Detectable Impact of Local and Remote Anthropogenic Aerosols on the 20th Century Changes of West African and South Asian Monsoon Precipitation

Anthropogenic aerosols are a key driver of changes in summer monsoon precipitation in the
Northern Hemisphere during the 20th century. Here we apply detection and attribution methods to
investigate causes of change in the West African and South Asian monsoons separately and identify the
aerosol source regions that are most important for explaining the observed changes during 1920–2005.
Historical simulations with the GFDL-CM3 model are used to derive fingerprints of aerosol forcing from
different regions. For West Africa, remote aerosol emissions from North America and Europe (NAEU)
are essential in order to detect the anthropogenic signal in observed monsoon precipitation changes.
The changes are significantly underestimated in the model, however. While natural (volcanic) forcing
seems to also play a role, the dominant contribution is found to come from aerosol-induced changes in the
interhemispheric temperature gradient and associated meridional shifts of the Intertropical Convergence
Zone. For South Asia, in contrast, changes in observed monsoon precipitation cannot be explained
without local emissions. Here the findings show a weakening of the monsoon circulation, driven by the
increase of remote NAEU aerosol emissions until 1975, and since then by the increase in local emissions
offsetting the decrease of NAEU emissions. The results show that the aerosol forcing from individual
emission regions is strong enough to be detected over internal variability. They also underscore the
importance of the spatial pattern of global-aerosol emissions, which is likely to continue to change
throughout the expected near-future decline in global emissions.