UC Berkeley

Several lines of evidence - paleoclimate observations, model simulations, and theory - suggest that thermal forcing originating from the extratropics have a profound effect on tropical rainfall. Cold thermal forcing located in the Northern extratropics cools air and surface temperatures over the entire hemisphere, generating an interhemispheric temperature asymmetry that shifts the latitudinal position of the Intertropical Convergence Zone southwards and weaken rainfall over Northern Hemisphere summer monsoon regions. This change is associated with a northward anomalous cross-equatorial atmospheric energy transport induced by differences in the energy flux entering each hemisphere. I summarize how this 'interhemispheric pattern' could be usefully applied to understanding multidecadal and longer-timescale forced changes in tropical rainfall over the 20th and 21st centuries, focusing on three specific causes: anthropogenic aerosols, the late 1960s climate shift, and hemispheric asymmetric feedbacks to greenhouse warming.