Disentangling the influence of local and remote anthropogenic aerosols on South Asian monsoon daily rainfall characteristics

Abstract

Wet and dry periods within the South Asian summer monsoon season can have acute societal impacts. Recent studies have identified changes in daily rainfall characteristics of the monsoon, but the underlying causes are poorly understood. In particular, although the dominant role of anthropogenic aerosols in shaping historical changes in seasonal-mean monsoon rainfall has been documented, their influence on daily-scale rainfall remains unconstrained. Using an ensemble of single-forcing climate simulations, we find that anthropogenic aerosols have a stronger influence on late-20 th century changes in the frequency of wet events, dry events and rainless days, compared with other climate forcings. We also investigate the role of aerosol-cloud interactions ("indirect effects") in the total aerosol response, and the contribution of aerosols emitted from South Asia versus from remote sources. Based on additional simulations with the GFDL-CM3 climate model, we find that the simulated aerosol response over South Asia is largely associated with aerosol-indirect effects. In addition, local aerosols suppress wet-event frequency and enhance dry-event frequency over eastern-central India, where increases in aerosol loading are the largest. Remote aerosols cause a north-south dipole pattern of change in mean rainfall over India and fewer rainless days over western India. However, the overall spatial response of South Asian rainfall characteristics to total aerosol forcing is substantially influenced by the non-linear climate response to local and remote aerosols. Together, our results suggest that understanding the influence of different aerosol emissions trajectories on the regional climate dynamics is critical for effective climate-risk management in this populated, vulnerable region.

Indian summer monsoon: Extreme events, historical changes, and role of anthropogenic forcings

“…While aerosols and GHG have been the most widely‐studied external forcings on the South Asian summer monsoon, the role of widespread agricultural expansion and intensification in driving the 20th century weakening are starting to get attention (Krishnan et al, ; Lee et al, ; Niyogi, Kishtawal, Tripathi, & Govindaraju, ; Paul et al, ; Shukla et al ; D. Singh, McDermid, et al, ). Recent studies have shown the potential for land‐cover change from agricultural expansion to contribute to the weakening of the monsoon.…”

“…Unlike most other regions where GHG are likely to be the dominant forcing in the future, South Asia will continue to experience these multiple, diverse, and spatially heterogeneous forcings (Figure ), which have likely already influenced several characteristics of the monsoon (Bollasina, Ming, & Ramaswamy, , ; Douglas, Beltrán‐Przekurat, Niyogi, Pielke, & Vörösmarty, ; Guo, Turner, & Highwood, ; Lau & Kim, ; Z. Li et al, ; Meehl, Arblaster, & Collins, ; Paul, Ghosh, Oglesby, Pathak, & Chandrasekharan, ; Puma & Cook, ; Ramanathan et al, ; Ramanathan & Carmichael, ; Salzmann, Weser, & Cherian, ; Shukla, Puma, & Cook, ; D. Singh, Bollasina, Ting, & Diffenbaugh, ). Regional aerosols are projected to increase for at least the next few decades and agricultural activities will continue to intensify, to meet the rising food and energy demands of the growing population (Defries, Bounoua, & Collatz, ; R. Kumar et al, ; Tilman et al, ; van Vuuren et al, ).…”

Dimethylsulfide (DMS), marine biogenic aerosols and the ecophysiology of coral reefs

“…On a global scale, atmospheric DMS concentrations are predicted to increase by 41 % with a tripling of atmospheric CO 2 , increasing mean aerosol optical depth (AOD) by 3.5 % and cooling the Northern and Southern Hemisphere by 0.4 and 0.8 K, respectively (Gabric et al, 2013). Other studies simulate a global reduction of the DMS flux by 10 %-18 % (Kloster et al, 2007;Six et al, 2013) and 48 % (Schwinger et al, 2017) under various ocean acidification scenarios by the end of the 21st and 22nd centuries, respectively. Modelled scenarios predict an additional global warming of 0.23-0.48 K due to a decline in DMS-derived sulfate aerosols and cloud albedo (Schwinger et al, 2017;Six et al, 2013), although significant regional variability in the response of DMS emissions to climate perturbations was present.…”

“…Other studies simulate a global reduction of the DMS flux by 10 %-18 % (Kloster et al, 2007;Six et al, 2013) and 48 % (Schwinger et al, 2017) under various ocean acidification scenarios by the end of the 21st and 22nd centuries, respectively. Modelled scenarios predict an additional global warming of 0.23-0.48 K due to a decline in DMS-derived sulfate aerosols and cloud albedo (Schwinger et al, 2017;Six et al, 2013), although significant regional variability in the response of DMS emissions to climate perturbations was present. In the Southern Ocean, DMS emissions increased in response to warming, counteracting the predicted regional decrease in emissions in response to OA (Schwinger et al, 2017).…”

“…Conversely, Fiddes et al (2018) found that DMS-derived sulfate particles enhanced LLC cover and lifetime and consequently suppressed rainfall in parts of the tropics. When high concentrations of fine particles rapidly grow to CCN activation sizes, cloud droplet size decreases, cloud lifetime increases and rainfall is suppressed (Rosenfeld et al, 2007;Dave et al, 2019;Singh et al, 2018). However, when CCN minimum size is not met, water vapour remains in the atmosphere, suppressing local rainfall yet enhancing rainfall downwind when conditions for particle growth are more favourable (Andreae and Rosenfeld, 2008;Fan et al, 2018;Grandey and Wang, 2015;Lin et al, 2018;Guo et al, 2016).…”