Atmospheric Responses to the Redistribution of Anthropogenic

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Journal of Geophysical Research: Atmospheres

RESEARCH ARTICLE Atmospheric responses to the redistribution 10.1002/2015JD023665 of anthropogenic aerosols Key Points: Yuan Wang1, Jonathan H. Jiang1, and Hui Su1 • The emission shift contributes to the “ ” “ ” dimming in Asia and brightening 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA in the US and Europe • Atmospheric meridional circulations are weakened by the redistributed aerosols Abstract The geographical shift of global anthropogenic aerosols from the developed countries to the Asian • Aerosol effects from different regions continent since the 1980s could potentially perturb the regional and global climate due to aerosol-cloud-radiation contribute distinctively to the interactions. We use an atmospheric general circulation model with different aerosol scenarios to investigate circulation modiﬁcation the radiative and microphysical effects of anthropogenic aerosols from different regions on the radiation budget, precipitation, and large-scale circulations. An experiment contrasting anthropogenic aerosol scenarios in 1970 and 2010 shows that the altered cloud reﬂectivity and solar extinction by aerosols results in regional Correspondence to: surface temperature cooling in East and South Asia, and warming in the US and Europe, respectively. These Y. Wang, [email protected] aerosol-induced temperature changes are consistent with the relative temperature trends from 1980 to 2010 over different regions in the reanalysis data. A reduced meridional streamfunction and zonal winds over the tropics as well as a poleward shift of the jet stream suggest weakened and expanded tropical circulations, Citation: Wang, Y., J. H. Jiang, and H. Su (2015), which are induced by the redistributed aerosols through a relaxing of the meridional temperature gradient. Atmospheric responses to the Consequently, precipitation is suppressed in the deep tropics and enhanced in the subtropics. Our assessments redistribution of anthropogenic of the aerosol effects over the different regions suggest that the increasing Asian pollution accounts for the aerosols, J. Geophys. Res. Atmos., 120, 9625–9641, doi:10.1002/2015JD023665. weakening of the tropics circulation, while the decreasing pollution in Europe and US tends to shift the circulation systems southward. Moreover, the aerosol indirect forcing is predominant over the total aerosol Received 13 MAY 2015 forcing in magnitude, while aerosol radiative and microphysical effects jointly shape the meridional energy Accepted 24 AUG 2015 distributions and modulate the circulation systems. Accepted article online 27 AUG 2015 Published online 30 SEP 2015

1. Introduction Atmospheric aerosols from natural or anthropogenic sources have profound impacts on the regional and global climate [Andreae and Rosenfeld, 2008]. Currently, the radiative forcing of aerosols in the climate system remains highly uncertain, representing the largest uncertainty in climate predictions [Myhre et al., 2013]. In addition to the complicated chemical and physical properties of aerosols [Zhang et al., 2015; Wang et al., 2013] and the various mechanisms of aerosol-cloud-precipitation interactions [Rosenfeld et al., 2014; Altaratz et al., 2014], the inhomogeneous and fast varying distribution of aerosols in space substantially contributes to the uncertainties of the aerosol forcing assessment. In particular, the anthropogenic emissions of aerosols and precursor gases have undergone dramatic changes during the past few decades. As the world’s most populous continent, Asia has experienced a quasi-exponential growth in industrialization, which has led to a rapid increase in emissions of gas-phase and particulate pollutants to the atmosphere. Conversely, emission levels have been stabilized or substantially reduced in the traditionally developed countries of North America and Europe since the 1960s [Smith et al., 2011], due to stricter government environmental policies. This global redistribution of anthropogenic aerosols could potentially emerge as a critical player in global climate change due to its signiﬁcanteffectsonregionalradiativebudget,aswellasthe dynamical and microphysical evolution of cloud systems. The impacts of changes in anthropogenic emissions from preindustrial (PI) to present-day (PD) conditions on the global climate and on large-scale circulations have been extensively investigated by previous modeling studies. For instance, Ming and Ramaswamy [2011] employed an atmosphere-ocean coupled general circulation model (AOGCM) to study the responses of the tropical circulation and hydrological cycle to the inter-hemispherically asymmetrical aerosol forcings by contrasting the PD and PI aerosol scenarios. They found a weakened (enhanced) Hadley circulation in the Northern (Southern) Hemisphere due to the elevated aerosol levels and the associated radiative cooling in the Northern Hemisphere since the 1850s. Such a ﬂ ©2015. American Geophysical Union. modulation results in northward energy uxes across the equator and a southward shift in tropical rainfall. All Rights Reserved. Similar climate responses in the form of a southward shift of the Intertropical Convergence Zone (ITCZ) to

