The influence of aerosol forcing on the South Asian monsoon in CMIP5
historical simulations
Andrew Turner
This study examines CMIP5 historical simulations, in which greenhouse gases
(GHGs), aerosols and other forcers are changed over the 20th century in
accordance with observed values. While strong GHG forcing may be expected to
increase monsoon rainfall, the observed raingauge record for India suggests
declining monsoon rainfall since the 1950s. Our analysis shows that CMIP5
experiments in which only aerosol forcings were used as perturbations during
the 20th century most closely match this downward trend. Furthermore, we find
a distinct difference between the groups of models that include aerosol indirect
effects (aerosol-cloud interactions) and those that do not. The models including
indirect effects show much greater negative radiative forcing over India and the
northern hemisphere as a whole, leading to declining monsoon rainfall. Those
models including only direct radiative effects of aerosol in fact show increasing
monsoon rainfall at the end of the 20th century.