A simple, analytic model is derived to study the physical mechanisms responsible for ITCZ narrowing under global warming. The model combines an energetic framework for the ITCZ with mock Walker circulation physics. Explicit expressions for the ITCZ area fraction are derived as a function of the zonal mean sea surface temperature (SST) profile, and bulk parameters representing the effects of radiation, convection and circulation. The bulk parameters are estimated from an ensemble of Atmospheric Model Intercomparison Project (AMIP) models. Under uniform surface warming, the simple model predicts a narrower and more intense ITCZ—in line with evidence from observations and climate models. The various physical contributions to ITCZ area changes under warming are decomposed using analytic approximations. Narrowing ITCZ tendencies are attributed to stronger clear-sky shortwave water vapor feedbacks, a smaller gross moist stability in convecting regions and stronger poleward moisture transports. Widening ITCZ tendencies are due to stronger clear-sky longwave water vapor feedbacks and weaker cloud-radiative effects in a warmer climate.
The simple process model is then used to diagnose the inter-AMIP-model spread in the tropical ascent area fraction change under warming. The AMIP inter-model spread is best captured by forcing the simple model with gross moist stability and clear-sky water vapor feedback parameters from the AMIP ensemble. This parameter perturbation exercise also captures the multi-model means of both the ascent area change and tropical mean precipitation change per unit surface warming seen among AMIP models. Furthermore, the simple model reproduces relationships between ascent area, and ascent area-averaged precipitation and strength, previously noted for complex climate models. The results here highlight that relatively simple physics underlie ITCZ narrowing under global warming. However, parametric/structural uncertainties in the treatment of convection and radiation can generate considerable inter-model spread in the ITCZ narrowing tendency with warming.