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the aerosol forcings since the preindustrial days were reported by Xie et al. [2013] which analyzed the simulation results from the Coupled Model Intercomparison Project Phase 5 (CMIP5). They also suggested that through the ocean-atmosphere feedbacks in the coupled models, the aerosol forcing in the Northern Hemisphere caused a reduction of surface temperature and wind speed over the Southern Ocean. In subtropics and extratropics, Ming et al. [2011] simulated a wintertime equatorward shift of the subtropical jet and midlatitude storm tracks in the Northern Hemisphere, particularly in the North Pacific, due to the pronounced cooling effect of aerosols. Rotstayn et al. [2013] further argued that anthropogenic aerosol effects in the Northern Hemisphere tend to weaken the subtropical jet in the Southern Hemisphere by decreasing the midtropospheric temperature gradient between low and middle latitudes. In recent decades, especially since 1980, the regional and global climate has experienced dramatic changes. Numerous studies linked the recent climate change to the variation of the aerosols in the different regions. At the global scale, Allen and Sherwood [2010] and Allen et al. [2012] attributed the observed tropical expansion in recent decades to the increases in heterogeneous warming caused by the elevated absorbing aerosols such as black carbon as well as tropospheric ozone based on global climate simulations. Murphy [2013] showed little net clear-sky radiative forcing from the recent (2000–2012) regional redistribution of aerosols using satellite observation and a radiative transfer model, while the modeling study by Yang et al. [2014] suggested a global cooling (À0.015 K/decade) driven by the Asian aerosols since the 1970s. The influence of local changes in aerosol amount and types on the regional radiation budget and hydrological cycle is expected to be more prominent than that on global mean. Over South Asia, Bollasina et al. [2011] used a series of climate model experiments to investigate the responses of the South Asian monsoon to enhanced aerosol forcing. They concluded that the recent widespread drought in South Asia is an outcome of a slowdown of the tropical meridional overturning circulation, which can be attributed mainly to anthropogenic aerosol emissions. Over East Asia, long-term in situ measurements and regional cloud-resolving simulations suggested that the increases in anthropogenic aerosols serving as cloud condensation nuclei (CCN) can suppress light precipitation, enhance heavy precipitation, invigorate the convective system, and elevate lightning activities in China [Qian et al., 2009; Wang et al., 2011; Fan et al., 2012]. Moreover, modeling studies suggested that the observed tendency of “southern flood and northern drought” during the weakened East Asian summer monsoon was caused by the reduction in the land-sea thermal contrast due to the aerosol forcing over northern China [Wu et al., 2013; Song et al., 2014]. Over the North Pacific, multiscale modeling studies suggested Asian pollution outflow accounts for an enhanced amount of deep convective clouds, increased precipitation, and invigorated storms during the wintertime [Wang et al., 2014a, 2014b]. Over Central Europe, regional modeling simulations constrained by reanalysis data showed that aerosol reduction and the associated radiative effects are responsible for about 80% of the atmospheric brightening and 23% of the surface warming since the 1980s [Nabat et al., 2014, Cherian et al., 2014]. Most of the previous modeling studies focused on the regional changes in cloud systems and atmospheric states due to local aerosol perturbations using regional climate modeling systems. Hence, there is a funda- mental need to comprehensively assess the impacts of the observed shift of global anthropogenic aerosol distributions on the global radiation budget, hydrological cycle, and circulation systems. However, it is challenging to achieve this assessment by analyzing all of the CMIP5 simulation results. In the current CMIP5 GCMs, the representations of aerosols and parameterizations of aerosol-cloud interactions vary substantially in the degree of complexity, which profoundly affects quantifications of aerosol effects [Ekman, 2014]. NCAR-DOE Community Earth System Model (CESM) employs sophisticated parameterizations of aerosol effects in terms of an online prognostic aerosol module and an explicit aerosol activation scheme, but its simulation results are not included in some recent CMIP5 analyses [i.e., Song et al., 2014]. As a first step in a series of research efforts, we employ the atmospheric component of CESM in this study to illustrate fast responses in the climate system, without consideration of ocean feedbacks, due to dramatic changes in aerosol distributions over the globe. Serving as important references, the fast responses to aerosol perturbations reported in this study will be systematically compared with ocean-mediated responses from slab ocean configurations like Ganguly et al. [2012a, 2012b] as well as fully coupled transient simulations like Bollasina et al. [2011] in our future studies. Such comparisons can facilitate the attribution of the aerosol-induced changes to fast atmospheric responses or slow feedbacks from air-sea interactions. In this study, the possible global impacts of the aerosol changes over particular regions will be assessed individually, and we will also separate and compare the aerosol indirect (microphysical) effects from the aerosol direct (radiative) effects.

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This paper is structured as follows: the experiment design and the important aspects of the aerosol effects pertaining to our numerical model are provided in section 2, the simulated impacts of the redistributed aerosols from 1970 to 2010 from a suite of sensitivity experiments are discussed in section 3, and a conclusion and discussions on the merit and the limitation of this modeling study are provided in section 4.

Figure 1. Temporal evolutions of the SO2 emissions from anthropogenic 2. Model Description sectors (such as energy and industry) over South/East Asia, Europe, and North America. The atmospheric component of the CESM version 1.0.4, i.e., CAM5.1, is adopted in this study. It includes sub- stantial improvements and updates for the dynamics and the physical parameterizations compared to previous versions. For example, a new moist turbulence scheme [Bretherton and Park, 2009] explicitly simulates cloud-radiation-turbulence interactions in planetary boundary layer as well as the aerosol effects on stratus. Large-scale cloud and precipitation processes are parameterized with a prognostic two-moment bulk stratiform cloud microphysics scheme [Morrison and Gettelman, 2008]. Deep convection is parameterized by the Zhang and McFarlane [1995] scheme with modiﬁcations of Neale et al. [2008] to use dilute convective available potential energy for closure. Note that aerosol indirect effects on deep convective clouds are still not explicitly considered in CAM5.1. In addition, parameterizations of homogeneous ice nucleation and heterogeneous immersion nucleation in cirrus clouds [Liu and Penner, 2005] are implemented in the microphysics scheme, making it possible to explicitly simulate the effects of sulfate and dust aerosol serving as ice nuclei on cold clouds. Aerosol radiative effects in shortwave and longwave are taken into account by the Rapid Radiative Transfer Method for GCMs (RRTMG) radiative transfer scheme [Iacono et al., 2008] The modal aerosol module with three modes (MAM3) is available in CAM5.1, which provides internally mixed representations of number concentrations and mass for Aitken, accumulation, and coarse aerosol modes [Liu et al., 2012]. Various types of aerosol with different hygroscopicities and optical properties are considered in MAM3, including sulfate, black carbon (BC), primary organic matter, secondary organic aerosol, dust, and sea salt. The aerosol module accounts for most of the important processes associated with atmospheric aerosols, including emission, nucleation, coagulation, condensational growth, gas- and aqueous-phase chemistry, dry deposition, in-cloud and below-cloud scavenging, and reproduction from evaporated cloud droplets. The multicomponent aerosol activation parameterization is based on theschemeofAbdul-Razzak and Ghan [2000]. Anthropogenic emissions are adopted from the Intergovernmental Panel on Climate Change (IPCC) AR5 historic emission data set developed for the CMIP5 [Lamarque et al., 2010], which covers the time period of 1850–2010. Production of sea salt from ocean and mineral dust aerosols from desert will be online calculated following the parameterizations by Mårtensson, 2003 and Yoshioka et al. [2007], respectively.

