Stratospheric vs. tropospheric effects on circulation changes in a simple GCM

Comprehensive General Circulation Model (GCM) integrations show consistently that increased CO2 concentrations in the future lead to upper tropospheric heating in the tropics, surface warming over the arctics, meridional shift of the surface westerlies, cooling of the stratosphere, and a strengthening of the global circulation. In such comprehensive GCM simulations, however, it is not clear how to determine the individual sources of such changes, or to separate cause and effect. We use an idealized dry GCM, in which thermodynamic forcings can be closely controlled, with a well-resolved stratosphere to determine the relative influence of the stratosphere on the overall expected changes. We pay particular attention to surface westerly shifts and their relation to a warmer tropical troposphere, cooling of the stratosphere, and arctic amplification. The stratospheric Brewer-Dobson circulation is of particular import because of its importance in troposphere-stratosphere coupling, as it is related to the intensity of tropical upwelling and the strength of the polar vortex, and hence can make a noticeable difference to global meridional mass transport and the latitude of the surface westerlies.