Potential Impact of South Asian Anthropogenic Aerosols on Northern Hemisphere Climate

Abstract:

South Asia has one of the world’s highest aerosol loading due to the dramatic increase of anthropogenic emissions from the 1950s associated with rapid urbanization and population growth. The possible large-scale impact of the late 20^th century increase of South Asian aerosol emissions on climate away from the source regions was studied by means of historical ensemble experiments 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, in response to anomalous surface cooling in the northern Indian Ocean, aerosols cause a westward shift of convection over the eastern Indian Ocean and compensating subsidence to the west and over the Maritime continent. During the summer, aerosols are collocated with rainfall, and cause a widespread drying over South Asia mostly by indirect effects. In both cases, 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. Depending on the region, the induced anomalies may have opposite signs between the two seasons, and may thus contribute to reinforcing or dampening those due greenhouse gases. These results underscore 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.