Midlatitude Interactions Expand the Hadley Circulation

Abstract The Hadley circulation describes a planetary-scale tropical atmospheric flow, which has a major influence on climate. Contemporary theoretical understanding is based upon angular momentum conservation, the basic dynamical constraint governing the state of the flow pattern, and scaling relationships characterizing the macroturbulence created by synoptic eddies. However, despite the degree of success in representing the Hadley circulation, the canonical theoretical model of Held and Hou does not treat interactions with other regions, particularly the midlatitudes. Here, we extend their model to include the influence of midlatitude large-scale atmospheric dynamics, which we treat using the planetary-scale heat equation with a parameterized poleward heat flux driven by synoptic eddies. The energy flux balance within the Hadley cell includes the poleward heat flux at the poleward edge of the cell, which is controlled by the baroclinic instability of the subtropical jet. We find that an increase (decrease) in the poleward heat flux leads to a strengthening (weakening) tropical convection, driving an equatorward (poleward) shift of the edge of the Hadley cell. Thus, our theoretical solutions suggest that global warming, which can reduce the baroclinicity of the subtropical jet, can lead to the poleward expansion of the Hadley cell due to the change in energy flux balance within it. Significance Statement The Hadley circulation is a principal dynamical feature in the tropics, acting as an engine that transfers heat and momentum to high latitudes. At the same time, the extratropical regions can influence the Hadley circulation. Under global warming, high latitudes warm faster than low latitudes, reducing the equator to pole temperature difference and the poleward heat flux in the midlatitudes. The theory described here shows that the decrease in the poleward heat flux can lead to the expansion of the Hadley cell and a poleward shift of subtropical dry zones.