A groundbreaking study published in the journal Science Advances suggests that the catastrophic Australian bushfires in 2019-2020 precipitated a chain of climate interactions leading to a chilling effect on the Tropical Pacific. This cooling, thousands of miles away from the burning Australian shores, eventually triggered the formation of a multi-year La Niña event, a phenomenon that has only recently dissipated.
La Niña events, characterized by cooler-than-average sea surface temperatures in the equatorial Pacific, have historically exerted substantial influence on North America’s winter climate.
These weather oscillations typically induce drier and warmer conditions in the southwest U.S., foster increased precipitation in the Pacific Northwest, and bring about colder temperatures in Canada and northern parts of the U.S. Due to the potential for their emergence to be anticipated months in advance, La Niña events play a crucial role in seasonal climate forecasting.
However, the recent wave of La Niña events, observed over three consecutive winters starting from 2020-21, signifies a rare climate phenomenon. The pattern is only the third streak of its kind in the historical record, dating back to 1950. Moreover, unlike previous instances, this La Niña chain did not succeed a potent El Niño event, which characteristically manifests as a warming of the Tropical Pacific.
“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 study lead author John Fasullo, a scientist at the National Center for Atmospheric Research (NCAR).
The researchers set out to investigate the potential climate impacts of the Australian fires, which charred an estimated 46 million acres. With large-scale events like these or major volcanic eruptions known to shift the odds towards a La Niña event, the team leveraged an advanced computer model, the Community Earth System Model version 2, to carry out two sets of simulations.
Both simulation sets started from the same point, August 2019, prior to the escalation of the Australian fires. One incorporated the observed wildfire emissions from the catastrophic blazes, while the other simulated average wildfire emissions typically used in long-term climate model simulations.
Contrary to the direct sunlight-reflecting action of high-altitude volcanic emissions, wildfire emissions have a different modus operandi. These fire-originated aerosols, enveloping the Southern Hemisphere, intensified the brightness of cloud decks, particularly off the coast of Peru.
The amplified brightness led to a cooling and drying of the air, consequently shifting the junction of the northern and southern trade winds. This overall climatic shift incited a sustained cooling of the Tropical Pacific ocean, a critical component in the formation of La Niña events.
Discussing the complexity of the climate interactions, Fasullo said, “It’s a Rube Goldberg of climate interactions that we were only able to identify because our model now represents specific details in the evolution of smoke and cloud-aerosol interactions, a recent improvement to its capabilities.”
The research, primarily funded by the U.S. National Science Foundation (NCAR’s sponsor), NASA, and the U.S. Department of Energy, has far-reaching implications for future climate forecasts. It throws light on the failure of several forecasts to predict the onset of the three-year La Niña, which had been indicating “neutral” conditions as late as June 2020.
The research emphasizes the significance of utilizing coupled Earth system models, which consider both atmospheric and oceanic components, as essential forecasting tools. Furthermore, it points to the need for accurate representation of wildfire emissions in both seasonal and long-range climate projections.
“As the climate changes, the emissions from wildfires will also change,” said Fasullo. “But we don’t have that feedback in the model. It is the goal of our current work to incorporate these effects as realistically as possible.”
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This material is based upon work supported by the National Center for Atmospheric Research, a major facility sponsored by the National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.
La Niña can have wide-reaching effects that influence weather patterns and climate conditions worldwide. Here’s a broad overview of some of the impacts typically associated with La Niña:
North America
La Niña tends to bring colder and snowier winters to the northern tier of the United States and drier, warmer conditions to the southwestern states. The Pacific Northwest may experience wetter weather, while the Southeast could be warmer and drier. Canada often sees colder temperatures during a La Niña event.
South America
La Niña can result in excessive rainfall over Colombia, Ecuador, and northern Peru, increasing the risk of flooding. In contrast, it can cause drier conditions in southeastern South America, including parts of Brazil, Argentina, Paraguay, and Uruguay, leading to drought.
Asia and Australia
Southeast Asia, especially Indonesia, the Philippines, and Australia, often see more rainfall during a La Niña event. This increased rainfall can sometimes result in flooding. Conversely, India may experience a weaker monsoon season.
Africa
East Africa, including Kenya, Somalia, and Tanzania, may receive above-average rainfall during the short rains of October to December, which can lead to flooding. Meanwhile, southern Africa can experience drier than normal conditions, increasing the risk of drought.
Atlantic hurricanes
La Niña can contribute to an intensification of the Atlantic hurricane season because it decreases the vertical wind shear over the Caribbean Sea and tropical Atlantic Basin, which makes conditions more favorable for tropical storm development.
These are general tendencies and can be influenced by other climatological factors. It is also important to note that while La Niña events can often be predicted months in advance, there is still a level of uncertainty associated with predicting specific weather outcomes during these events. Climate scientists are continually working to improve our understanding and prediction of these global effects.
