A Multiyear Tropical Pacific Cooling Response to Recent Australian Wildfires in CESM2
The Australian bushfire season of 2019-20 was exceptional in size and duration. It also preceded an exceptionally long and unique La Niña event lasting from 2020-2023. This work explores the climate response to the fires and seeks to understand the specific links between the fires and La Niña.
In response to the fires, an increase in biomass aerosol burdens across the southern hemisphere is simulated through late 2019 and early 2020, accompanied by an enhancement of cloud albedo, particularly in the southeastern subtropical Pacific Ocean (SESP). In turn, the surface in the SESP cools, the boundary layer dries, and the moist static energy of the flow advected low-level flow into the equatorial Pacific is reduced. These responses instigate coupled feedbacks in which the Intertropical Convergence Zone migrates northward and sea surface temperature in the Niño3.4 region cools, with a strengthening of the Walker circulation amplifying the cooling. A subsequent multi-year ensemble-mean cooling of the tropical Pacific is simulated through the end of 2021, suggesting an important contribution to the prolonged La Niña event in nature.
Our analysis reveals an important and previously unidentified role of wildfire in the climate system. The role of fire is typically not considered in seasonal climate prediction systems, suggesting a potentially useful enhancement of seasonal prediction skill with its incorporation in some circumstances. The continued intensification of wildfires under climate change suggests an increasing influence of similar effects in the future, with a range of effects yet to be explored.