Understanding El Niño Southern Oscillation (ENSO)
The El Niño Southern Oscillation (ENSO) is Earth’s most significant climate fluctuation on an interannual scale.
Its phases—warm El Niño and cold La Niña—cause shifts in ocean surface temperatures and alter wind and rainfall patterns across the tropics.
While El Niño usually lasts a year, La Niña often develops post an El Niño phase and can persist for two years or more.
This phenomenon, known as a multiyear La Niña event, has far-reaching impacts, including increased wildfires, flooding, and altered hurricane, cyclone, and monsoon patterns.
Expect More Multiyear La Niña Events
A recent international study involving researchers from China, Australia, and the U.S. has suggested that the frequency of multiyear La Niña events is likely to increase due to global warming.
The study, published in Nature on July 26, used data from various climate models collected by the Coupled Model Intercomparison Project Phase 6 (CMIP6).
The researchers found a significant increase in the projected frequency of multiyear La Niña events over the next century.
In a low greenhouse gas emission scenario, the frequency was estimated to be 19 ± 11%, rising to 33 ± 13% under a high emission scenario.
Underlying Mechanisms and Impact
Dr. Jia Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) explained the mechanisms underlying this projected increase.
“A strong El Niño in the boreal winter triggers a negative North Pacific Meridional Mode (NPMM)-like response in the subtropical North Pacific, leading to a La Niña in the subsequent winter,” said Dr. Fan.
The NPMM-like response produces patterns corresponding to weak negative wind stress curl off-tropics, resulting in a slow recharge of the upper ocean, conducive to the first-year La Niña condition persisting into the second year.
Under global warming, El Niño becomes more efficient at triggering a multiyear La Niña due to more effective tropical-subtropical interaction, which essentially results from a Pacific mean-state warming pattern.
Compared with the twentieth century, faster warming in the subtropical northeastern Pacific makes the NPMM-like response to El Niño’s convective anomalies both more sensitive and extending farther north.
Moreover, these anomalies are intensified by quicker warming in the equatorial eastern Pacific. The resulting easterlies extend farther north, causing an even slower heat recharge of the equatorial Pacific.
This leaves a colder upper-ocean condition, making it easier for cold SST anomalies to persist, leading to more multiyear La Niña events.
“Weather extremes as seen during the 2020–2022 La Niña will probably occur more frequently in the near future,” warned Dr. Geng Tao from Ocean University of China.
Dr. Jia emphasized that the study’s findings underscore the urgency to reduce greenhouse-gas emissions to mitigate the adverse impacts of increased multiyear La Niña events.
The study was published in Nature.
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Source: Chinese Academy of Sciences