Climate Impacts of Inter-hemispherically Asymmetric Radiative Forcing

It has been demonstrated in a number of modeling studies that the tropical circulation of the atmosphere is sensitive to changes in the inter-hemispheric energy budget. Examples of perturbations to the inter-hemispheric energy budget include changes in the Atlantic Meridional Overturning Circulation or spatially heterogeneous changes in the distribution of radiatively active aerosols. Whether aspects of the response of the circulation to the forcing are dependent on the forcing's latitudinal position is the subject of this study. Here we investigate the impact of the location of the asymmetric perturbation on the tropical circulation in the context of the gross moist stability framework using simulations from both an idealized moist model and a comprehensive GCM. In each model we artificially impose a negative radiative forcing between 0 and 30 degrees North and 30 and 90 degrees North, and discuss changes in precipitation patterns, meridional moist static energy transport, zonal mean mass streamfunction, and gross moist stability. In the idealized moist model, for a given asymmetric negative radiative forcing, the ITCZ shift is greater when the forcing is placed in the tropics instead of the extratropics. This difference in behavior can partially be explained by the fact that in the tropical case we decrease the net shortwave radiation at the equator, while in the extratropical case it is left unchanged. From Bischoff and Schneider (2014), decreasing the shortwave radiation at the equator increases the sensitivity of the ITCZ position to the moist static energy flux there. However we show that this increase in sensitivity cannot fully explain the differences seen between the idealized tropical and extratropical cases. In the full GCM, due to zonal asymmetries, the response in the tropical and extratropical cases is more complicated; an interesting result from those cases in the full GCM is that over the Sahel and Maritime Continent we see opposing precipitation responses. Further investigation is required to determine how these differing rainfall patterns can be traced back to the location of the forcing.