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

Abstract The Hadley circulation (HC), a dominant driver of energy and moisture transport in low latitudes, varies in meridional extent on time scales 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 time scales. 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 time scales. The 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. Significance Statement The Hadley circulation is a major tropical atmospheric overturning circulation, with rising air near the equator (creating rainbands) and sinking air in the subtropics (creating dry regions). Its width changes over time, influencing rainfall and drought patterns globally. While an energy balance framework exists for understanding shifts in the rising branch, the controls on the Hadley circulation’s overall width have remained unclear. This study extends the energy balance approach to the descending branches, showing that their location—and thus the Hadley circulation’s width—is controlled by two factors: the energy received by the tropics and the energy transported away by atmospheric eddies. Different factors are dominant on different time scales: El Niño causes contraction due to increased tropical energy input, while long-term warming leads to expansion due to increased eddy energy transport. Understanding these controls is crucial for predicting future climate change and its regional impacts.