The effect of regional changes in anthropogenic aerosols on rainfall of the East Asian Summer Monsoon

Abstract

The response of East Asian Summer Monsoon (EASM) precipitation to long term changes in regional anthropogenic aerosols (sulphate and black carbon) is explored in an atmospheric general circulation model, the atmospheric component of the UK High-Resolution Global Environment Model v1.2 (HiGAM). Separately, sulphur dioxide (SO<sub>2</sub>) and black carbon (BC) emissions in 1950 and 2000 over East Asia are used to drive model simulations, while emissions are kept constant at year 2000 level outside this region. The response of the EASM is examined by comparing simulations driven by aerosol emissions representative of 1950 and 2000. The aerosol radiative effects are also determined using an off-line radiative transfer model. During June, July and August, the EASM was not significantly changed as either SO<sub>2</sub> or BC emissions increased from 1950 to 2000 levels. However, in September, precipitation is significantly decreased by 26.4% for sulphate aerosol and 14.6% for black carbon when emissions are at the 2000 level. Over 80% of the decrease is attributed to changes in convective precipitation. The cooler land surface temperature over China in September (0.8 °C for sulphate and 0.5 °C for black carbon) due to increased aerosols reduces the surface thermal contrast that supports the EASM circulation. However, mechanisms causing the surface temperature decrease in September are different between sulphate and BC experiments. In the sulphate experiment, the sulphate direct and the 1st indirect radiative effects contribute to the surface cooling. In the BC experiment, the BC direct effect is the main driver of the surface cooling, however, a decrease in low cloud cover due to the increased heating by BC absorption partially counteracts the direct effect. This results in a weaker land surface temperature response to BC changes than to sulphate changes. The resulting precipitation response is also weaker, and the responses of the monsoon circulation are different for sulphate and black carbon experiments. This study demonstrates a mechanism that links regional aerosol emission changes to the precipitation changes of the EASM, and it could be applied to help understand the future changes in EASM precipitation in CMIP5 simulations

Preferred response of the East Asian summer monsoon to local and non-local anthropogenic sulphur dioxide emissions

“…This excessive equatorial Indian Ocean rainfall appears to be an inherent feature of the Met Office Unified model, with preferential model convection over areas with large amounts of available heat and moisture (Martin et al 2010). This bias is also found in other versions of the Met Office Hadley Centre Global Environmental Model (Levine and Turner 2012;Guo et al 2013). Despite these deficiencies, the simulated precipitation over East Asia compares fairly well with observations with an area averaged model precipitation of 2.99 mm day −1 over North China (35°N-45°N, 100°E-125°E) and 8.93 mm day −1 over South China (22.5°N-32.5°N, 100°E-120°E), in comparison with area averaged values of 2.70 and 6.58 mm day −1 in observations (Fig.…”

“…In this study, we investigate the transient responses to an abrupt change in sulphur dioxide emissions from specific regions (Asia and Europe) in order to study physical processes of the EASM changes, using simulations with an atmospheric general circulation model. We chose to study the impacts of sulphur dioxide emissions since previous studies indicated that regional sulphate aerosols have a more important impact on the EASM than BC (e.g., Liu et al 2009;Guo et al 2013). A similar approach was used in Dong et al (2009) to investigate the transient adjustment of the atmosphere and land surface in response to an instantaneous doubling of CO 2 , and in Dong et al (2014) to investigate transient adjustment of the West African Monsoon to abrupt changes in regional sulphur dioxide emissions.…”

“…Subsequently, however, this cold and dry anomaly is advected into Asia, where it induces atmospheric and surface feedbacks over Asia and the Western North Pacific (WNP), which also weaken the EASM. In spite of very different perturbations to the local aerosol burden in response to Asian and European sulphur dioxide 1 3 , decadal changes in sensible heat flux over the Tibetan Plateau (Ding et al 2009;Duan et al 2013), anthropogenic aerosol (Xu 2001;Menon et al 2002;Huang et al 2007;Liu et al 2009Liu et al , 2011Chen et al 2012;Zhang et al 2012;Guo et al 2013;Hwang et al 2013;Jiang et al 2013;Ye et al 2013;Wang et al 2013;Zhou et al 2013;Polson et al 2014;Song et al 2014), and natural decadal variability (Lei et al 2011).…”

“…The addition of non-Asian aerosols generated an enhancement and broadening of cooler temperatures over Europe and Asia relative to the ambient oceans, supporting stronger northerly flows that further suppress Asian monsoon rainfall, highlighting the importance of the non-Asian aerosols in exacerbating the impact of Asian aerosols on monsoon rainfall across Asia. Liu et al (2009) and Guo et al (2013) investigated the role of both sulphur dioxide and black carbon (BC) emissions over Asia on the EASM and concluded that regional sulphate aerosols have a more important impact on the EASM than BC. Guo et al (2013) further found that the impacts of aerosols are more significant during the withdrawal phase of the EASM (September) rather than active phase (June, July, August).…”

“…Liu et al (2009) and Guo et al (2013) investigated the role of both sulphur dioxide and black carbon (BC) emissions over Asia on the EASM and concluded that regional sulphate aerosols have a more important impact on the EASM than BC. Guo et al (2013) further found that the impacts of aerosols are more significant during the withdrawal phase of the EASM (September) rather than active phase (June, July, August).…”