasonal climate predictions and long-range climate projections.

Nonetheless, King agrees the study is “really interesting, the kind that tries to get at the heart of what’s going on in the climate system; a modelling study that’s trying to disentangle things where there’s really complex interactions going on.”

Led by the National Centre for Atmospheric Research (NCAR), the US researchers concluded that smoke from the Australian bushfires of 2019-20 had cooled the oceans to nudge the Tropical Pacific into a rare multi-year La Niña event, the third since Australian records began in 1950.

All fires release particulates or aerosols into the atmosphere. Once in the stratosphere, the smoky particles can affect Earth’s energy budget both directly, by scattering and/or absorbing incoming solar radiation, and indirectly, by influencing cloud properties.

“The … wildfire season was exceptional in both its severity and particulate emissions,” the researchers write, “driving the maximum in observed aerosol optical depth in the southern hemisphere over recent decades.”

By examining the effects of the smoke on sea surface temperatures in the south-eastern Pacific, the researchers concluded the smoky aerosols had interacted with clouds, which acted to cool the ocean surface, and thereby helped the drenching La Niñas to form.

They modelled the atmospheric and ocean processes involved using the Community Climate System Model version 2 (CESM2), which can represent interactions between biomass aerosols and clouds.

The authors said it was important to use a coupled Earth system model as a forecast tool, one which includes the atmosphere and the ocean.

The research also underscored the importance of including realistic wildfire emissions in seasonal climate predictions and long-range climate projections, with due implications also for climate change modelling.

What drives climate in the Pacific?

The main driver is ENSO, but other drivers are also important.

El Niño Southern Oscillation (ENSO) describes the cycling between an El Niño phase and its opposite phase La Niña, the result of changing atmospheric and ocean temperature conditions in the Pacific.

Both straddle “neutral” ENSO years between.

In El Niño years, there is warming of the central and eastern tropical Pacific Ocean, which, according to the Bureau of Meteorology, leads to a major shift in weather patterns across the Pacific. Under these conditions, trade winds can weaken or reverse and the surface water of the central Pacific warms, clouds and rainfall over the north of Australia become subdued, and “below average winter–spring rainfall for eastern parts of the country, and a drier start to the northern wet season,” result.

The cycle recurs roughly every three to eight years, and is associated with a weaker Walker Circulation, an east-west circulation of the atmosphere above the tropical Pacific.

When the cycle reverses to La Niña, trade winds strengthen and the temperature of the warm water north of Australia increases. This enhances cloud and rainfall to the north of Australia, yielding “above average winter–spring rainfall for eastern and central parts of the country, and a wetter start to the northern wet season”.

In an effort to forecast ENSO outcomes, the Australian Bureau of Meteorology maintains an ENSO Outlook, currently set on El Niño WATCH status, meaning there is about a 50% chance of an El Niño developing in 2023.

“A lot of the time, the Pacific Ocean is in a state where it’s kind of leaning towards El Niño or La Niña, and then … something going on in the atmosphere can just nudge it in one direction or another.”

Linking ENSO causally to the wildfires of 2019-20, however, is even more complex, says King.

“With the Pacific Ocean we already see other complex processes occurring,” King says.

“Even if we had no fires, we’d have La Niñas and El Niños occurring anyway.

“When we have La Niña events, we have cooler waters in the eastern and Central Pacific, [and] warmer waters in the West Pacific, and … typically, more precipitation around Australia.

“A lot of the time, the Pacific Ocean is in a state where it’s kind of leaning towards El Niño or La Niña, and then … something going on in the atmosphere can just nudge it in one direction or another.”

Importantly, other phenomena are at play in addition to ENSO. Some readers might be familiar with the Indian Ocean Dipole, or IOD, a climate pattern affecting the Indian Ocean. During a positive IOD, warm waters are pushed west, while colder waters are brought to the surface from the depths of the Eastern Indian Ocean. The pattern is reversed during a negative phase of the IOD, with consequent effects for prevailing conditions.

And there are more systems, some perhaps less familiar, including extratropical influences “SAM” and “MJO”, King explains.

“The Southern Annular Mode, or SAM, is associated with shifts in [the] positions of storms, low pressure systems that cross to the South of Australia.

“When we have what’s called negative Southern Annular Mode conditions, we typically see those storms a bit further north.

“When we have positive SAM conditions, the storms are typically further south, so we get less rainfall.”

Then there is the Madden-Julian Oscillation (MJO), characterised by movements of convection around the tropics.

“You have more convective clouds and more storms,” says King, “and then not far to the west and east you might have drier conditions and more stable conditions.

“It’s a movement of this cloud system and associated drier conditions around the equator.”

What about climate change?

In terms of Australia, King says it’s hard to say how much of the changes being observed are related to decadal climate variability, or to human influence on the climate.

“We’ve warmed the planet, we’ve altered the system quite a lot through our greenhouse gas emissions, which could then affect ENSO and ensure relationships to regional climates.

“There is research on the change in ENSO as the planet warms, and that research seems to suggest that ENSO might be getting more extreme, and more extreme El Niño events, more extreme La Niña events, bigger swings in the Pacific.”

“Over the last 50 years or so, strong El Niño and La Niña events have occurred more often.”

Such research surfaced again this week in a new study by CSIRO researchers published in the journal Nature Reviews Earth and Climate.

Led by CSIRO Chief Research Scientist Oceans and Atmosphere Dr Wenju Cai, the research had aimed to investigate the effect on ENSO of increased greenhouse emissions.

“Over the last 50 years or so, strong El Niño and La Niña events have occurred more often,” write two authors of the paper, lead Cai, and co-author Agus Santoso.

“Was climate change playing a role? Our research set out to answer this question.

“Decades of observations of climate change show sea surface temperatures are warming; in many oceans across the world, including the Pacific, this has caused the sea surface to warm faster than the water below.”

Previous research in this space had projected how El Niño and La Niña might change in the future but it was unable to tell whether human-caused climate change had already affected them.

The CSIRO research analysed simulations from 43 “climate models”, or computer simulations of the Earth’s climate system.

Simulations from 1901-1960 were compared with those from 1961-2020, with most results showing an increase in variability of ENSO after 1960.

“We then examined climate simulations over hundreds of years before humans started ramping up greenhouse gas emissions,” write Cai and Santoso, “and compared these to the simulations after 1960.

“This analysis showed even more clearly the very strong variability in the El Niño-Southern Oscillation after 1960.”

The researchers concluded that human-caused greenhouse gas emissions were the culprit.

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