An Energetic Framework for Understanding Hadley Circulation Width Variations: Seasonal Cycle, ENSO, and Global Warming

The Hadley Circulation (HC), a dominant driver of energy and moisture transport in low latitudes, varies in meridional extent on timescales ranging from seasonal to multidecadal. For example, the subtropical termini of the descending HC branches shift latitudinally by 10° over the seasonal cycle, contrasting with a 34° seasonal shift of the Intertropical Convergence Zone (ITCZ). While diagnostic energetic frameworks have successfully provided insight into ITCZ migrations, this study applies a similar framework to understand HC extent variations across timescales. The framework relates the HC extent to the tropical net energy input (NEI) and the eddy energy export from the HC to the extratropics. Variations in HC extent can therefore be decomposed into contributions from NEI and eddy energy export changes, which play different roles on different timescales. Analysis of observations and climate simulations shows that HC contraction during El Niño is primarily related to increased tropical NEI, while HC expansion under global warming is primarily related to increased eddy energy export. This energetic framework, while not fully predictive because it excludes factors such as the angular momentum balance, offers a new perspective for understanding variations in HC extent.