Competing roles of anthropogenic aerosols and greenhouse gases in June–August baroclinicity and storm-track trends

Abstract. Northern Hemisphere summer (June–August) storm tracks have been extensively studied over recent decades, with evidence for weakening trends in parts of the Northern Hemisphere and strengthening trends in the Southern Hemisphere. However, the physical drivers of these trends, and their strong regional variability, are still debated. Here, we investigate recent changes in mid-latitude baroclinicity by analysing trends in Eady Growth Rate and baroclinic mean available potential energy (MAPE) in ERA5 reanalysis data and CMIP6 single-forcing large-ensemble experiments over both the industrial and satellite eras. We further decompose baroclinic MAPE and Eady Growth Rate into their physical components and compare the baroclinicity trends with trends in storm-track intensity. We find that changes in the meridional temperature gradient provide the dominant contribution to recent baroclinicity trends. Significant regional asymmetries are identified between the North Pacific and North Atlantic baroclinicity trends, with the strongest recent weakening occurring over the North Pacific storm-track region. Greenhouse-gas forcing produces a robust positive Southern Hemisphere baroclinicity trend and a mixed weakening response in the Northern Hemisphere, while aerosol perturbations generate substantial regional responses in both hemispheres and partially offset the greenhouse-gas-driven baroclinicity signal. In particular, changes in aerosol emissions during the satellite era are associated with enhanced negative baroclinicity trends over the North Pacific and contribute to the emergence of the greenhouse-gas-driven storm-track response. These results highlight the competing roles of greenhouse gases and aerosols in shaping recent mid-latitude circulation changes, and demonstrate that both forcings must be considered when interpreting historical storm-track trends and projecting their future behaviour.