The effect of South American biomass burning aerosol emissions on the regional climate
Authors: Thornhill Gillian D. , Ryder Claire L. , Highwood Eleanor J. , Shaffrey Len C. , Johnson Ben T. 
Published: April 2018

Abstract. The impact of biomass burning aerosol (BBA) on the regional climate in South
America is assessed using 30-year simulations with a global atmosphere-only
configuration of the Met Office Unified Model. We compare two simulations of
high and low emissions of biomass burning aerosol based on realistic
interannual variability. The aerosol scheme in the model has hygroscopic
growth 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 find
that the difference in the September (peak biomass emissions month) BBA
optical depth between a simulation with high emissions and a simulation with
low emissions corresponds well to the difference in the BBA emissions between
the two simulations, with a 71.6 % reduction from high to low emissions
for both the BBA emissions and the BB AOD in the region with maximum
emissions (defined by a box of extent 5–25∘ S, 40–70∘ W,
used for calculating mean values given below). The cloud cover at all
altitudes in the region of greatest BBA difference is reduced as a result of
the semi-direct effect, by heating of the atmosphere by the BBA and changes
in the atmospheric stability and surface fluxes. Within the BBA layer the
cloud is reduced by burn-off, while the higher cloud changes appear to be
responding to stability changes. The boundary layer is reduced in height and
stabilized by increased BBA, resulting in reduced deep convection and reduced
cloud cover at heights of 9–14 km, above the layer of BBA. Despite the
decrease in cloud fraction, September downwelling clear-sky and all-sky
shortwave radiation at the surface is reduced for higher emissions by
13.77 ± 0.39 W m−2 (clear-sky) and
7.37 ± 2.29 W m−2 (all-sky), whilst the upwelling shortwave
radiation at the top of atmosphere is increased in clear sky by
3.32 ± 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 the
aerosol layer by 18 % in the high emissions case. The mean surface
temperature is reduced by 0.14 ± 0.24 ∘C and mean
precipitation is reduced by 14.5 % in the peak biomass region due to both
changes in cloud cover and cloud microphysical properties. If the increase in
BBA occurs in a particularly dry year, the resulting reduction in
precipitation may exacerbate the drought. The position of the South Atlantic
high pressure is slightly altered by the presence of increased BBA, and the
strength of the southward low-level jet to the east of the Andes is
increased. There is some evidence that some impacts of increased BBA persist
through the transition into the monsoon, particularly in precipitation, but
the differences are only statistically significant in some small regions in
November. This study therefore provides an insight into how variability in
deforestation, realized through variability in biomass burning emissions, may
contribute to the South American climate, and consequently on the possible
impacts of future changes in BBA emissions.