To illustrate the overall magnitude of emission shift, the historic and projected temporal evolutions of the SO2 emission from 1950 to 2050 are shown in Figure 1 based on the IPCC AR5 anthropogenic emission data set in combination with the representative concentration pathways (RCP 8.5) [Riahi et al., 2011]. As the most impor-

tant precursor gas for sulfate aerosol, evolutions of SO2 emission reﬂect that anthropogenic emissions over Europe and North America have been sharply reduced since 1970 by 73% and 63%, respectively, due to the energy usage transformation in Central Europe like the “Black Triangle” region (Germany-Porland-

Czech Republic) and the legislative controls, such as the Clean Air Act in the US. Meanwhile, SO2 emissions grew fast over developing Asian counties like China and India along with the booming local economy. By

the end of 2010, SO2 emissions have been elevated by four and eight times over East Asia and South Asia, respectively, compared to the emission levels in 1970. As projected by the RCP 8.5, such an emission shift from developed countries to developing ones is expected to last for another 10–20 years. Therefore, it is a

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critical moment now to investigate the regional aerosol-cloud interactions and the atmospheric responses to the unique inhomogeneous aerosol forcings. In this study, the emission scenarios at the years 1970 and 2010 are chosen to represent the historical day (HD) and the present-day (PD) aerosol conditions, respectively. The model’s resolution is 1.25° in longitude and 0.9° in latitude with 30 vertical levels from surface to 3 hPa. The sea surface temperature and sea ice are prescribed by the year 1982–2001 climatology for both PD and HD experiments. The modeling experiment for each anthropogenic emission scenario consists of seven ensemble simulations starting from randomly perturbed initial meteorological ﬁelds. Only aerosols forcings are different between PD and HD, while all other forcings like greenhouse gases are identical. For each ensemble member, the model ran for 10 years, and the results from the last 5 years in each run are analyzed. Four regions (countries) are targeted in this study, including East Asia (China), South Asia (India), Europe, and North America (US).

3. Results 3.1. Model Evaluation Before sensitivity experiments, CAM5 PD simulations of aerosol, cloud properties, and top-of-atmosphere (TOA) radiation fluxes are evaluated with available satellite measurements. At the global scale, CAM5 captures the major hot spots of AOD around the world in comparison of the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements, including the mineral dust over Sahara, sea salt over Southern Ocean, and the anthropogenic pollution over East and South Asia, as shown in Figures 2a and 2b. The overall correlation coefficient between the AOD spatial patterns from model and satellite is 0.4. The global mean in AOD is 0.13 in CAM5, which is comparable to 0.15 averaged from 6 year MODIS measurements from 2005 to 2010. However, at finer scales, the magnitude of AOD in CAM5 is lower than MODIS for most of the nondust regions. This is consistent with the previous study by Shindell et al. [2013] that systematically evaluated AOD simulations in CMIP5 models. The underestimated AOD over remote maritime areas in CAM5 was discussed in Wang et al. [2013] and was attributed to the unrealistic wet removal processes in convective clouds of CAM5. Over continents, biases on AOD could stem from underestimations in the emissions inventory and unresolved subgrid variations of relative humidity due to the model’s coarse resolution. Better than the AOD simulations, spatial patterns of total cloud fraction in CAM5 are comparable to the MODIS retrievals with a correlation coefficient of 0.7. The model reproduces the distributions of tropical convective clouds, clouds with midlatitude storm tracks, and nimbostratus clouds over Southern Ocean (Figures 2c and 2d). The model- simulated cloud properties such as liquid water path (LWP) and ice water content have been validated by satellite measurements [Jiang et al., 2012]. Precipitation in CAM5 is evaluated by the Tropical Rainfall Measuring Mission (TRMM) measurements which have spatial coverage from 50°S to 50°N (Figures 2e and 2f). Globally, the rainfall patterns show a good agreement between CAM5 and TRMM. Precipitation rate is biased high over a narrow belt along the Central Pacific ITCZ and underestimated over the Western Pacific warm pool. The precipitation patterns and the magnitude with midlatitude cyclones are well captured by CAM5. The TOA shortwave radiation fluxes simulated by CAM5 also show a good agreement with the Clouds and the Earth’s Radiant Energy System (CERES) measurements in terms of the spatial patterns (correlation coefficient of 0.8) and magnitude. The overall performance of CAM5 is reasonable as suggested by the previous inter-model comparison study [Jiang et al., 2012], which makes it eligible to assess atmospheric equilibrium and transient responses to aerosol changes.

3.2. Responses to Aerosol Forcings from 1970 to 2010 Under the influence of the shifted anthropogenic emissions, the differences and fractional changes in aerosol optical depth (AOD) between PD and HD are pronounced mainly over the Northern Hemisphere. As shown in Figures 3a and 3b, the significant AOD increases occur in East China, India, and Southeast Asian countries. AOD over the downwind regions of the pollution centers like North Pacific and North Indian Ocean is also elevated. In the Central Europe and Northeast US, the AOD reductions vary by 20–60%. It is important to note that because of the emission reduction in Europe, the aerosol loading over the Arctic region decreases by up to 40%. The AOD variations over North Africa and Middle East are mainly caused by the varied dust loading under the modulated regional meteorological conditions. The changes in the absorption AOD in Figure 3c indicated the anthropogenic sources of black carbon and light absorbing organic carbon. It is clear that increases in the absorbing aerosol amount mainly occur in the East and South Asia with the highest

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Figure 2. Model evaluation using satellite measurements. Climatology of AOD from (a) CAM5 PD simulations and (b) MODIS L3 measurements; climatology of total cloud fraction from (c) CAM5 PD simulations and (d) MODIS L3 measurements; climatology of total precipitation from (e) CAM5 PD simulations and (f) TRMM measurements; climatology of outgoing shortwave radiation at TOA from (g) CAM5 PD simulations and (h) CERES measurements.

