ITCZ shifts in simulations of global warming

We introduce a new theoretical framework to help interpret why comprehensive climate models differ in their projections of how much the ITCZ will shift with global warming. The framework is based on fundamental energetic constraints of the system: since both moisture transports and energy transports within the deep tropical atmosphere are governed by the Hadley circulation, a southward shift of the ITCZ is associated with a northward transport of moist static energy. This situation is typically associated with enhanced heating of the Southern Hemisphere, often due to hemispheric differences in aerosols, clouds, surface albedo changes.

We show that the cross-equatorial energy flux is strongly anticorrelated with ITCZ shifts in several different warming scenarios, justifying the energetic framework. We then perform partitioning and attribution of the energy flux changes using the exact energy budget and an energy balance model. In many models extratropical forcing is the primary driver of ITCZ shifts, with clouds and surface albedo changes of key importance. In 20th century simulations, aerosol cooling of the Northern Hemisphere causes the ITCZ to shift southward in all models, of greatly varying degree.