Reanalysis data show a significant weakening of summertime circulation in the Northern Hemisphere midlatitudes in the satellite era with important 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 summertime circulation weakening. Regionally, aerosol dominates the Pacific storm track weakening whereas greenhouse gas dominates the Atlantic. Using a regional energetic framework, we interpret how aerosol forcing weakens the storm tracks and why the impact is the largest in the Pacific. Decreasing aerosol emissions over Eurasia and North America increases (clear-sky) shortwave radiation. The atmospheric energy input is exported downstream via stationary circulation, poleward of the Pacific and Atlantic storm tracks. This land-ocean energy coupling is similar to the spring-to-summer seasonal transition and is larger over the Pacific due to greater energy input over Eurasia. In addition, increasing aerosol emissions over South and East Asia decreases shortwave radiation, and the energy export downstream via stationary circulation weakens. This occurs equatorward of the Pacific storm track, which further weakens it. Our results show aerosols are a dominant driver of regional circulation trends during the Northern Hemisphere summertime in the satellite era and show a regional energetic framework explaining the underlying processes.
