Aerosol effects on summer monsoon over Asia during 1980s and 1990s

Abstract

The Community Earth System Model is used to study the aerosol climate effects during the 1980s and 1990s in which the anthropogenic SO2 emissions decreased in North America and Western Europe and increased in East and South Asia. From the 100 year simulations, aerosol forcing results in cooler (−0.13 K) and drier (−0.01 mm/day) atmosphere with less shortwave radiation flux at the surface (−0.37 W/m2). The clear‐sky shortwave radiation flux decreased over East Asia (−0.81 W/m2) and South Asia (−1.09 W/m2), but increased over Western Europe (+1.16 W/m2) and North America (+0.39 W/m2), consistent with aerosol loading changes. While changes in spatial distributions of all‐sky shortwave radiation and surface temperature are closely related to cloud changes, the changes in wind and precipitation do not correspond to aerosol loading changes, indicating the complexity of aerosol‐cloud circulation interactions. The East and South Asia monsoons were generally weakened due mainly to southward shift of the 200 hPa East Asia Jet (EAJ) and decrease in 850 hPa winds; annual precipitation decreased by 2% in South Asia but increased by 2% in Yangtze‐Huai River Valley over East Asia. The uncertainties associated with aerosol climate effects are addressed within the context of model variability and the global warming effect. For the latter, while the aerosol effects decrease the greenhouse warming on the global mean, the regional responses are different. Nevertheless, the characteristics of aerosol climate effects, including the southward 200 hPa EAJ and weakened South Asia monsoon, still persist when the climate becomes warmer, although the strength and the geographical distribution are slightly modulated.

Impacts of recent decadal changes in Asian aerosols on the East Asian summer monsoon: roles of aerosol–radiation and aerosol–cloud interactions

“…Atmospheric aerosols and monsoon circulation over East Asia are strongly coupled. The distribution of aerosol is determined by the atmospheric circulation, and deposition processes, while the circulation is affected by the aerosol (Liu et al 2011;Tao et al 1 3 2012; Liao et al 2015;Wu et al 2015;Li et al 2016;Tsai et al 2016). Aerosol emission changes have the potential to affect both regional and global climate by altering the radiation budget directly and interacting with clouds through microphysical processes, thereby causing subsequent changes in surface temperature, and the local and global hydrological cycle (e.g., Giorgi et al 2003;Rosenfeld et al 2008;Stevens and Feingold 2009;Liu et al 2011;Tao et al 2012;Levy et al 2013;Stjern and Kristjansson 2015;Wang et al 2015;Kim et al 2016;Xie et al 2016a, b;Lau and Kim 2017;Li et al 2016Li et al , 2018Wilcox et al 2018).…”

“…This is known as the aerosol indirect effects or aerosol-cloud interaction (ACI) (Boucher et al 2013). The radiative and cloud processes can interact with each other and produce complex aerosol effects on clouds, precipitation, and extreme precipitation, both locally and remotely to emission regions (e.g., Chou et al 2005;Bollasina et al 2014;Dong et al 2016;Tsai et al 2016;Xie et al 2016a, b;Wang et al 2017a;Zhang et al 2017;Lin et al 2018;Wilcox et al 2018).…”

“…Previous studies have demonstrated that anthropogenic aerosols are an important driver of changes in the East Asian climate (Liu et al 2011;Guo et al 2013;Song et al 2014;Liao et al 2015;Wu et al 2015;Jiang et al 2015;Stjern and Kristjansson 2015;Wang et al 2015Wang et al , 2017aDong et al 2016;Kim et al 2016;Zhang and Li 2016;Lau and Kim 2017;Tsai et al 2016;Zhang et al 2017). However, many questions remain open.…”

“…Most early studies of the influence of AA on the East Asian summer monsoon used either Atmospheric General Circulation Models (AGCMs) (Liu et al 2011;Guo et al 2013;Jiang et al 2015;Wang et al 2015;Dong et al 2016;Xie et al 2016a, b) or Coupled General Circulation Models (CGCMs) (e.g., Bollasina et al 2014;Song et al 2014;Stjern and Kristjansson 2015;Tsai et al 2016;Zhang and Li 2016;Lau and Kim 2017;Wang et al 2017a, b, c;Zhang et al 2017). In AGCMs, aerosol induced ocean feedbacks are not considered while the CGCMs exhibit significant biases in the mean state (e.g., Wang et al 2014).…”

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

“…Modeling studies suggest a role for both local and nonlocal aerosols in weakening monsoons in Asia (Cowan & Cai, ; Dong et al, ; Guo et al, ) and West Africa (Dong, Sutton, et al, ), though local aerosol emissions are suggested to be predominantly responsible for the drying of the South Asian monsoon (Bollasina et al, ; Ganguly et al, ; Guo et al, ; Salzmann et al, ). Some other studies looked at the effect of the simultaneous increase of Asian emissions and decrease of North American and European emissions (Tsai et al, ), but the relative influence of aerosols from different regions on the various monsoon systems still remains largely unclear.…”