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. The impact of biomass burning aerosol (BBA) on the regional climate in SouthAmerica is assessed using 30-year simulations with a global atmosphere-onlyconfiguration of the Met Office Unified Model. We compare two simulations ofhigh and low emissions of biomass burning aerosol based on realisticinterannual variability. The aerosol scheme in the model has hygroscopicgrowth and optical properties for BBA informed by recent observations,including those from the recent South American Biomass Burning Analysis(SAMBBA) intensive aircraft observations made during September 2012. We findthat the difference in the September (peak biomass emissions month) BBAoptical depth between a simulation with high emissions and a simulation withlow emissions corresponds well to the difference in the BBA emissions betweenthe two simulations, with a 71.6 % reduction from high to low emissionsfor both the BBA emissions and the BB AOD in the region with maximumemissions (defined by a box of extent 5–25 ∘  S, 40–70 ∘  W,used for calculating mean values given below). The cloud cover at allaltitudes in the region of greatest BBA difference is reduced as a result ofthe semi-direct effect, by heating of the atmosphere by the BBA and changesin the atmospheric stability and surface fluxes. Within the BBA layer thecloud is reduced by burn-off, while the higher cloud changes appear to beresponding to stability changes. The boundary layer is reduced in height andstabilized by increased BBA, resulting in reduced deep convection and reducedcloud cover at heights of 9–14 km, above the layer of BBA. Despite thedecrease in cloud fraction, September downwelling clear-sky and all-skyshortwave radiation at the surface is reduced for higher emissions by13.77  ±  0.39 W m −2 (clear-sky) and7.37  ±  2.29 W m −2 (all-sky), whilst the upwelling shortwaveradiation at the top of atmosphere is increased in clear sky by3.32  ±  0.09 W m −2 , but decreased by - 1.36 ± 1.67  W m −2 when cloud changes are included. Shortwave heating rates increase in theaerosol layer by 18 % in the high emissions case. The mean surfacetemperature is reduced by 0.14  ±  0.24  ∘ C and meanprecipitation is reduced by 14.5 % in the peak biomass region due to bothchanges in cloud cover and cloud microphysical properties. If the increase inBBA occurs in a particularly dry year, the resulting reduction inprecipitation may exacerbate the drought. The position of the South Atlantichigh pressure is slightly altered by the presence of increased BBA, and thestrength of the southward low-level jet to the east of the Andes isincreased. There is some evidence that some impacts of increased BBA persistthrough the transition into the monsoon, particularly in precipitation, butthe differences are only statistically significant in some small regions inNovember. This study therefore provides an insight into how variability indeforestation, realized through variability in biomass burning emissions, maycontribute to the South American climate, and consequently on the possibleimpacts of future changes in BBA emissions.

The effect of South American biomass burning aerosol emissions on the regional climate