The catastrophic Australian bushfires in 2019 to 2020 contributed to ocean cooling thousands of miles away, ultimately nudging the tropical Pacific into a rare multiyear La Niña event that dissipated only recently.
The research was led by scientists at the National Center for Atmospheric Research (NCAR) and published in Science Advances.
La Niña events tend to impact the winter climate over North America, causing drier and warmer than average conditions in the Southwest U.S., wetter weather in the Pacific Northwest and colder temperatures in Canada and the northern U.S. Because the emergence of La Niña can often be predicted months in advance, it's an important phenomenon for seasonal climate forecasts.
"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 NCAR scientist John Fasullo, lead author of the study.
The research was funded by the U.S. National Science Foundation, which is NCAR's sponsor, NASA and the U.S. Department of Energy.
La Niñas are not uncommon, but an occurrence for three consecutive winters is rare. The recent run of La Niñas, beginning in the winter of 2020 and continuing through last winter, is only the third string of three in the historical record, which dates to 1950.
The recent La Niña streak is also unusual because it is the only one that did not follow a strong El Niño - a warming instead of cooling in the tropical Pacific with similar but opposite climate impacts.
Scientists have previously established that events in the Earth system, including large volcanic eruptions in the Southern Hemisphere, can shift the odds toward a La Niña emerging. In the case of a volcano, emissions spewed high into the atmosphere can result in the formation of light-reflecting particles called aerosols, which can cool the climate and ultimately create favorable conditions for La Niña.
Given the massive scale of the Australian fires - which burned an estimated 46 million acres - Fasullo and his co-authors wondered what climate impacts the resulting emissions might have had.
The research team found that the emissions from the wildfires, which quickly encircled the Southern Hemisphere, kicked off a chain of climate interactions. Unlike a volcanic eruption, the bulk of wildfire emissions did not make it high enough in the atmosphere to cool the climate by directly reflecting sunlight.
Instead, the aerosols that formed from the emissions brightened the cloud decks across the Southern Hemisphere and especially off the coast of Peru, which cooled and dried the air in the region, ultimately shifting the zone where the northern and southern trade winds come together. The net result was a cooling of the tropical Pacific Ocean, where La Niñas form, over multiple years.
