Publishing in ScienceAdvances, researchers simulated the way emissions produced by the wildfire smoke affected clouds worldwide. The fires began raging in summer 2019 and quickly became Australia’s costliest-ever natural disaster. They also affected the atmosphere worldwide: Within about a month, the fire’s emissions had encircled the globe, the researchers write.

A climate chain reaction had begun. The emissions fueled aerosol particles that brightened clouds in the Southern Hemisphere, where the air then became cooler and drier through late 2019 and early 2020. In turn, the region where the northern and southern trade winds meet shifted, and sea surface temperatures cooled across the tropical Pacific Ocean. Factors converged to push the ocean into a La Niña event that continued through last winter.

La Niña events aren’t uncommon, but consecutive ones are. The climate events are part of a three-phase climate phenomenon known as the El Niño-Southern Oscillation, or ENSO. The global circulation of trade winds through the atmosphere causes the Pacific to shift between La Niña, El Niño and neutral states that in turn affect the weather across North America.

Usually, La Niña follows El Niño, in which the tropical Pacific warms, the northern United States gets drier and warmer, and the southern part of the country experiences cooler, wetter weather.

“Many people quickly forgot about the Australian fires, especially as the covid pandemic exploded, but the Earth system has a long memory, and the impacts of the fires lingered for years,” said John Fasullo, a project scientist at the National Center for Atmospheric Research who led the study, in a news release.

The La Niña event is finally over, and climate researchers expect newly confirmed El Niño conditions to prevail this winter.