15 NOVEMBER 2020 C H E N E T A L . 9783 Regional Climate Responses in East Asia to the Black Carbon Aerosol Direct Effects from India and China in Summer a a a,b a a a a HUIMIN CHEN, BINGLIANG ZHUANG, JANE LIU, SHU LI, TIJIAN WANG, XIAODONG XIE, MIN XIE, a a MENGMENG LI, AND MING ZHAO a School of Atmospheric Sciences, CMA-NJU Joint Laboratory for Climate Prediction Studies, Jiangsu Collaborative Innovation Center for Climate Change, Nanjing University, Nanjing, China; b Department of Geography and Planning, University of Toronto, Toronto, Canada (Manuscript received 3 October 2019, in final form 5 July 2020) ABSTRACT: Black carbon (BC) aerosol is a significant and short-lived climate forcing factor. Here, the direct effects of BC emissions from India (IDBC) and China (CNBC) are investigated in East Asia during summer using the state-of-the-art regional climate model RegCM4. In summer, IDBC and CNBC account for approximately 30% and 46% of the total BC emissions in Asia, respectively. The total BC column burden from the two countries and corresponding TOA effective 2 2 radiative forcing are 1.58 mg m 2 and 11.87 W m 2 in East Asia, respectively. The regional air temperature increases over 2 0.3 K at maximum and precipitation decreases 0.028 mm day 1 on average. Individually, IDBC and CNBC each can bring about rather different effects on regional climate. IDBC can result in a cooling perturbation accompanied by a substantially increased cloud amount and scattering aerosol loading, resulting in a complex response in the regional precipitation, while CNBC can lead to regional warming, and further induce a local flood in northern China or drought in southern China depending on the opposite but significant circulation anomalies. CNBC plays a dominant role in modulating the regional climate over East Asia due to its higher magnitude, wider coverage, and stronger climate feedback. The direct effect of the total BC from both countries is not a linear combination of that of IDBC and CNBC individually, suggesting that the regional climate responses are highly nonlinear to the emission intensity or aerosol loading, which may be greatly related to the influences of the perturbed atmospheric circulations and climate feedback. KEYWORDS: Numerical analysis/modeling; Regional models; Aerosol radiative effect; Aerosols/particulates 1. Introduction et al. 2013; Zhuang et al. 2014; Wang et al. 2015; K. Li et al. Black carbon (BC) aerosols can significantly impact climate 2016). BC imposes a positive direct radiative forcing (DRF) at change at both regional and global scales via the direct, indi- the top of the atmosphere (TOA), ranging from the order of 2 2 2 rect, and snow albedo effects (Twomey 1974; Albrecht 1989; 10 1 Wm 2 on a global scale to 100 Wm 2 in polluted urban Menon et al. 2002; Lau et al. 2006; Forster et al. 2007; Randles areas (Zhuang et al. 2013; Boucher et al 2013; Myhre et al. and Ramaswamy 2008; Zhuang et al. 2010; Wang et al. 2011; 2013; Zhuang et al. 2014). The regional mean of BC DRF was 2 Wilcox 2012). The BC potential warming effect plays an im- estimated to be 10.81 W m 2 over East Asia (Zhuang et al. 2 portant role in global and regional warming (Myhre et al. 2013) and 11.22 W m 2 over China (K. Li et al. 2016), while 2 2013). The convergence and upward motion anomalies caused the global mean is assessed as 10.71 W m 2 (Bond et al. 2013). by BC can result in a more unstable stratification of the at- Therefore, the CO2 greenhouse effect, which is offset by the mosphere (Randles and Ramaswamy 2008; Zhuang et al. negative TOA DRF of scattering aerosols (Kiehl and Briegleb 2013), which further leads to changes in thermal-dynamic 1993), could be enhanced by the BC direct effect (Wang et al. processes and the hydrologic cycles (Menon et al. 2002; Lau 2015; Zhuang et al. 2018). The BC-induced radiation per- and Kim 2006; Bollasina et al. 2008; Wu et al. 2008; Zhuang turbation alters the regional or global climate because the et al. 2018, 2019). perturbation can impact the thermodynamic conditions and BC aerosols are known to influence the regional climate hydrological cycle (Lohmann et al. 2000; Kristjánsson 2002; over East Asia in summer to a large degree (Guo et al. 2013; Qian et al. 2003; Cook and Highwood 2004; Wu et al. 2008; Zhou et al. 2014; Song et al. 2014; Wang et al. 2015; Zhuang Bollasina et al. 2008; Zhuang et al. 2010, 2013). Moreover, the et al. 2018) through affecting Earth’s radiation budget, which is strongest and most extensive radiative forcing by both scat- referred to as the BC direct effect. According to both obser- tering and absorbing aerosols