Anthropogenic aerosol forcing has significantly weakened regional summertime storminess in the Northern Hemisphere in the satellite era

Abstract

Reanalysis data show the summertime circulation in the Northern Hemisphere midlatitudes has weakened significantly in the satellite era. Recent work shows the circulation weakening is not significantly affected by Arctic Amplification and Arctic Sea ice loss, but did not examine the role of other anthropogenic forcings such as aerosols. Here we use Detection and Attribution Model Intercomparison Project (DAMIP) simulations, which capture the weakening trend in reanalysis data, to quantify the impact of anthropogenic forcing due to aerosols and greenhouse gases. The DAMIP simulations show aerosol forcing dominates the weakening of the circulation across the Eurasia-Pacific sector, including the Pacific jet and storm track. Aerosol and greenhouse gases contribute equally to weakening the Atlantic jet and storm track. We use an energetic framework to understand the impact of aerosols on the storm track. In particular we show aerosol forcing leads to an increasing surface shortwave radiation trend over Western Europe and a decreasing surface shortwave trend over South and East Asia. These shortwave trends induce a weakening trend of the equator-to-pole energy gradient that leads to a weaker downstream storm track. Overall, our results show aerosol forcing is a dominant factor in regional circulation trends during Northern Hemisphere summertime in the satellite era. They have important implications for interpreting summertime heatwave trends in the Northern Hemisphere midlatitudes during summertime.