What caused a 3-year La Niña? Supercomputer reveals wildfires thousands of miles away

For instance, La Niña events have a significant impact on the winter climate over North America, leading to drier and warmer than average conditions in the southwest U.S., wetter weather in the Pacific Northwest and Canada, winter temperatures that are warmer than normal in the South and cooler than normal in Canada and the northern U.S., and a more severe hurricane season.

Interesting Engineering (IE) reached out to the study's lead author, John Fasullo from NCAR (National Center for Atmospheric Research), to delve deeper into the significance of this discovery and its implications for our understanding of the complex interplay between climate events.

A La Niña cooling effect like no other

La Niñas occurring for three consecutive winters are a rare phenomenon, although La Niñas themselves are not uncommon. The recent streak of La Niñas, spanning from the winter of 2020-21 to this last winter, is particularly noteworthy as it is only the third occurrence of a "triple dip" La Niña in the meteorological record dating back to 1950.

What makes this recent La Niña streak even more unusual is that it deviates from the typical pattern where La Niñas follow strong El Niño events, which involve warming instead of cooling in the Tropical Pacific and have similar yet opposite climate effects.

Scientists have previously established that certain events within the Earth system, such as large volcanic eruptions in the Southern Hemisphere, can increase the likelihood of a La Niña emerging. Volcanic emissions that reach high altitudes form light-reflecting particles called aerosols, which have a cooling effect on the climate and create favorable conditions for La Niña to develop.

Given the immense scale of the Australian fires, estimated to have burned around 46 million acres, Fasullo and his co-authors were intrigued by the potential climate impacts of the resulting emissions.

Unveiling insights through Cheyenne supercomputer simulations

"We used NCAR's climate model, CESM2, to quantify the influence of the fires on La Niña," Fasullo told IE. 

The CESM2, or Community Earth System Model, version 2, is an advanced computer model developed by the National Center for Atmospheric Research (NCAR). It is a comprehensive Earth system model designed to simulate and study the interactions between various components of the Earth's climate system.

The CESM2 incorporates multiple modules representing the atmosphere, ocean, land surface, sea ice, and other components, allowing scientists to simulate and analyze complex Earth system processes and their interconnections. 

Subsequently, researchers can investigate a wide range of climate-related phenomena, including changes in temperature, precipitation patterns, ocean circulation, atmospheric composition, and the impacts of human activities on the Earth's climate.

The team ran two simulations on the Cheyenne system. This supercomputer is located at the NCAR-Wyoming Supercomputing Center (NWSC), a state-of-the-art facility dedicated to high-performance computing and data analysis for atmospheric and Earth system research.

All the simulations were initiated in August 2019, before the Australian bushfires reached their historic scale. However, only one set of simulations took into account the emissions from the wildfires as observed through satellite data. 

The remaining simulations followed the standard practice of using average wildfire emissions, typically done when conducting long-term climate model simulations.

"Originally, it was part of a project to estimate the climate response to COVID pandemic-related emissions reductions. As it turns out, the fires were the major climate event of 2020," Fasullo said.

"The smoke from the bushfires was carried around the Southern Hemisphere by winds"

The research team made a significant discovery regarding the emissions released by the wildfires that rapidly spread throughout the Southern Hemisphere. 

Unlike volcanic eruptions, the majority of wildfire emissions do not ascend to high altitudes, therefore not triggering the direct sunlight reflection needed to cool the climate. 

However, the emissions did lead to the formation of aerosols, which brightened the cloud decks across the Southern Hemisphere and particularly near the coast of Peru. 

"We mainly used the CESM2, though the aerosols from the smoke were also clearly visible from satellite," said Fasullo. 

Fasullo told IE, "The smoke from the bushfires was carried around the Southern Hemisphere by winds."

"[The winds] provided surfaces on which cloud drops could form. This makes the cloud drops smaller and brighter, and thus reflects more sunlight back to space, thereby cooling the surface," he explained. 

The team argued this phenomenon resulted in the cooling and drying of the air in the region, ultimately causing a shift in the convergence zone where the northern and southern trade winds meet. Consequently, the Tropical Pacific Ocean, the breeding ground for La Niñas, experienced a cooling effect that persisted over multiple years.

Adding the missing Piece: wildfires in climate forecasts

"La Niña influences the chances of floods and droughts around the globe," Fasullo emphasized.

"This work suggests that wildfires should be incorporated into seasonal prediction systems. Currently, they are not included in seasonal climate prediction," he added.

The study's recent press release emphasized an intriguing finding: in June 2020, shortly before the onset of the three consecutive La Niñas, some seasonal forecasts were still indicating "neutral" conditions in the Tropical Pacific. This meant that neither a La Niña nor an El Niño was expected. However, contrary to these forecasts, a robust three-year La Niña event unfolded.

According to Fasullo, his research provides insight into the reason behind the inaccurate forecast. It underscores the significance of employing a coupled Earth system model that incorporates both the atmosphere and the ocean for more accurate predictions.

This research also pinpoints the significance of accurately representing wildfire emissions in climate predictions for both short-term and long-term projections.

For instance, in many climate models, the emissions from biomass burning are predetermined and not influenced by interactions within the model itself. This means that even if the model simulates a period of hot and dry conditions, it does not necessarily increase simulated wildfires and emissions. 

"As the climate changes, the emissions from wildfires will also change," Fasullo said in a press release. "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."

In other words, better accounting for the dynamic relationship between climate conditions and wildfire emissions within the models is crucial to improve climate projections.

"Prediction can play a key role in mitigating the impacts of La Niña as it helps stakeholders (e.g., water managers, agricultural interests, etc.) plan for the influences of both El Niño and La Niña," Fasullo concluded.