concentration in East China. Europe experiences a slight decrease in BC, while there is no reduction in the BC over North America. From the chemical composition analysis of the fine-mode aerosols over four target regions (Figure 4), we find out that sulfate aerosols account for the most variations of the aerosol mass from HD to PD in all four regions among all six types of aerosols. The net aerosol forcing at TOA caused by the shifted aerosol distribution is À0.23 W/m2 in global mean, which implies that the elevated pollution in Asian countries exerts larger influence on the radiation budget than the aerosol reduction in Europe and US. More specifically, the global mean TOA shortwave aerosol forcing is À0.31 W/m2, and the longwave aerosol forcing is +0.08 W/m2. The global map of aerosol net forcing at TOA in Figure 5a shows the positive radiation anomaly over Europe and US due to the aerosol reductions. Interestingly, the largest decrease in TOA net radiation flux does not occur over the pollution centers like China and India where aerosol concentrations have been increased most dramatically. Such phenomena can be explained by the compensating effects from the increased longwave radiation forcing and the strong

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radiative absorption in the atmosphere over Asia [Ghan et al., 2012]. As shown in Table 1, TOA longwave forcings are +1.47 W/m2 and +2.22 W/m2 for East and South Asia, respectively, both of which partially offset the shortwave forcings. The enhanced longwave warming effects are mainly caused by the elevated ice content with high clouds over East and South Asia (Table 1). Meanwhile, if we contrast the shortwave fluxes between the TOA and the surface, we find that there is a pronounced warming in the atmosphere over the Asian countries, +3.5 W/m2 for East Asia and +2.2 W/m2 for South Asia. Such a warming effect from absorbing aerosols in the atmosphere also acts to offset a part of the net cooling at TOA. The contributions to the aerosol forcings from aerosol direct radiative effects and from aerosol-cloud interactions can be inferred by decomposing the aerosol forcing into clear-sky and cloudy conditions. It is interesting to see that the relative contributions from different aerosol effects are distinctive over the different regions, as shown in Table 1. In comparison of the clear-sky shortwave aerosol forcing and aerosol-induced shortwave Figure 3. CAM5 simulated (a) difference and (b) fractional change of aero- cloud forcing (SWCF), we find that over sol optical depth, and (c) difference of absorbing aerosol optical depth East and South Asia, the aerosol- between PD and HD. Black dots indicate significance larger than 90%. induced cloud forcings are one to two Black boxes indicate four target regions of our interest, including the US times larger than the aerosol forcings (60°–135°W, 25°–45°N), Europe (0°–45°E, 40°–65°N), South Asia (65°–90°E, 5°–35°N), and East Asia (90°–125°E, 20°–45°N). at clear-sky conditions, while in Europe, the clear-sky aerosol radiative forcing (1.02 W/m2) is close to the aerosol- induced cloud forcing (1.54 W/m2). More comprehensive analysis and comparison between aerosol radiative and microphysical effects will be conducted with the additional experiments in section 3.4. With the modulated radiation budget at the different regions, the surface temperatures (Ts) respond accordingly. Ts increases by 0.13 and 0.11 K for Europe and US, and decreases by 0.13 and 0.1 K for East Asia and South Asia, respectively. Even though the relative Figure 4. CAM5 simulated six types of aerosol compositions in the accu- changes of emission and AOD are lar- mulation mode for PD and HD. ger in East Asia than those in Europe

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Figure 5. CAM5 simulated changes in (a) TOA radiation net ﬂux, (b) liquid water path, (c) surface temperature, and (d) low cloud fraction between PD and HD.

(Table 1), the absorbing aerosol components buffer the surface temperature changes to the aerosol perturbation in East Asia, resulting in a change of similar magnitude compared to Europe. Considering the observed global warming rate is about 0.12 K/decade caused by the greenhouse gases since the 1950s [Hartmann et al., 2013], the temperature changes induced by the redistributed aerosols are of climatic signiﬁcance over those four regions. As a back-of-the-envelop calculation, the aerosol effects from 1970 to 2010 may cause a

Table 1. Simulated Effects of Redistributed Aerosols on the Various Atmospheric Properties Over Four Targeted Regionsa PD-HD Difference

East Asia South Asia Europe US

Variables Difference Fracb (%) Difference Frac (%) Difference Frac (%) Difference Frac (%)

AODVISc 0.07 50.31 0.04 36.72 À0.05 À32.45 À0.02 À21.68 AODABS 1.06EÀ02 84.03 5.79EÀ03 55.55 À2.15EÀ03 À20.16 2.84EÀ04 8.13 Cloud number (#/cm3) 5.65 37.05 2.23 51.17 À6.50 À36.75 À1.55 À22.65 CLDTOT (%) 0.18 0.31 0.39 0.72 À0.70 À1.19 À0.41 À0.71 CLDLOW (%) À0.12 À0.37 À0.06 À0.26 À0.68 À1.76 À0.34 À1.06 CLDMED (%) 0.35 1.21 0.39 1.97 À0.65 À1.92 À0.17 À0.69 CLDHGH (%) 0.28 0.74 0.54 1.32 À0.41 À1.15 À0.33 À0.87 LWP (g/m2) 0.012 13.70 0.005 8.18 À0.009 À14.49 À0.003 À7.90 IWP (g/m2) 9.05EÀ06 0.05 5.88EÀ04 2.45 À7.10EÀ05 À0.42 À4.16EÀ04 À2.28 Precipitable water À0.03 À0.11 0.09 0.25 À0.03 À0.18 0.02 0.11 2 FLNSurf (W/m ) 0.71 À1.25 1.05 À1.65 À1.36 2.20 À0.49 0.74 2 FLNTOA (W/m ) 1.47 À0.62 2.22 À0.89 À0.87 0.38 À0.76 0.32 2 FLNCTOA (W/m ) 0.36 À0.14 0.39 À0.14 À0.47 0.19 À0.23 0.09 2 FSNSurf (W/m ) À6.19 À3.87 À5.47 À2.81 3.86 2.96 1.71 1.06 2 FSNTOA (W/m ) À2.65 À1.09 À3.23 À1.13 3.16 1.65 1.85 0.81 2 FSNCTOA (W/m ) À0.79 À0.26 À0.77 À0.22 1.62 0.70 0.69 0.26 LWCF (W/m2) 1.1 4.31 1.88 5.84 À0.41 À2.02 À0.51 À2.37 SWCF (W/m2) À1.86 2.97 À2.47 4.28 1.54 À3.89 1.16 À2.74 Precipstrat (mm/d) À2.39EÀ02 À1.57 À5.13EÀ02 À5.86 5.38EÀ04 0.05 1.07EÀ03 0.09 Precipconv (mm/d) À6.33EÀ02 À3.31 À1.01EÀ01 À2.52 9.63EÀ03 1.59 À3.21EÀ03 À0.29 TSurf (K) À0.13 À0.04 À0.1 À0.03 0.13 0.05 0.11 0.04 aBold numbers indicate significance of t test is larger than 90%. bFractional change is defined as (PD À HD) / HD. cAcronym definition: AODVIS—aerosol optical depth at 550 nm; AODABS—absorbing aerosol AOD; CLDTOT—total cloud fraction; CLDLOW—low cloud fraction; CLDMED—middle cloud fraction; CLDHGH—high cloud fraction. For those fluxes terms, S—shortwave, L—longwave, C—clear sky, and CF—cloud forcing; positive values for downward fluxes and negative values for upward fluxes.

