A La Niña‐Like Climate Response to South African Biomass Burning Aerosol in CESM Simulations

Abstract

The climate response to atmospheric aerosols, including their effects on dominant modes of climate variability like El Niño–Southern Oscillation (ENSO), remains highly uncertain. This is due to several sources of uncertainty, including aerosol emission, transport, removal, vertical distribution, and radiative properties. Here, we conduct coupled ocean‐atmosphere simulations with two versions of the Community Earth System Model (CESM) driven by semiempirical fine‐mode aerosol direct radiative effects without dust and sea salt. Aerosol atmospheric heating off the west coast of Africa—most of which is due to biomass burning—leads to a significant atmospheric dynamical response, including localized ascent and upper‐level divergence. Coupled Model Intercomparison Project version 6 (CMIP6) biomass burning simulations support this response. Moreover, CESM shows that the anomalous aerosol heating in the Atlantic triggers an atmospheric teleconnection to the tropical Pacific, including strengthening of the Walker circulation. The easterly trade winds accelerate, and through coupled ocean‐atmosphere processes and the Bjerknes feedback, a La Niña‐like response develops. Observations also support a relationship between south African biomass burning emissions and ENSO, with La Niña events preceding strong south African biomass burning in boreal fall. Our simulations suggest a possible two‐way feedback between ENSO and south African biomass burning, with La Niña promoting more biomass burning emissions, which may then strengthen the developing La Niña.

The 2017 Mega-Fires in Central Chile: Impacts on Regional Atmospheric Composition and Meteorology Assessed from Satellite Data and Chemistry-Transport Modeling

“…In the long run the radiative impact of aerosols emitted by biomass burning over the Amazon rainforest is found to improve the net primary productivity of biomass located downwind, due to changes in the more scattered nature of the light reaching the canopy, which in turn induces modifications in biochemical processes [33]. In South Africa, a Niña-like climate response to biomass burning is found, affecting the Walker circulation in the tropical Pacific through atmospheric teleconnections [34]. In the Southeast Atlantic region, biomass burning aerosols create a cyclonic anomaly over the ocean but enhance tropospheric stability over the continent [35].…”

Satellite Observations Reveal Northern California Wildfire Aerosols Reduce Cloud Cover in California and Nevada Through Semi-Direct Effects

“…Previously, using prescribed aerosol simulations in the Community Earth System Model version 2 (CESM2), it was hypothesized that the large 2019 wildfires in Australia could have intensified that year's La Niña through aerosols directly cooling the ocean surface (Fasullo et al, 2021). Another CMIP6 study observed a similar effect on La Niña as a result of a teleconnection caused by an influx of absorbing aerosols into the atmosphere from South African wildfires (Amiri-Farahani et al, 2020). While studies such as these demonstrate that it is possible to model past effects of fires on local and global climate, without proper parameterization of BB aerosol emission, as well as parametrization of secondary dust aerosol emission from wildfire-cleared vegetation, the radiative forcing of future fires' primary and secondary aerosols will remain a source of uncertainty.…”