The Influence of Aerosol Absorption on the Extratropical Circulation

This study examines how aerosol absorption affects the extratropical circulation by analyzing the response to a globally uniform increase in black carbon (BC) simulated with an atmospheric general circulation model forced by prescribed sea surface temperatures. BC-induced heating in the free troposphere stabilizes the mid-latitude atmospheric column, which results in less energetic baroclinic eddies and thus reduced meridional energy transport at mid-latitudes. Upper tropospheric BC also decreases the meridional temperature gradient on the equatorward flank of the tropospheric jet and yields a weakening and poleward shift of the jet, while boundary layer BC has no significant influence on the large-scale circulation since most of the heating is diffused by turbulence in the boundary layer. The effectiveness of BC in altering circulation generally increases with height. Dry baroclinic eddy theories can explain most of the extratropical response to free troposphere BC. Specifically, the decrease in vertical eddy heat flux related to a more stable atmosphere is the main mechanism for re-establishing atmospheric energy balance in the presence of BC-induced heating. Similar temperature responses are found in a dry idealized model, which further confirms the dominant role of baroclinic eddies in driving the extratropical circulation changes. The strong atmospheric-only response to BC suggests that absorbing aerosols are capable of altering synopic-scale weather patterns. Its height dependence highlights the importance of better constraining modelsimulated aerosol vertical distributions with satellite and field measurements. 9