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Figure 6. 1980–2010 surface temperature anomaly trends over (a) East Asia, (b) South Asia, (c) US, and (d) Europe from ECMWF ERA-Interim. A global land-only mean of the surface temperature has been subtracted out over each region.

0.03 K/decade surface warming in Europe, which is about one fourth of the observed global warming rate. Such an estimation of the contribution of aerosols to the global warming in Europe is consistent with the previous regional modeling assessment, which is about 23% as estimated by Nabat et al. [2014]. Note that sea surface temperature is prescribed in our simulations, so these experiments still likely underestimate the responses of the surface temperature to the aerosol perturbations. To further corroborate the role of the aerosols in regulating the surface temperature trend, we explore the surface temperature record from European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data. Since the global mean surface temperature trend from 1980 to 2010 is mainly modulated by the greenhouse gas variations, we subtract out the global land-mean temperature trend over the continental regions to highlight the regional differences in the temperature trends. The time series and the linear trend of the surface temperature anomaly over the four target regions in Figure 6 show that Europe experienced the largest warming trend of the surface temperature compared to the other three regions. A warming trend is also found in the US but with a weaker magnitude. East and South Asia have decreasing rates of the surface temperature compared to the other regions of the world (Figures 6c and 6d). Even though there are various underlying factors that can modulate the surface temperature such as greenhouse gas forcing and surface heat capacity changes, the observed distinct and inhomogeneous temperature trends over different regions still suggest that the variations of aerosols could effectively induce local surface temperature changes and regulate the historical temperature trends. Such a finding is supported by the recent model-evaluation study [Ekman, 2014] that revealed that parameterizations of aerosol-cloud interactions in AOGCMs are crucial in simulating the historical surface temperature trends. By grouping and comparing CMIP5 models with respect to their sophistication in the representations of aerosols and aerosol indirect effects, Ekman [2014] found that the more accurate aerosol forcings from the realistic treatment of aerosol budget and the sophisticated parameterization of cloud droplet concentration as a function of both aerosol concentration and supersaturation help to reproduce the observed surface temperature trends from 1965 to 2004. As an important indicator of the aerosol indirect effect, LWP exhibits distinct changes between PD and HD. Figure 5b shows the patterns of LWP variations are well correlated with those of AOD changes with a global correlation coefficient of 0.7, implying the high sensitivity of LWP to the aerosol perturbations in CAM5 [Wang et al., 2011]. Different from the characteristic responses of LWP, the signals of cloud fraction seem to be quite complicated and even close to the noise level (Figure 5d). Only the regions with reduced aerosol amount like Central Europe, Northeast US, and Arctic experience significant reduction of the low cloud fractions, while the

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Figure 7. (a) Zonal mean of the changes in the TOA radiation between PD and HD. Latitude-vertical curtain proﬁles of the PD-HD changes (color contours) and HD climatology (contour lines) in (b) air temperature, (c) meridional streamfunction, and (d) transient eddy kinetic energy.

low cloud fractions in other parts of the world like Asia show little response to aerosol perturbations. One possible explanation is that the cloud occurrences over lower latitudes in East and South Asia are mainly regu- lated by large-scale conditions other than the microphysical factors. A quantitative comparison of the responses of cloud number concentration, LWP, and cloud fraction in Table 1 further reveals that cloud microphysical properties, such as droplet number concentration, size, and water content, are more sensitive to the aerosol perturbation than cloud macrophysical properties like cloud type and fraction, in terms of the larger fractional changes and statistical significance. Large-scale circulations are mainly governed by the energy distribution within the climate system [Ming and Ramaswamy, 2011]. The zonal mean radiation budget at TOA in Figure 7a shows that the shifted anthropogenic emissions mainly perturb the meridional distribution of radiation in the Northern Hemisphere. The elevated aerosol loading over Asian countries results in the energy deficit over tropics from 0°N to 30°N and high latitudes 50°–70°N. The reductions of aerosols over the US and Europe are responsible for the energy surplus in the extra-tropics (30°–50°N) and Arctic region. Longwave radiation anomaly acts differently with shortwave, but its magnitude is smaller than that of the shortwave radiation. The redistributed energy in the atmosphere induces the variation of temperature gradient and large-scale circulation. The increase in the temperature above 150 hPa over the tropics (Figure 7b) and the associated reduction in tropopause height are attributed to the weakened tropical convections induced by the redistributed aerosols. The significant cooling within PBL in the lower latitudes and warming in the higher latitudes tend to relax the meridional temperature gradient near the surface. As shown in Figure 7c, the reduced meridional streamfunction in the low latitudes clearly illustrates the weakening of the Hadley circulation. Comparing the south and north branches of the Hadley cell, the weakening is more significant in the north branch. Accordingly, the midlatitude circulation is slowed down in the Northern Hemisphere. The transient eddy kinetic energy (TEKE) corresponds to the jet streams and baroclinic eddy activities in the upper troposphere. The changes in TEKE from HD to PD in Figure 7d show that the jet stream in the Northern Hemisphere is shifted toward the Arctic, which supports the widening and expansion of the Hadley cell after it gets to slow down under the influence

