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La Nina events will increase for years due to global warming: Study

The Southern Oscillation (ENSO) is a critical annual climate phenomenon that alternates between the warm El Niño and cold La Niña

By Ground report
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La Nina events will increase for years due to global warming: Study

The Southern Oscillation (ENSO) is a critical annual climate phenomenon that alternates between the warm El Niño and cold La Niña phases, influencing sea surface temperatures and weather patterns across tropical regions. Unlike El Niño, which usually lasts for a year, La Niña can persist for two or more consecutive years, leading to severe impacts like forest fires, floods, storms, and changing monsoon patterns.

Global warming intensifies multi-year La Niña

A recent study published in the journal Nature, conducted by researchers from China, Australia, and the US, reveals alarming findings that multi-year La Niña events are expected to become more frequent due to global warming.

Analyzing climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), the researchers predict the likelihood of multi-year La Niña events to increase by 19 percent to 33 percent over a 100-year period, depending on greenhouse gas emissions scenarios.

Projected increase in frequency of multiyear La Niña events under future greenhouse-gas forcing. Credit: IOCAS

The study unravels the mechanism behind this projected increase. Under current climate conditions, a robust El Niño during the winter months in the northern regions triggers a negative North Pacific Southern Mode (NPMM)-like feedback in the subtropical North Pacific. This leads to an upsurge in Southern Sea Surface Temperature (SST) during the subsequent winter and disrupts easterly winds, eventually giving rise to La Niña.

However, global warming accentuates the link between El Niño and multi-year La Niña due to intensified tropical-subtropical interactions, mainly caused by Pacific warming. In particular, the rapid warming in the subtropical northeast Pacific creates a more sensitive NPMM-like response to El Niño convective anomalies, extending farther north.

Current patterns

Niwa scientist Daithi Stone, who was not involved in the study, describes La Niña as driving a pile up of warm water towards Indonesia, which is caused by the easterly trade winds and results in this body of water gathering deep below the surface.

Stone said that every now and then, the water returns towards Central America in "what we call an El Niño event". Since the hot water rises to the surface and spreads, it releases heat into the atmosphere.

El Niño typically raises the mercury in North, Central, and South America and dries out Southeast Asia and north Australia, creating conditions for bushfires, while also bringing thunderstorms to the eastern Pacific.

Stone said, "In New Zealand, La Niña affects us a little bit more, making us a little bit unusual."

Stone said that warm water sloshes back and forth across the equatorial Pacific every few years. The pattern is irregular. At best, scientists can predict about 12 months in advance whether the Pacific will be in La Niña, El Niño, or a neutral state, he added.

Back-to-back La Niñas tend to come after a strong El Niño event, noted the researchers – including Wenju Cai, the former chief research scientist at CSIRO.

The specific mechanics are complex, but essentially, these winds would slow the reheating of the deep waters. It would take the Pacific longer to “recharge” and tip into a different phase of the cycle, he said.

“The system requires back-to-back La Niña events to do so. Thus, back-to-back La Niña events occur more often under greenhouse warming.”

“Our finding is another line of evidence that extremes are likely to occur more often in a warming climate unless actions are taken to cut emissions,” Cai added.

La Niña risks increase with warming

The warming also amplifies easterly wind anomalies in the equatorial eastern Pacific, retarding the heat recharge of the equatorial Pacific, and favoring the continuation of cold SST anomalies, thus sustaining La Niña for several years.

The joint impact from the two mean-state warming maxima under greenhouse warming (a) is an NPMM-like pattern that is more sensitive to El Niño’s warming anomalies and extends farther north (b) in the 21st century. Credit: IOCAS

The implications of these findings are significant, as researchers suggest that extreme weather events akin to the 2020-2022 La Niña could occur more frequently in the future. Consequently, the study emphasizes the urgent need to reduce greenhouse gas emissions to mitigate the adverse impacts of increasing multi-year La Niña events.

Geng Tao from the Ocean University of China, the first author of the study, underscores the importance of this research, while Jia Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), a co-corresponding author of the study, advocates for emissions reduction to tackle the potential challenges posed by prolonged La Niña events.

As the world grapples with the consequences of climate change, understanding and addressing the implications of the Southern Oscillation and its impact on multi-year La Niña events is of paramount importance for global climate resilience.

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