The South Asian Monsoon Response to Remote Aerosols: Global and Regional Mechanisms

Abstract

The South Asian summer monsoon has been suggested to be influenced by atmospheric aerosols, and this influence can be the result of either local or remote emissions. We have used the Hadley Centre Global Environment Model Version 3 (HadGEM3) coupled atmosphere-ocean climate model to investigate for the first time the centennial-scale South Asian precipitation response to emissions of sulfur dioxide (SO 2 ), the dominant anthropogenic precursor of sulfate aerosol, from different midlatitude regions. Despite the localized nature of the regional heating that results from removing SO 2 emissions, all experiments featured a similar large-scale precipitation response over South Asia, driven by ocean-modulated changes in the net cross-equatorial heat transport and an opposing cross-equatorial northward moisture transport. The effects are linearly additive, with the sum of the responses from the experiments where SO 2 is removed from the United States, Europe, and East Asia resembling the response seen in the experiment where emissions are removed from the northern midlatitudes as a whole, but with East Asia being the largest contributor, even per unit of emission or top-of-atmosphere radiative forcing. This stems from the fact that East Asian emissions can more easily influence regional land-sea thermal contrasts and sea level pressure differences that drive the monsoon circulation, compared to emissions from more remote regions. Our results suggest that radiative effects of remote pollution should not be neglected when examining changes in South Asian climate and that and it is important to examine such effects in coupled ocean-atmosphere modeling frameworks.Plain Language Summary Atmospheric aerosols have been shown to exert a strong influence on global and regional climate through their radiative effects. The South Asian summer monsoon, a climate phenomenon on which billions of lives depend, has been suggested to be influenced by aerosols, and this influence can be the result of either local or remote emissions. We have used a global climate model to investigate how sensitive is the monsoon to emissions from remote industrialized regions such as Europe, the North America, and East Asia. Despite the localized nature of the regional heating that results from removing emissions in these regions, all of them featured a similar large-scale rainfall change remotely over South Asia. This similarity reflects a common underlying mechanism, which involves a displacement of the tropical rain belt driven by aerosol effects on the temperature contrast between the two hemispheres of the Earth. We find that the effects of the different regions on South Asia are linearly additive, with East Asia being the largest contributor. This stems from the fact that its emissions can more easily influence regional land-sea thermal contrasts and sea level pressure differences over Asia that drive the summer monsoon circulation, compared to emissions from more remote regions. Our results suggest that influences of remote pollution should not be ...

Accelerated increases in global and Asian summer monsoon precipitation from future aerosol reductions

“…The anomaly from SSP5-8.5 is also significantly larger than that from SSP2-4.5. This suggests that anthropogenic aerosol plays a limited role in the evolution of global mean near-surface temperature on these timescales, supporting the conclusions of Shindell and Smith (2019). However, as will be discussed in Sect.…”

“…For precipitation, the regional response is particularly pronounced for the Asian summer monsoon (Levy et al, 2013;Westervelt et al, 2015;Acosta Navarro et al, 2017;Bartlett et al, 2018;Samset et al, 2018). Here, precipitation is sensitive to changes in remote aerosol through its control on the interhemispheric temperature gradient and intertropical convergence zone (ITCZ) location and the atmospheric wave pattern over Eurasia and to local aerosol changes, which further modify the local monsoon circulation (Polson et al, 2014;Dong et al, 2016;Guo et al, 2016;Shawki et al, 2018;Undorf et al, 2018). A number of studies have suggested that historical aerosol increases are a key driver of the observed decrease in Asian summer monsoon precipitation (Lau and Kim, 2010;Song et al, 2014;Li et al, 2015;Liu et al, 2019), and projected increases in precipitation due to aerosol reductions are consistent with this.…”

“…In opposition to the CMIP5 generation findings summarised above, Shindell and Smith (2019) recently dismissed the possibility that future aerosol reductions might lead to rapid increases in the magnitude or rate of global-mean warming, even in scenarios with aggressive clean air policies, based on simulations with a reduced-complexity impulse response model (Smith et al, 2018). Yet, this conclusion may not hold when using a fully coupled global climate model (GCM) and when investigating changes beyond global mean temperature.…”

Research and Education in India on Climate Change

“…Several faculty members of IISc have been experts at various Assessement Reports of the IPCC (past and present). Publications from the IISc in the period July 2018 -June 2019 include: South asian monsoon response to remote aerosols 41 . Research at IISc Connected with climate change, monsoon etc., is also going on in its Centre for Atmospheric and Oceanic Sciences (CAOS), its Centre for Earth Sciences (CEaS), and the Centre for Sustainable Technologies (CST), in addition to the Divecha Centre (DCCC).…”

Review of Zanis et al.

“…Which is fine, given the focus of the paper on fast responses, but it is worth stressing this again here. The studies by Ganguly et al (2012) and Shawki et al (2018) provide nice insight into the differing fast and slow effects of aerosols on the South Asian monsoon, as well as the complementary global and regional mechanisms that are at play.…”