A model investigation of aerosol-induced changes in boreal winter extratropical circulation

The authors examine the key characteristics of the boreal winter extratropical circulation changes in response to anthropogenic aerosols, simulated with a coupled atmosphere-slab ocean general circulation model. The zonal-mean response features a pronounced equatorward shift of the Northern Hemisphere subtropical jet owing to the midlatitude aerosol cooling. The circulation changes also show strong zonal asymmetry. In particular, the cooling is more concentrated over the North Pacific than over the North Atlantic despite similar regional forcings. With the help of an idealized model, the authors demonstrate that the zonally asymmetrical response is linked tightly to the stationary Rossby waves excited by the anomalous diabatic heating over the tropical east Pacific. The altered wave pattern leads to a southeastward shift of the Aleutian low (and associated changes in winds and precipitation), while leaving the North Atlantic circulation relatively unchanged. Despite the rich circulation changes, the variations in the extratropical meridional latent heat transport are controlled strongly by the dependence of atmospheric moisture content on temperature. This suggests that one can project reliably the changes in extratropical zonal-mean precipitation solely from the global-mean temperature change, even without a good knowledge of the detailed circulation changes caused by aerosols. On the other hand, such knowledge is indispensable for understanding zonally asymmetrical (regional) precipitation changes.