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Multiyear La Ni帽a events have become more common over the last 100 years, according to a new study led by University of Hawai'i (UH) at M膩noa atmospheric scientist Bin Wang. Five out of six La Ni帽a events since 1998 have lasted more than one year, including an unprecedented triple-year event. The study was published in .

"The clustering of multiyear La Ni帽a events is phenomenal given that only ten such events have occurred since 1920," said Wang, emeritus professor of atmospheric sciences in the UH M膩noa School of Ocean and Earth Science and Technology.

El Ni帽o and La Ni帽a, the warm and cool phases of a recurring climate pattern across the tropical Pacific, affect weather and ocean conditions, which can, in turn, influence the marine environment and fishing industry in Hawai'i and throughout the Pacific Ocean. Long-lasting La Ni帽as could cause persistent climate extremes and devastating weather events, affecting community resilience, tourist industry and agriculture.

Determining why so many multiyear La Ni帽a events have emerged recently and whether they will become more common has sparked worldwide discussion among climate scientists, yet answers remain elusive.

Wang and co-authors examined 20 La Ni帽a events from 1920-2022 to investigate the fundamental reasons behind the historic change of the multiyear La Ni帽a. Some long-lasting La Ni帽as occurred after a super El Ni帽o, which the researchers expected due to the massive discharge of heat from the upper-ocean following an El Ni帽o. However, three recent multiyear La Ni帽a episodes (2007鈥�08, 2010鈥�11, and 2020鈥�22) did not follow this pattern.

They discovered these events are fueled by warming in the western Pacific Ocean and steep gradients in sea surface temperature from the western to central Pacific.

"Warming in the western Pacific triggers the rapid onset and persistence of these events," said Wang. "Additionally, our study revealed that multiyear La Ni帽a are distinguished from single-year La Ni帽a by a conspicuous onset rate, which foretells its accumulative intensity and climate impacts."

Results from complex computer simulations of climate support the observed link between multiyear La Ni帽a events and western Pacific warming.

The new findings shed light on the factors conducive to escalating extreme La Ni帽a in a future warming world. More multiyear La Ni帽a events will exacerbate adverse impacts on communities around the globe, if the western Pacific continues to warm relative to the central Pacific.

"Our perception moves beyond the current notion that links extreme El Ni帽o and La Ni帽a to the eastern Pacific warming and attributes the increasing extreme El Ni帽o and La Ni帽a to different sources," Wang added.

"The knowledge gained from our study offers emergent constraints to reduce the uncertainties in projecting future changes of extreme La Ni帽a, which may help us better prepare for what lies ahead."