| Summary: | Aerosols play an important role in the Earth's climate system by scattering and absorbing solar radiation and by interacting with clouds. Using the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation models (AGCMs), we investigate aerosol transport and removal processes and the effects of aerosols on large-scale circulation. We also quantify how aerosols have affected twentieth-century climate. Arctic haze refers to the accumulation of aerosols in the Arctic, which results from the long-range transport of aerosols originating in mid-latitude industrial regions. We use the GFDL AM3 model to elucidate the factors driving the seasonal cycle of Arctic haze. The transport of aerosols into the Arctic is shown to vary little throughout the year, with the seasonal cycle of Arctic haze instead attributed mostly to the changes in wet removal, which becomes less efficient during summertime. The results suggest that future changes in precipitation can potentially alter the concentrations of Arctic aerosols, with implications for air quality and climate. The climate effects of absorbing and scattering aerosols differ greatly. We examine the effects of absorbing aerosols on atmospheric circulation using the GFDL AM2 model. Absorbing aerosols in the free troposphere stabilize the mid-latitude atmospheric column, which decreases baroclinic eddy activity and thus reduces meridional energy transport at mid-latitudes. The effectiveness of absorbing aerosols in altering circulation generally increases with their height. Scattering aerosols, on the other hand, cool the climate and alter atmospheric circulation in a nearly opposite way to greenhouse gases. Using GFDL AM2 coupled to a slab ocean model, we find that the circulation responses to scattering aerosols and greenhouse gases are not linearly additive. The nonlinearity mainly arises from cloud radiative responses. Large uncertainty remains in the climate impacts of aerosols. We use GFDL AGCMs to decompose historical land temperature change due to anthropogenic aerosols into fast and slow components. It is found that the fast response to aerosols significantly contributes to the observed European warming in recent decades but results in cooling over Asia. We demonstrate that AGCM simulations of the fast response are useful for empirically constraining historical aerosol forcing.
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