in the Northern Hemisphere vation and simulation, BC reduces the incoming shortwave occurs in summer (Ghan et al. 2001; Zhang et al. 2008, 2009; Yu radiation that reaches the surface more effectively than scat- et al. 2013; Zhuang et al. 2018), and East Asia experiences tering aerosols (Forster et al. 2007; Boucher et al. 2013; Bond heavy rainfall in summer because of the Asian monsoon. As a result, the weather and climate system are more sensitive to aerosol effects during this season (Zhang et al. 2009). Lau et al. Denotes content that is immediately available upon publica- (2006), Lau and Kim (2006), and Meehl et al. (2008) suggested tion as open access. that increased absorbing aerosols (including BC) could inten- sify the circulation of the Indian summer monsoon with the Corresponding author: Bingliang Zhuang, [email protected] advancing rainy season, thus leading to the increased precipitation DOI: 10.1175/JCLI-D-19-0706.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/03/21 09:47 PM UTC 9784 JOURNAL OF CLIMATE VOLUME 33 over South Asia and decreased precipitation over East Asia. Manoj et al. (2011) indicated that BC can also be conducive to the Indian summer monsoon transition from break to active spells. Furthermore, Wang et al. (2015) and Zhuang et al. (2018) implied that BC can favor the circulation development of East Asian summer monsoon, and further result in regional droughts and floods. Additionally, the atmospheric stability can be modified under the BC direct effect (Randles and Ramaswamy 2008; Wu et al. 2008; Zhuang et al. 2013; Ding et al. 2016; S. Li et al. 2016). All these studies have addressed the significant role of BC in modulating regional climate, especially in high BC regions (e.g., China and India) or in sensitive climate systems (e.g., the summer monsoon). China has frequently experienced episodes of severe air pol- lution over the recent decades because of rapid industrialization and urbanization (Yang et al. 2018), which are accompanied with substantially increased emissions of aerosols including BC (Qin et al. 2001; Cao et al. 2006; Zhang et al. 2009; Li et al. 2017). Observations in China showed that the annual mean of surface m 23 2 FIG. 1. Seasonal mean of surface BC concentrations ( gm ) over BC concentration was up to 10 mgm 3 in urban areas and up to 2 the study domain. 4 mgm 3 at rural sites (Zhang et al. 2008, 2012). BC can consid- erably contribute to haze pollution episodes (Ding et al. 2016). Observations in India, where national BC emissions are second 2. Methodology highest among Asian countries, suggested that BC loadings there 2 could reach 15 mgm 3 in summer at urban sites despite a wide a. Description of the regional climate model RegCM4 range of spatial and temporal variations (Babu and Moorthy Because of their higher spatial resolutions, regional climate 2002; Rai et al. 2002; Tripathi et al. 2005; Pant et al. 2006). models are more capable of capturing small-scale character- Furthermore, BC in the Indo-Gangetic Plain (IGP) of northern istics of the climate than global climate models (Denis et al. India, a regional pollution hotspot, strengthened the environ- 2002). In this study, we employ the latest version of the regional mental sensitivity of the domain (Gautam et al. 2011; Giles et al. climate model RegCM4 (Solmon et al. 2012; Giorgi et al. 2012), 2011; Praveen et al. 2012; Lal et al. 2013). The seasonal mean of which has been widely used to investigate the interactions surface BC concentration in summer is illustrated in Fig. 1, between aerosols, climate, and biogeochemical cycles (Zhou 2 showing substantially high BC (exceeding 8 mgm 3)inChinaand et al. 2014; Yin et al. 2015; Z. Q. Li et al. 2016; Zhuang et al. India. Owing to their potential effects on global warming and 2018, 2019; Xie et al. 2020). regional air quality, BC emissions must be reduced. BC emissions RegCM4 provides a much more comprehensive description from China and India have different temporal and spatial varia- on air pollutants than RegCM3, including a gas phase chem- tions, and their BC DRF and associated climate responses are istry module with the Carbon Bond Mechanism, version Z also different, which in turn changes the spatial pattern of forcing (CBMZ; Shalaby et al. 2012) and a volatility basis set model due to other aerosol species in different ways. Notably, the cli- (VBS; Yin et al. 2015) to address trace gases and secondary mate effects may not be entirely dependent on BC loadings organic aerosols, respectively. In addition to the scheme of (Sadiq et al. 2015), which can bring large uncertainty in estimating Qian et al.