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of the emission shift. The reduced TEKE and decreased zonal winds in the lower troposphere over tropics further reveal the weakened Walker circulation along with the slowdown of the Hadley cell. Changes in precipitation are closely linked to variations of the large-scale circulations. Since aerosol microphysical effects on the deep convective clouds are still missing in the convection parameterization of CAM5, the responses of the convective precipitation mainly reflect the modulated atmospheric dynamics and instability induced by aerosol radiative effects. The changes in the global stratiform and convective precipitation with the shifted emission sources in Figure 8 show that there is significant suppression of the convective precipitation along ITCZ including the Central Pacific, West Pacific warm pool, and central Africa in PD. The precipitation over the East and South Asia continent is also significantly reduced. Conversely, the large-scale stratiform precipitation which is explicitly linked with CCN concentration exhibits rela- tively weak sensitivity to aerosol perturbations. Globally, there is no clear pattern for stratiform precipitation fl Figure 8. CAM5 simulated precipitation changes from (a) convective changes under the in uences of redis- sources and (b) large-scale stratiform sources between PD and HD. (c) tributed aerosols. Over the four target Zonal mean of the total precipitation changes (black line) in comparison regions, the change of the stratiform to the simulated HD climatology (blue line). precipitation is weakly anticorrelated with the aerosol changes, i.e., increased stratiform precipitation over Asia and decreased stratiform precipitation over Europe and the US (Table 1). Also note that with the one-degree model resolution, the global amount of large-scale precipitation from stratiform clouds is much smaller than that of convective precipitation from convection parameterizations. Therefore, the total precipitation changes are dominated by the aerosol radiative effects on convective clouds through aerosol-radiation-convection interactions. The latitudinal profile of the zonal mean precipitation in Figure 8c clearly shows the suppressed precipitation in the deep tropics (5°S–15°N) and the enhanced precipitation in the subtropics (5°–15°S, 15°–30°N). Those precipitation changes corroborate the weakening of both ascending and descending branches of the Hadley circulation, which corresponds to the “wet” and “dry” regions at the different latitudes, respectively. Such an aerosol effect may mitigate the previously reported greenhouse gas effect on the hydrological cycle, i.e., the “wet” gets wetter and the “dry” gets drier [Held and Soden,2006;Chou et al.,2009;Lau and Kim,2015].Alsonotethatthefixed sea surface temperature in our experiments may limit the variations of water vapor availability and precipitation especially over the ocean. In South and East Asia, the precipitation is found to be significantly reduced in PD. We examine the seasonal changes of the precipitation in South and East Asia. The 4.5% and 4.0% reductions of the convective precipitation Asia are found during the summer monsoon season [June–July–August (JJA)] in South and East Asia, respectively. Those reduction rates and magnitude are highest among the four seasons and explain most of the annual precipitation reductions (À3.3% and À2.5%) over the two regions. The significant JJA precipitation reductions suggest the weakening of the East Asian and South Asia summer monsoons by the

WANG ET AL. REDISTRIBUTION OF ANTHROPOGENIC AEROSOLS 9634 Journal of Geophysical Research: Atmospheres 10.1002/2015JD023665

Figure 9. (a) AOD and (c) surface temperature changes in the PD(Asia) experiment which only considers the emission changes over Asia. (b) AOD and (d) surface temperature changes in the PD(EU) experiment which only considers the emission changes over US and Europe.

redistributed anthropogenic aerosols. The reduced surface temperature over the Asian continent and the decrease in the land-ocean thermal contrast may explain the weakened East and South Asian summer monsoons with elevated local pollution. Similarly, Ganguly et al. [2012b] reported a 12% reduction of precipitation with summer monsoon in South Asia using CAM5 coupled with a slab ocean model. The larger precipitation reduction in that study may stem from the larger aerosol perturbation between PD and preindustrial conditions as well as the stronger feedbacks from the prognostic sea surface temperature. 3.3. Assessing the Global Impacts of Aerosols From Different Regions To better understand the roles of the aerosols from different regions, we perform additional experiments that only consider the aerosol increase in Asia or aerosol decrease in Europe and US. One experiment “PD(Asia)” has the present-day aerosol emission scenario only over South and East Asia and the historical emission scenarios for the rest of the world. The other experiment “PD(EU)” has the present-day aerosol emission scenario only over Europe and US and the historical emission scenarios for the rest of the world. Figures 9a and 9b show the corresponding AOD changes in the two experiments. The responses of the surface temperature are quite localized, which closely follow the pattern of the local AOD variations (Figures 9c and 9d); i.e., the Asian pollution leads to the signiﬁcant cooling over Asia, and the aerosol reduction in Europe and US results in the warming locally. The precipitation responses to the aerosol forcing from different regions are intriguing and distinctive from the temperature responses. Even though we perturb the aerosols over the different regions in the different magnitude, PD(Asia) and PD(EU) exhibit similar patterns of the total precipitation changes (Figure 10). The changes in the zonal mean precipitation clearly show three common characteristic patterns between the two experiments, including a precipitation increase in 10°S–5°N, a decrease in 5°N–20°N, and another increase in 20°N–30°N. The variations of the TOA energy budget and the large-scale circulations from the two experiments are investigated to explain the zonal mean precipitation changes. TOA radiation analyses in Figures 11a and 11b show the distinctive radiation budgets between PD(Asia) and PD(EU). In PD(Asia), the increase of anthropogenic aerosols in Asia exerts a large cooling effect in tropics and subtropics of the Northern Hemisphere but a more intensive cooling around 60 N due to the modulated high-latitude clouds. In PD(EU), the reduction of aerosols in Europe and US results in a strong warming effect in 30°–80°N. The aerosol perturbations in the two experiments both induce the signiﬁcant changes in meridional circulation in the Northern Hemisphere but in different manners as indicated by the changes in the meridional streamfunction in Figures 11c and 11d. For the Asian-aerosol-only case in PD(Asia), due to the cooling in the tropics and subtropics in the

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Figure 10. (a) Zonal mean of total precipitation changes in the PD(Asia) experiment which only considers the emission changes over Asia. Same in Figure 10b but for the PD(EU) experiment which only considers the emission changes over US and Europe. Black lines denote precipitation differences between different experiments, and blue lines denote the simulated HD climatology.

