| Summary: | Anthropogenic aerosols have been identified as an important driver of global and regional climate. Globally, aerosols are estimated to have offset much of the positive forcing due to greenhouse gases; regionally, their effect can be dominating, and can potentially drive climate anomalies far from the emission sources due to changes in the atmospheric circulation. Aerosols emitted from North America (NA) and Europe (EU) dominated the global aerosol loading until the late twentieth century. Despite recent progress, our knowledge of the climate imprint of NA and EU aerosols is still incomplete, especially regarding the decades before the mid-twentieth century, in which emissions were still lower and did not yet change as rapidly as later, but might have been more effective due to non-linearities in the aerosol-cloud interactions. The overarching goal of this work is thus to determine robust features of the impact of NA and EU aerosols on regional and large-scale climate and to advance current understanding of the underlying mechanisms, compared to those generated by other forcing agents as well as aerosols from other geographical regions. The study focuses mainly on the period of increasing sulphur dioxide (SO2) emissions -precursor of sulphate aerosols, the most abundant anthropogenic aerosol species- from NA and EU sources (1850-1975), and on identifying the aerosol impact over the Atlantic and Eurasian domain, where North American and European aerosols are presumed to have relevant impact. Along with observations, existing historical simulations from a range of coupled climate models are studied and complementary experiments performed and analysed. First, the boreal summer climate response to North American and European (NAEU) anthropogenic aerosol emissions during the twentieth century is characterised using a suite of models from the Coupled Model Inter-Comparison Project 5 (CMIP5). Supported by the co-variability of aerosol optical depth and near-surface climate, long-term variations in aerosol-only and all-forcing simulations are attributed to NAEU aerosol forcing if they undergo a significant reversal coinciding with the peak in NAEU SO2 emissions, measured by inter-model agreement on the sign of linear trends before and after 1975. Regionally, robust aerosol impact is found on Eurasian near-surface temperature, pressure, and diurnal temperature range; remotely, robust aerosol impact is found on the Inter-Tropical Convergence Zone (ITCZ) position and the subtropical jet stream. The contribution of anthropogenic aerosol forcing to the forced component of simulated inter-decadal climate variability of European-mean near-surface temperature is furthermore estimated to be more than a third throughout the twentieth century. Observed variations also of European-mean sea level pressure and diurnal temperature range tend to agree better with simulations that include aerosols. These findings highlight significant aerosol impact on Eurasian climate already in the first half of the twentieth century. The aerosol impact on observed West African and South Asian monsoon precipitation is then investigated by using a detection and attribution (D&A) approach. The aerosol source regions (NAEU, South Asia, or China) which are most important for explaining the observed 1920-2005 changes are identified. For this, fingerprints of the response to regional-aerosol forcing are derived from historical simulations with the GFDL-CM3 model along with CMIP5 simulations. It is found that in precipitation observations for West Africa, the only anthropogenic forcing which can be detected are NAEU emissions. In precipitation observations for South Asia, in contrast, local emissions are the only external forcing detected. Changes in West Africa are related to a meridional shift in the ITCZ due to aerosol-induced changes in the inter-hemispheric temperature gradient. Changes in South Asia, in contrast, are associated with a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975 and since then by the increase in local emissions offsetting the decrease in NAEU emissions. These findings show for the first time that the aerosol forcing from individual emission regions is strong and distinct enough to be detected in the presence of internal variability. Finally, the dynamical impact of NA and EU sulphate aerosol emissions is fully analysed in the coupled Community Earth System model (CESM1-CAM5), focusing on the Atlantic. For this, multi-member ensemble simulations covering the period 1850- 1975 are performed, and the response to emissions from NA and EU is contrasted. The results show that sulphate aerosols from either source cause a long-term cooling of North Atlantic sea-surface temperatures (SSTs), with the patterns a combination of atmospheric aerosol effects and an aerosol-induced strengthening of the Atlantic Meridional Overturning Circulation (AMOC). The North Atlantic response to NA emissions is larger than that to EU emissions, with stronger indirect aerosol effects due to a wider aerosol spread over the Atlantic and collocation with climatological cloud cover. A southward shift of the ITCZ, affecting tropical precipitation globally, is also found. The (multi)decadal variability components of Atlantic SSTs and of the AMOC are furthermore both found to be externally forced. A suppression of Atlantic Tropical Hurricane frequency and a north-eastward shift of Atlantic extra-tropical storms in response to both NA and EU emissions are finally shown. The analysis provides novel insights into the mechanisms of aerosol impact on the Atlantic. Overall, the results from this work represent a significant contribution to advance our understanding of the historical impact of anthropogenic aerosols over the entire twentieth century and in particular that of aerosols from NA and EU by finding robust signals across models, using statistically rigorous methods to detect forced impact in observations, and analysing new model experiments. The findings emphasise the importance of historical anthropogenic aerosol emissions already before the late twentieth century and shed light on differences in the climate response to aerosols depending on their emission region, which will also be relevant for understanding future patterns of change related to further emission reductions.
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