Mechanisms for the atmospheric circulation response to idealized thermal forcings in a simple GCM

We explore various dynamical mechanisms to explain the large-scale atmospheric circulation response to idealized thermal forcings in a simple general circulation model. These thermal forcings are designed to mimic two key aspects of anthropogenic climate change: (1) warming in the tropical troposphere, which in our model drives a poleward shift of the extratropical tropospheric storm tracks and a weakened stratospheric Brewer-Dobson circulation; and (2) cooling in the polar stratosphere, which also drives a poleward shift of the storm tracks that is sensitive to the location of the forcing in the stratosphere. We examine several widely-known hypotheses to explain this response, including: (a) changes in static stability associated with the vertical gradient of the thermal forcing; (b) lifting of the tropopause; and (c) changes in upper-level winds and the associated changes in eddy phase speeds. We also propose and test a new mechanism.