Multiyear El Niño and La Niña events likely to increase, researchers say
Alanna Madden
(CN) — Climate scientists are bracing for potentially lengthy El Niño and La Niña events, according to a new study revealing how the underlying mechanism for climate variability is responding to increased greenhouse gas emissions in unpredicted ways and inducing El Niño-like conditions after volcanic eruptions.
The research published in Nature Wednesday details recently discovered trends of the “Pacific Walker Circulation,” (PWC) an atmospheric phenomenon relating to east-west circulation along the equatorial Pacific. The pattern plays an atmospheric role in the El Niño–Southern Oscillation, the dominant mode of global interannual climate variability that comprises two phases: El Niño and La Niña.
El Niño brings about warmer, reoccurring climate patterns across the tropical Pacific that affect weather patterns globally. La Niña, on the other hand, consists of cooler climate trends that worsen drought in the western U.S. and exacerbate the Atlantic hurricane season. The shift between the two phases occurs irregularly every two to seven years, affecting ocean surface temperature along with tropical wind and rainfall patterns.
Despite the Walker Circulation’s crucial role in the global climate, researchers say its response to anthropogenic influences and its natural variability is poorly understood. The study’s authors write that while most observational datasets indicate that the circulation strengthened between 1992 and 2011 — creating more “La Niña-like” conditions — such data is at odds with climate models predicting that it will eventually weaken due to global warming.
“The tropical Pacific has an outsized influence on global climate,” said Sloan Coats, study co-author and assistant professor of earth sciences at the University of Hawai‘i at Mānoa's School of Ocean and Earth Science and Technology, in a statement. “Understanding how it responds to volcanic eruptions, anthropogenic aerosols and greenhouse gas emissions is fundamental to confidently predicting climate variability.”
To understand whether greenhouse gases have affected the Walker Circulation, the international team of researchers sourced biologic and geologic signatures from ice cores, trees, lakes, corals and caves to get a picture of Pacific weather patterns over the last 800 years. The signatures, composed of oxygen and hydrogen isotopes, allowed the team to track how the circulation has changed and compare trends relating to the rise in greenhouse gases.
“We found that the overall strength hasn’t changed yet, but instead, the year-to-year behavior is different,” said lead author Georgy Falster, a research fellow at the Australian National University and the ARC Centre of Excellence for Climate Extremes, in a statement.
The team found that the length of time for the Walker Circulation to switch between El Niño-like and La Niña-like phases has slowed slightly over the industrial era between 1850 and 2000.
“That means in the future we could see more of these multiyear La Niña or El Niño events as the atmospheric flow above the Pacific Ocean switches more slowly between the two phases,” Falster said.
Such events could intensify regional risks of drought, fire, rains and floods in a pattern already devastating many parts of the world. The team also found that volcanic eruptions affected the circulation, consistently weakening it to cause El Niño-like conditions.
The good news is that the researchers have yet to find any significant change in the Walker Circulation’s strength — a surprising result, Stevenson said, given that most climate models suggest it will weaken by the end of the 21st century.
“Although there is no significant PWC trend since the onset of anthropogenic forcing (around 1850), an anomalous PWC strengthening trend over the past couple of decades, as well as an industrial-era shift towards lower-frequency variability, suggests that the PWC may be responding to anthropogenic forcing, albeit in ways that are not consistently reproduced by climate model simulations,” the authors wrote.
Coats describes the study as one that provides longterm context for a fundamental component of the tropics’ atmosphere-ocean system.
“Understanding how the Pacific Walker Circulation is affected by climate change will enable communities across the Pacific and beyond to better prepare for the challenges they may face in the coming decades,” Coats said.
Investigating how climate change affects the Walker Circulation is also crucial for creating reliable predictions, Stevenson explained, adding that if researchers don’t know what happened in the real world, they wouldn’t know whether their models accurately predict future changes, impacts and risks.
Moving forward, the team is investigating potential culprits for documented circulation changes while a doctoral student of Stevenson creates a model that integrates isotope ratios. Developing the model, researchers say, will predict measurements that can allow them to test different hypotheses.