Climate response and ocean adjustment to interannual fluctuations of biomass burning aerosol emissions in the CESM2 Large Ensemble Simulation

We consider the hypothesis that the rectified effects of high-amplitude interannual variability in biomass burning (BB) events can impact energetically the mean climate state through multi-decadal memory residing in the oceans heat reservoir. This is investigated utilizing the 100-member CESM2 Large Ensemble. These simulations largely follow standard historical and SSP3-7.0 forcing protocols over 1850-2100, using a mix of micro- and macro-perturbations for initialization. Two separate sets of aerosol emissions from BB are used to force the first and second 50 members. For the first set, the original CMIP6 protocols are applied. The second set is forced by temporally smoothed CMIP6 BB emissions (SMBB) that were designed to maintain net emissions. Smoothing is applied using an 11-year running-mean filter over the period that incorporates the Global Fire Emissions Database (GFED) in the CMIP6 forcing protocols (19972014), impacting the variability of the BB fluxes over the years 19902020. Differences in the top-of-atmosphere radiation fluxes between the first set (CMIP6-50) and the second set (SMBB-50) of members reveal significant additional relative heat absorption in the CMIP6-50 simulations during the GFED period with the highly-variable forcing. The globally integrated heat input during the GEFD period for the CMIP6-50 members relative to the SMBB-50 members is comparable in amplitude to the reduction of heat uptake during a major volcanic eruption, despite the net amount of BB aerosol emissions being nearly conserved between CMIP6 and SMBB. While a small fraction (<10%) of the relative heating for the CMIP6-50 members warms the atmosphere and melts the Arctic sea ice, most of the heat is absorbed by the oceans over the Northern Hemisphere subtropical western boundary current regions, as well as near sea ice edge areas. The heat subducted into the deeper ocean results in a long-term multi-decadal rectified response to the interannual fluctuations. Differences are identified in the Pacific and Atlantic heat reservoir responses, reflecting differences in their meridional overturning circulations. These results underscore the need to improve the underlying process representation of the forcing from BB in models as an important priority for Earth System Model development.