Figure 11. (a) Zonal mean of the changes in the TOA radiation between PD(Asia) and HD. Latitude-vertical curtain proﬁles of the PD(Asia)-HD changes (color contours) and HD climatology (contour lines) of the (c) meridional streamfunction and (e) transient eddy kinetic energy in the PD(Asia) experiment which only considers the emission changes over Asia. Same in Figures 11b, 11d, and 11f but for the PD(EU) experiment which only considers the emission changes over US and Europe.

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Table 2. Same With Table 1 but for the Aerosol-Indirect-Effect-Only Experiments (AIE)a PD-HD Difference

East Asia South Asia Europe US

Variables Difference Frac (%) Difference Frac (%) Difference Frac (%) Difference Frac (%)

CLDTOT (%) 0.18 0.32 1.26 2.54 À1.0 À1.58 À0.44 À0.78 CLDLOW (%) 0.37 1.55 0.14 0.77 À1.0 À2.35 À0.07 À0.24 CLDMED (%) 0.29 0.97 0.75 3.82 À0.48 À1.42 À0.09 À0.51 CLDHGH (%) À0.14 À0.36 1.44 3.60 À0.66 À1.68 À0.54 À1.49 LWP (g/m2) 0.01 14.11 4.5EÀ3 10.40 À0.01 À16.37 À2.5EÀ3 À5.60 IWP (g/m2) 1.7EÀ4 0.69 1.6EÀ3 7.01 2.1EÀ4 À1.11 À3.0EÀ4 À1.76 Precipitable water À0.10 À0.48 0.23 0.72 À0.09 À0.63 À0.04 À0.20 2 FLNSurf (W/m ) 0.68 À1.00 1.09 À1.47 À1.49 2.56 À0.35 0.51 2 FLNTOA (W/m ) 1.43 À0.62 3.08 À1.22 À0.87 0.39 À0.70 0.29 2 FLNCTOA (W/m ) 0.60 À0.23 0.60 À0.21 À0.24 0.10 À0.16 0.06 2 FSNSurf (W/m ) À2.91 À1.81 À3.68 À1.80 3.53 2.85 1.19 0.68 2 FSNTOA (W/m ) À2.97 À1.28 À3.53 À1.24 3.39 1.87 1.18 0.48 2 FSNCTOA (W/m ) À0.02 À0.01 0.21 0.06 À0.04 À0.02 0.12 0.04 LWCF (W/m2) 0.83 3.16 2.48 8.98 À0.63 À2.83 À0.54 À2.38 SWCF (W/m2) À2.95 5.00 À3.73 7.56 3.42 À7.35 1.06 À2.15 Precipstrat (mm/d) À4.4EÀ02 À2.57 À1.6EÀ2 À2.12 2.2EÀ2 1.82 1.7EÀ3 0.16 Precipconv (mm/d) À8.9EÀ02 À4.31 4.7EÀ2 1.42 1.1EÀ2 1.63 1.0EÀ2 0.72 TSurf (K) À0.31 À0.11 À0.08 À0.03 0.08 0.03 0.02 0.01 aBold numbers indicate signiﬁcance of t test is larger than 90%.

Northern Hemisphere, the northern branch of the Hadley cell becomes weaker. The boundaries of the Hadley cell extend toward the high latitudes, as suggested by the poleward shifts of the TEKE in both hemispheres in Figure 11e. For the Europe-US-aerosol-only case, the most prominent change in the meridional streamfunction is the significant north-to-south cross-equator flux, which corresponds to the southward shift of the Hadley cell. As an analogue to the previous finding about the northward shift of the tropical circulation under the significant cooling in the Northern Hemisphere from the PD versus PI aerosol conditions [Ming and Ramaswamy, 2011], the southward shift of the latitudinal circulations is attributed to the energy surplus in the Northern Hemisphere from the aerosol reductions in Europe and US. The southward shift in the latitudinal circulations can also be identified from the shift in the locations of the midlatitude jet stream in both hemispheres (Figure 11f). The distinctive responses of the large-scale circulations to the different aerosol forcing imposed from different regions underline the importance of accurately considering the aerosol spatial variations in the climate assessment. 3.4. Separating the Aerosol Indirect Forcing from Direct Forcing In CAM5 simulations, aerosol effects on radiation and cloud formation at the grid scale are explicitly repre- sented. Since the cloud adjustment to aerosol perturbations has the single largest uncertainty in the aerosol forcing assessment [Myhre et al., 2013], it is valuable to isolate the aerosol indirect effect, i.e. aerosol serving as CCN/IN to affect cloud formation, from the overall effect of aerosols and to assess its climatic influence under the background of the globally shifted emissions. Following the aerosol forcing decomposition method by Ghan et al. [2012], we turn off all the radiation calculations relevant to the aerosol properties in the model and perform additional ensemble simulations with only aerosol indirect effect (AIE). The global mean in the TOA radiation flux changes for AIE is À0.32 W/m2, which represents an even larger radiative cooling induced by aerosols than À0.23 W/m2 in aerosol-all-effect (AAE) simulations. Over the four target regions, TOA aerosol forcings are close between AIE and AAE, but the forcings over surface and in the atmosphere exhibit significant differences over different regions, as shown in Table 2. AIE induces much larger shortwave cloud forcings over South/East Asia and Europe than those in AAE. It implies the aerosol direct and semidirect effects could exert a negative influence on the aerosol-induced cloud formation by suppres- sing (enhancing) convection and cloud development under the polluted (pristine) conditions. Particularly in East Asia, surface temperature becomes cooler in AIE than that in AAE, indicating a more prominent “brightening” effect of pollution in East Asia by only considering the aerosol-cloud interactions. All other three regions show the weaker temperature changes when the aerosol radiative effects are excluded.

WANG ET AL. REDISTRIBUTION OF ANTHROPOGENIC AEROSOLS 9637 Journal of Geophysical Research: Atmospheres 10.1002/2015JD023665

Figure 12. Same with Figure 7 but for aerosol-indirect-effect-only (AIE) experiment.

