Hadley Cell Expansion: Separating Eddy and Mean Flow Responses to Forcings

The drivers of Hadley cell expansion remain elusive, in part because the circulation sits at the nexus of tropical and extratropical dynamics. There are many convincing mechanisms, including the influence of extratropical waves and moist thermodynamics, but given the strong coupling within the earth system it can be difficult to parse which might be most important. Here we apply a simple scheme within an idealized model that can directly isolate the eddy and mean flow responses to forcings, and use it to explore a few simplified global and local forcing scenarios. Hadley cell expansion appears to be driven entirely by changes in the eddy flow field, and is opposed by zonal-mean thermodynamic changes. While this technique is currently limited to an idealized model, it can be extended, with some work, to more comprehensive earth system models to study other aspects of global variability and change.