The dynamical mechanisms through which stratospheric forcing can influence tropospheric annular modes are explored. A torque is applied to the stratosphere of an idealized general circulation model, and, under some circumstances, a robust tropospheric response is observed. These tropospheric responses, while initiated by stratospheric forcing, are maintained locally by interactions with transient eddies, and they closely resemble the intrinsic annular modes of the model. Manipulations of the model are consistent in showing that planetary waves, and not only the zonally symmetric secondary circulations induced by stratospheric forcing, are important for transmitting dynamical signals to the troposphere. Specifically, it is found that the tropospheric response is significantly reduced when planetary waves are suppressed in the stratosphere by additional damping or when the strength of the stratospheric jet is increased. Wave diagnoses indicate that the confinement of these waves within the troposphere, when stratospheric winds are enhanced, leads to increased planetary wave deceleration of the zonal winds in the high-latitude upper troposphere.
<HR ALIGN="center" WIDTH="30%">