Explaining Tropical Expansion Using an Energy Balance Framework

We examine tropical width changes through a diagnostic framework based on the energy balance. Given an observed or modeled change in tropical width, the framework can decompose it into a sum of changes in tropical net energy input, atmospheric storage, or poleward energy transport. This allows us to understand the magnitude of (1) tropical expansion in 4xCO2 CMIP6 experiments and (2) tropical contraction during El Nino events. We also examine model spread in tropical width and relate it to inter-model differences in the spatial pattern of warming, the surface energy budget, and clouds.