Impact of Anthropogenic Aerosols on the Northern Hemisphere Monsoon and Potential Northern Hemisphere Teleconnections from South Asian Aerosols

The Northern Hemisphere monsoons are an integral component of Earth's hydrological cycle and affect the lives of billions of people. Observed precipitation in the monsoon regions underwent substantial changes during the second half of the 20th century, with human activity, in the form of anthropogenic aerosols, suggested to have played an important role. This study will address two important questions related to the impact of anthropogenic aerosols on regional climate. In the first part, CMIP5 climate models are used to investigate the influence of individual climate forcings (i.e. greenhouse gas, anthropogenic aerosol and natural) on the Northern Hemisphere monsoon precipitation by means of a robust statistical analysis. The results of a detection and attribution analysis show that the observed temporal changes can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. Anthropogenic aerosols, not greenhouse gas or natural forcing, have been the dominant influence on monsoon precipitation over the second half of the 20th century. The second part of the study will specifically focus on South Asian aerosols and investigate the extent to which the late 20th century increase of South Asian emissions may result in changes of climate away from the source regions. To this purpose, historical ensemble experiments were conducted with a state-of-the-art coupled climate model with fully interactive aerosols and a representation of both direct and indirect aerosol effects. The key characteristics of the northern hemisphere responses are examined separately for winter and summer, and show that regional aerosols induce significant planetary-scale teleconnection patterns. In both seasons, the large-scale aerosol imprint originates from substantial changes in the regional precipitation distribution: during the winter, aerosols cause a westward shift of convection over the eastern Indian Ocean-Maritime Continent, during the summer aerosols are collocated with rainfall, and cause a widespread drying over South Asia mostly by indirect effects. The impact of the regional diabatic heating anomaly propagates remotely by exciting a northern hemisphere wave-train which, enhanced by regional feedbacks, leads to remarkable changes in near-surface climate, including circulation and temperature, over Eurasia, the northern Pacific and North America. This underscores the potential influence of Asian aerosols on global climate, which is a compelling problem as regional aerosol loading will continue to be large in the coming decades.