  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. However, despite the degree of success
in representing the Hadley circulation, the canonical theoretical model does
not treat interactions with other regions, particularly the mid-latitudes.
Here, we extend the original model of Held and Hou (1980) to include the
influence of mid-latitude 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 sub-tropical jet. We find that
an increase (decrease) in the poleward heat flux leads to a strengthening
(weakening) of 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.
