Anthropogenic Aerosols Have Significantly Weakened the Regional Summertime Circulation in the Northern Hemisphere...

Reanalysis data show a significant weakening of summertime circulation in the Northern Hemisphere (NH) midlatitudes in the satellite era with implications for surface weather extremes. Recent work showed the weakening is not significantly affected by changes in the Arctic, but did not examine the role of different anthropogenic forcings such as aerosols. Here we use the Detection and Attribution Model Intercomparison Project (DAMIP) simulations to quantify the impact of anthropogenic aerosol and greenhouse gas forcing. The DAMIP simulations show aerosols and greenhouse gases contribute equally to zonal-mean circulation weakening. Regionally, aerosol dominates the Pacific storm track weakening whereas greenhouse gas dominates in the Atlantic. Using a regional energetic framework, we show why the impact of aerosol is the largest in the Pacific. Reduced sulfate aerosol emissions over Eurasia and North America increase (clear-sky) surface shortwave radiation and turbulent fluxes. This enhances land-to-ocean energy contrast and energy transport via stationary circulations to the ocean. Consequently, energy converges poleward of oceanic storm tracks, demanding weaker poleward energy transport storm tracks, and the storm tracks weaken. The impact is larger over the Pacific following the larger emission decrease over Eurasia than North America. Similar yet opposite, increased aerosol emissions over South and East Asia decrease shortwave radiation and weaken land-to-ocean energy transport. This diverges energy equatorward of the Pacific storm track, further weakening it. Our results show aerosols are a dominant driver of regional circulation weakening during the NH summertime in the satellite era and a regional energetic framework explaining the underlying processes.