AbstractA general circulation model was used in perpetual January mode, to study the wintertime climatological sensitivity to changes in the horizontal hyper‐Laplacian diffusion scheme at both spectral T21 and T42 horizontal resolutions.Analysis of the global energy cycle revealed a large impact especially on the conversion of eddy to zonal kinetic energy. Less diffusion resulted in less zonal available potential energy, weaker baroclinic conversions and caused the barotropic conversion of eddy to zonal kinetic energy to become significantly weaker. It was found that by weakening the diffusion in the T21 model, the eddy kinetic energy increased and became more realistic yet at the same time the conversion of eddy to zonal kinetic energy decreased and became less realistic.Weakening the horizontal diffusion caused a northern hemisphere tropospheric response similar to that obtained by increasing the upper‐level orographic gravity wave drag in the model. The zonal‐mean zonal winds weakened poleward of the subtropical jets, the Ferrel cells weakened and the troposphere warmed up at high latitudes. The weakening of the zonal wind and the Ferrel cells was associated with a weakening of the upper‐level convergence of the poleward eddy momentum flux and the wave‐mean flow interaction was demonstrated using Eliassen–Palm diagnostics. A substantial part of the change in the poleward eddy momentum flux was due to large changes in the poleward momentum flux of the transient eddies in the mid‐latitude stormtracks and changes in their horizontal phase tilt. A large northern hemisphere stationary‐wave change appeared in these regions and had a similar structure in the Atlantic sector to that seen when strengthening upper‐level