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Atmospheric Responses to the Redistribution of Anthropogenic PUBLICATIONS 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 WANG ET AL. REDISTRIBUTION OF ANTHROPOGENIC AEROSOLS 9625 Journal of Geophysical Research: Atmospheres 10.1002/2015JD023665 the aerosol forcings since the preindustrial days were reported by Xie et al. [2013] which analyzed the simula- tion 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 subtro- pics 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] sug- gested 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 emis- sions. 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 mon- soon 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 model- ing 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
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