Radiative vs Dynamical Effects in Abrupt Climate Change Simulations

The main temperature trends predicted in comprehensive General Circulation Model (GCM) integrations of increased greenhouse gas (GHG) emission scenarios are robust in multimodel assessments, and include: 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 Brewer-Dobson circulation. In order to determine the individual contribution of radiative and dynamical effects in both the stratosphere and troposphere, we use an idealized dry GCM with a well-resolved stratosphere in simple perturbation experiments. In particular, we show that the polar amplification and the radiative forcing in the stratosphere induce a similar response throughout the atmosphere. This response counteracts the effect of upper tropospheric tropical heating, although the latter dominates in combined simulations. In addition, the purely radiative perturbation in the stratosphere due to increased GHG concentrations alone increases the Brewer-Dobson circulation, and with our model framework we can quantify the relative importance of the stronger circulation and radiative forcing by artificially keeping the circulation constant while changing the radiative state of the stratosphere.