The zonal mean TOA forcing induced by the aerosol indirect effects (Figure 12a) varies at different latitudes with a similar pattern to that in AAE simulations in Figure 8a. In the Northern Hemisphere, the peak of zonal mean TOA shortwave radiative cooling caused by the Asian pollution in AIE is about À1.6 W/m2, which is larger than À1.3 W/m2 in AAE. It can be explained by the excluded aerosol absorption in AIE. Meanwhile, by only considering the aerosol microphysical effects, the net TOA radiative warming at 30°–50°N becomes much weaker and less significant. With similar meridional patterns of the TOA aerosol forcing, AIE induces circulation changes in a similar manner compared to AAE but with the smaller strength. As shown in Figure 12b, the diminished warming effect over US and Europe in AIE results in a weaker reduction of the near surface temperature gradient over subtropics (around 30°N) than that in AAE (Figure 7b). Hence, the overall weakening effect on the tropical circulation in AAE becomes less significant in AIE (Figure 12c). Meanwhile, there exists a large statistically significant decrease in the temperatures at the upper troposphere and lower stratosphere (UTLS, 100–300 hPa) over 60°N–90°N in AIE (Figure 12b) but absent in AAE (Figure 7b), which collocates closely with a strong increase in TEKE (Figure 12d). The much larger net TOA radiative warming over 60°N–90°N in AIE can partially explain the enhancement of the north polar circulation, the elevation of the tropopause height, and the reduction in temperature at UTLS over the Arctic. The changes in midlatitude circulations as indicated by the poleward expansion of the midlatitude jet stream also interact with the dynamics in the Arctic region. The dramatic differences in the Arctic region between AIE and AAE indicate the profound impacts of aerosol direct effects over high latitudes through modulating the global circulations.

4. Conclusion and Discussions In this modeling study, we address the emerging issue on the climatic impacts of the geospatial redistribution of anthropogenic aerosols since the 1970s. A series of sensitivity simulations using the NCAR CAM5 model are conducted to investigate the atmospheric responses to perturbations of the aerosol amounts over different regions. The primary task of this study is to examine the changes in radiative forcing, clouds, precipitation, and large-scale

WANG ET AL. REDISTRIBUTION OF ANTHROPOGENIC AEROSOLS 9638 Journal of Geophysical Research: Atmospheres 10.1002/2015JD023665

circulations induced by the elevated pollution levels in the Asian developing countries as well as the anthropogenic emission reductions in developed countries in Europe and North America. By contrasting the simulations with aerosol emission scenarios in the years 1970 and 2010, which represent the historical day (HD) and present-day (PD) conditions, respectively, we find that there is about À0.23 W/m2 cooling at TOA in the global mean. Shortwave radiation contributes À0.31 W/m2 due to the enlarged aerosol scattering and cloud Figure 13. Comparison of aerosol radiative forcings (PD-HD) between reflectivity, while the longwave radiation aerosol-indirect-effect-only (AIE) experiment and all-aerosol-effect (AAE) 2 experiment. change is +0.08 W/m because of the increase in high clouds and column water vapor in the troposphere. Regionally, the elevated pollution levels in Asia give rise to À0.1 and À0.13 K surface cooling in South and East Asia, while the pollution reductions in Central Europe and US are responsible for +0.11 and +0.13 K increases in the surface temperature, respectively. Such changes in the surface temperature are supported by the different temperature trends over the different regions from ECMWF ERA reanalysis data. Under the background of global warming since the 1950s, the reanalysis data show that the warming trends in the surface temperature are larger in Europe and US than those in East and South Asia from 1980 to 2010. The comparison of the fractional changes of cloud properties between PD and HD reveals that cloud microphysical properties, such as cloud droplet number, size, and water content, are more sensitive to aerosol perturbations than cloud macrophysical properties like cloud fraction and distribution (Table 1). The convective precipitation is susceptible to the perturbation of the redistributed aerosols in CAM5, which is attributed to the altered atmospheric dynamics and circulations following the changes in the regional radiation budget. Conversely, the stratiform precipitation predicted by the microphysical scheme at the grid scale exhibits rela- tively less sensitivity to aerosols than convective precipitation. The suppressed precipitation over ITCZ and the enhanced precipitation over the subtropics indicate a weaker Hadley circulation under the influence of the shifted anthropogenic emissions. The zonal mean of the meridional streamfunction further reveals the slowdown of the latitudinal circulations in the Northern Hemisphere due to the energy deficitatlowerlatitudes and surplus at higher latitudes. Both the boundary of the Hadley cell and the midlatitude jet stream exhibit a poleward shift. Therefore, we conclude that the modulation of the large-scale circulations is subject to the variation of radiation budget and the energy distribution over the globe. By conducting the simulations considering only the East/South Asia PD emissions or only the Europe/US PD emissions, we individually assess the effects of aerosols from certain regions of the world. Similar to the results from the redistributed-aerosol case, the Asian-aerosol-only case predicts a weaker northern branch of the Hadley cell and a poleward extension of the Hadley cell due to the cooling in the tropics and subtropics. Differently, the aerosol reductions in Europe and US and associated warming effect induce an inter- hemispherical shift of the latitudinal circulation in terms of the stronger/weaker motions in the southern/northern branches of the Hadley circulation and the cross-equator mass fluxes. The distinctive responses of the large-scale circulations to the different aerosol forcings imposed from different regions suggest the potential importance of aerosol spatial variations to the climate change. The aerosol-indirect-effect-only experiment shows that aerosol-cloud interactions account for a larger por- tion of the aerosol forcing at TOA. Particularly, the aerosol indirect effect is critical for the “dimming” effect in East and South Asia, and the surface temperature reduction becomes even larger by only considering the aerosol indirect effect in East Asia (Figure 13). Excluding the aerosol direct effect can also dampen the warming effect over the US and Europe, which limits the influence of the redistribution of aerosols on the global circulations. Hence, aerosol direct and indirect effects work together to modulate the meridional energy distribution and alter the circulation systems.

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As a ﬁrst-order assessment, the current study focuses on the fast response of the atmospheric system to characteristic aerosol perturbations around the globe. The sensitivity of the aerosol effects may be constrained by the ﬁxed conditions of ocean and sea ice, since previous modeling study suggested a stronger effect of aerosols on the large-scale circulations with the interactive SST in a slab ocean setup [Allen and Sherwood, 2010]. Future studies will employ the comprehensive atmosphere-ocean fully couple general circulation model to systematically examine the transient and equilibrium response of the whole earth system to the shifted anthropogenic emissions and the redistributed aerosols.

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