The tropical response to extratropical thermal forcing:
Theory and application


We study the ITCZ response to extratropical thermal forcing in an idealized configuration to build up a theoretical basis and apply the theory to IPCC AR4 models forced with A2 scenario. First, in a simplified framework, it is shown that the ITCZ response is determined by the response in the atmospheric energy transport. The experiment is designed to perturb the ITCZ from the extratropics by subtracting heat from high latitudes in one hemisphere and simultaneously adding to high latitudes in the other hemisphere in GFDL AM2 in an aqua-planet configuration coupled to a slab ocean. We argue that the cloud radiative- and water vapor feedbacks are important. Second, the energy balance model (EBM) can predict the response in the atmospheric energy transport in the IPCC models when forced by ice-albedo, cloud forcing, aerosols and ocean heat uptake. The predicted cross-equatorial atmospheric energy transport can then be used to predict the ITCZ trend: in models with pre dicted southward (northward) cross-equatorial energy flux response from EBM, ITCZ tends to shift northward (southward). Moreover, we can do the attribution by comparing each forcing that is used to force the EBM. For example, in the 20th century, the largest forcing is the increase in the shortwave scattering, mostly due to sulfate aerosol increase in northern midlatitudes, which accounts for the northward cross-equatorial energy fluxes to shift the ITCZ to the south. However, in A2, due to differences in prescribed aerosol forcing in different models, the ITCZ exhibits no consistent trend.