Abstract
<div> <div> <div> <p>Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change.  However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, and the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response.  We use historical simulations in which aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to aerosol forcing uncertainty (−0.3 W m−2 to −1.6 W m−2).  Hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall.  Increasing the  scaling from 0.2 to 1.5 reduces the global monsoon area by 3% and the global monsoon intensity by 2% over 1950–2014, and changes the dominant influence on the 1950–1980 monsoon rainfall trend from greenhouse gas to aerosol.   Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall.</p> </div> </div> </div>
Highlights
Monsoon systems provide rainfall for billions of people, many of whom are dependent on the monsoon rains for survival
The cooling associated with these emissions is concentrated in the Northern Hemisphere and opposes the warming effect of greenhouse gases and reduces the temperature contrast between hemispheres and between land and sea
Understanding the interplay between aerosol forcing and monsoon properties in past simulations is important in order to constrain future monsoon projections, where anthropogenic aerosol reductions are likely to strengthen the monsoon, in addition to the strengthening anticipated in response to further increases in greenhouse gases
Summary
Monsoon systems provide rainfall for billions of people, many of whom are dependent on the monsoon rains for survival. Since the middle of the 20th century, large-scale AA-driven circulation changes have acted to weaken the monsoon and have dominated over the response to GHGs. Bollasina et al (2011), Polson et al (2014), Salzmann et al (2014), Song et al (2014) and Guo et al (2015) have all shown that increasing AA emissions played an important part in driving regional and global monsoon rainfall decrease during. The SMURPHS ensemble consists of a set of historical climate simulations with AA emissions scaled by various factors, chosen to span the range of uncertainty in present-day aerosol effective radiative forcing. This allows us to investigate the sensitivity of historical changes in the monsoon to the strength of aerosol forcing, without the complications arising from structural and parametric uncertainty found in a multi-model framework.
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