Interhemispheric gradient forcing in the 20th and 21st centuries and implications for tropical circulation

Building on previous research implicating interhemispheric gradients in tropical climate change, we examine the role of greenhouse and aerosol forcings in giving rise to these gradients over the 20th century and in 21st century projections. Zonal mean energy fluxes and transports in 20th century and SRES A1B scenario simulations in eight coupled global climate models are analyzed. The dominant change is a hemispherically symmetric monotonic increase in poleward atmospheric energy transport in the tropics during the 20th and 21st centuries, suggesting an intensification of the Hadley Cell. Against this steady intensification, we find a distinct hemispherically asymmetric mode of variability. Northward cross-equatorial atmospheric energy transport, indicating the latitudinal asymmetry of the Hadley Cell, increases during the late 20th and early 21st centuries. This suggests a southward shift of the Intertropical Convergence Zone (ITCZ) during this period. The northward cross-equatorial energy transport compensates for reduced incoming shortwave radiation caused by sulfate aerosol reflectance in the northern mid latitudes, which is stronger among models that include indirect aerosol effects. As aerosols decline in the mid twenty-first century, northward equatorial transport generally decreases; however, the cross-model variability is high, likely due to differences in cloud cover. Our analyses suggest that 20th and 21st century zonal mean tropical atmospheric circulation changes can be characterized by two significant modes: a steady increase in poleward transport caused by greenhouse warming; and a late 20th/ early 21st century southward ITCZ shift caused by northern hemisphere sulfate aerosols.