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 whose results compare well with results from an idealized dry general circulation model (GCM). Calculations are performed in which the heating is parameterized as a relaxation of the temperature back to a prescribed radiative-convective equilibrium (RCE) state whose maximum temperature occurs at varying distances from the equator, representing an idealized seasonal cycle. In the axisymmetric model, as in the GCM, qualitative changes in the dynamics occur as the RCE temperature maximum moves further from the equator. For more equatorially symmetric RCE temperature, eddy momentum fluxes play a large 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 sufficiently large asymmetry, the zonal momentum budget becomes more nonlinear, angular momentum is more nearly conserved, and the circulation is a stronger function 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.