A multiyear tropical Pacific cooling response to recent Australian wildfires in CESM2

Abstract

The climate response to biomass burning emissions from the 2019–2020 Australian wildfire season is estimated from two 30-member ensembles using CESM2: one of which incorporates observed wildfire emissions and one that does not. 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. In turn, the surface cools, the boundary layer dries, and the moist static energy of the low-level flow into the equatorial Pacific is reduced. In response, the intertropical convergence zone migrates northward and sea surface temperature in the Niño3.4 region cools, with coupled feedbacks amplifying the cooling. A subsequent multiyear ensemble mean cooling of the tropical Pacific is simulated through the end of 2021, suggesting an important contribution to the 2020–2022 strong La Niña events.

Physical science research needed to evaluate the viability and risks of marine cloud brightening

“…Fasullo et al . ( 116 )suggest, based on climate modeling studies, that smoke from the 2019/2020 wildfire season in eastern Australia brightened the clouds in the southeastern Pacific and cooled the ocean surface enough to contribute to the establishment of the unusually persistent La Niña event of 2020–2023 [see also ( 83 )].…”

Triple‐Dip La Niña in 2020–23: North Pacific Atmosphere Drives 2nd Year La Niña

“…Also, our experiment did not include realistic biomass burning emissions in 2019/20 Australia. It may increase the La Niña amplitude and improve reproducibility (Fasullo et al., 2023). The large‐ensemble hindcasts enable us to estimate the probability of La Niña with high confidence (Figure 3d).…”

“…However, the Atlantic Niño could be forced by multi‐year La Niña, implying it is the result but not the effect (Tokinaga et al., 2019). Also, it is reported that the Australian wildfire in 2019/20 contributed to bringing long‐lasting La Niña (Fasullo et al., 2023). However, their simulation also showed second‐year La Niña can occur without external forced components, implying internal variability was working.…”

Evaluating skill in predicting the Interdecadal Pacific Oscillation in initialized decadal climate prediction hindcasts in E3SMv1 and CESM1 using two different initialization methods and a small set of start years

“…Meehl et al, 2015;Wu et al, 2023) show anomaly pattern correlations near or above 0.5. The drop in skill for years after 2020 is likely related to the multi-year La Niña event that started in 2020 that could have been externally forced by smoke from the catastrophic large scale wild res in Australia in 2019-2020 (Fasullo et al, 2021(Fasullo et al, , 2023. Since none of the hindcasts considered here, including DPLE, included the Australian wild re smoke, and if the multi-year La Niña event was at least partly externally forced, the veri cation observational data would include the effects of the smoke, thus reducing hindcast skill of the model simulations that did not include the smoke.…”