The Influence of Anomalous Biomass Emissions on ENSO in CESM2

Abstract The influence of biomass burning (BB) emissions on the transient coupled evolution of ENSO is explored in CESM2 using composite satellite-retrieved emissions during recent El Niño and La Niña events. For both El Niño and La Niña, four 20-member ensembles are generated from CESM2 initial states that are predisposed to evolve into ENSO events. For each ENSO phase, one ensemble is forced with observed BB emissions while the other is forced with climatological emissions, with the response to anomalous BB emissions estimated from inter-ensemble differences. It is found that the regional BB anomalies associated with ENSO occur mainly during boreal fall and coincide with the climatological seasonal burden maxima, instigating a spatially and temporally complex climate response. The BB burdens are found to influence precipitation, cloud properties, and radiation in both the tropics and extratropics. At the onset of El Niño, BB anomalies increase precipitation in the northern branch of the ITCZ and enhance cloud albedo and amount across the Maritime Continent and eastern subtropical Pacific Ocean. Various other responses are identified at the onset and termination of La Niña. The net effect of these influences is to strengthen SST anomalies in the eastern Pacific Ocean during El Niño and broadly warm the tropical Pacific Ocean during La Niña, resulting in improved simulation of ENSO power, diversity, and asymmetry in CESM2.