Global warming could mean longer La Nina events, which bring cold, wet Canadian winters: study

FILE - The Manayunk neighborhood in Philadelphia is flooded Thursday, Sept. 2, 2021, in the aftermath of downpours and high winds from the remnants of Hurricane Ida. La Nina, the weather phenomenon linked to extreme weather events like floods, droughts and wildfires, is expected to last longer and become more frequent due to global warming, according to a new study. (AP Photo/Matt Rourke, File)

The world will likely see more multi-year La Niña events due to global warming, contributing to a higher risk of extreme weather events, according to a recent study published in the scientific journal Nature.

La Niña refers to a period of cooler than normal waters in the eastern and central Pacific Ocean. This weather phenomenon typically occurs approximately every three to five years and lasts one to two years, Environment and Climate Change Canada notes. 

La Niña is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern, which is characterized by the shifts in ocean temperatures and atmospheric conditions in the Pacific. The ENSO pattern alternates irregularly between warm El Niño and cold La Niña events, impacting global weather patterns, agriculture and ecosystems.

Based on multiple climate models collected by the Coupled Model Intercomparison Project Phase 6 (CMIP6), researchers from China, Australia and the U.S. reported a “significant increase" in the projected frequency of multi-year La Niña events over a 100-year period.

They projected an increase in frequency ranging from 19 per cent, plus or minus 11 per cent, in a low greenhouse gas emissions scenario to 33 per cent, plus or minus 13 per cent, in a high emissions scenario.

The study also explains the conditions underlying the projected increase.

"Under present-day climate conditions, a strong El Niño in the boreal winter induces a negative North Pacific Meridional Mode (NPMM)-like response in the subtropical North Pacific, producing a La Niña in the ensuing winter with meridionally extensive sea surface temperature (SST) and easterly wind anomalies," said Jia Fan, a co-author of the study from the Institute of Oceanology of the Chinese Academy of Sciences, in a news release.

These easterly wind anomalies are further intensified by faster warming in the equatorial eastern Pacific. The study suggests that the slower heat recharge associated with the northward-broadened easterly wind anomalies are what ultimately enables the cold anomalies of a La Niña to persist for longer than a year.

Compared to single-year La Niña events, multi-year La Niña events such as the one that lasted from 2020 to 2022 create higher or cumulative risk of extreme weather events, the study notes, including droughts, wildfires, flooding and altered patterns of hurricanes, cyclones and monsoons across the Pacific and Atlantic oceans.

La Niña increases Atlantic hurricane activity and worsens western drought. In Canada, La Niña winters are also typically wetter and colder, the National Oceanic and Atmospheric Administration says. 

"These findings suggest that weather extremes as seen during the 2020–2022 La Niña will probably occur more frequently in the near future," said Geng Tao from Ocean University of China, co-author of the study, in a news release.

The results of this study strengthen calls to reduce greenhouse-gas emissions “to alleviate the adverse impacts” of increased multi-year La Niña events, Fan said.