The role of individual forcings in driving the wave–like recent trends in northern hemisphere summer atmospheric circulation

Abstract. The summer climate in the Northern Hemisphere during recent decades has shown distinct trend patterns, with warming hotspots that spatially match with the ridges of a circumpolar atmospheric wave-5 pattern in the upper troposphere. The drivers behind this wave–like trend and warming pattern are not yet well understood. Through the use of the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) and the Atmospheric Model Intercomparison Project (AMIP) simulations, we study the contributions of different forcing components as well as the role of oceanic temperature variability to the observed changes. Analysis of the single-forcing experiments shows that in particular historical anthropogenic aerosol forcing leads to responses that have some pattern similarity with the observed changes in atmospheric circulation, diagnosed from the 200 hPa geopotential height (Z200) after removing the zonal mean. However, despite high pattern agreement, the magnitude of the trends is underestimated in models. Our results suggest that the observed spatial structure of trends in Z200 is at least partially caused by anthropogenic aerosol emissions and is not the result of global warming caused by greenhouse gas emissions, and highlight important inconsistencies between models and observations.