Single-layer axisymmetric model for a Hadley circulation with parameterized eddy momentum forcing

An axisymmetric single-layer model is used to study interactions of the Hadley circulation with extratropical eddies. Eddy momentum fluxes are parameterized using a simple closure motivated by calculations with an idealized dry general circulation model (GCM). Calculations are performed in which the heating is parameterized as Newtonian relaxation of temperatures toward a prescribed radiative-convective equilibrium (RCE) state. The latitude at which the maximum RCE temperature occurs is varied to represent seasonal variations. In the axisymmetric model, as in the GCM, qualitative changes in the zonal momentum budget occur as the RCE temperature maximum moves away from the equator past a threshold latitude. For RCE temperature maxima closer to the equator, eddy momentum fluxes play a dominant role in the zonal momentum budget, nonlinearity is weak, and the meridional circulation is a weak function of the degree of asymmetry about the equator. For RCE temperature maxima sufficiently far from the equator, the zonal momentum budget becomes more nonlinear, angular momentum is more nearly conserved, and the circulation is a stronger function of the degree of asymmetry about the equator. Since the axisymmetric model can capture this behavior while being much simpler than the GCM, it may be a useful step towards a more comprehensive theory of the zonal-mean general circulation.