Inter-hemispheric differences in energy budgets and cross-equatorial transport anomalies during the 20th century

Abstract

We analyze the evolution of inter-hemispheric asymmetries in the energy budgets (EBs) and near-surface temperature anomalies during the 20th century, as given in Coupled Model Inter-comparison Project, phase 5 (CMIP5) simulations. We also consider the cross-equatorial energy transports (CET) in the atmosphere and in the oceans, in order to evidence how EB asymmetries affect the redistribution of energy between the two hemispheres. Two different experimental settings have been considered, one including only the spatially homogeneous evolving greenhouse gas forcing (GHG), and another one a realistic superposition of all known evolving forcings (ALL), such as aerosols and volcanic eruptions. This study shows that, according to the CMIP5 models, the response of the climate system to the ongoing forcing during the 20th century has differed substantially from what would have resulted from an increase in GHG concentration alone. In the GHG ensemble the Northern Hemisphere (NH) warms more than the Southern Hemisphere (SH), while both hemispheres exhibit similar and positive EB anomalies at the TOA, mainly due to increasing shortwave absorption and with no significant variations of cross-equatorial energy transports. On the contrary, in the ALL ensemble the two hemispheres warm similarly, while the SH exhibits a positive EB anomaly twice as large as in the NH, due to a reduced LW emission (Outgoing Longwave Radiation, OLR) in the SH, with oceanic CET anomalies directed towards the NH. The EB asymmetry in ALL is ascribed to the asymmetry in OLR changes, which is explained by the different role of clouds in the two hemispheres. The ocean heat content (OHC) tendency per unit surface area is similar in the two hemispheres, so that the asymmetries in ALL EB determine CET changes. We evidence that CET changes in the ALL ensemble are associated with the inter-hemispheric asymmetry in the aerosol forcing, which is stronger in the NH than in the SH. We find no significant relation between CETs and inter-hemispheric near-surface temperature asymmetries in GHG, partly due to the large model spread. Generally, deficits in modeled CET for present-day conditions are not ascribed to forcings and feedbacks, rather they are intrinsic to the models.