Hadley cell dynamics in a primitive equation model. Part I: Axisymmetric flow

A strategy is adopted that applies the mean meridional circulation (MMC) equation to two different steady states of a primitive equation model. This allows for the investigation of the mechanisms behind the sensitivity of the Hadley cell structure to individual source terms in an axisymmetric circulation. Specifically, the strategy allows the MMC response to the individual source terms to be partitioned into direct and indirect components. The model's Hadley cell strengthens and broadens as the viscosity of the model is increased. It is found that a substantial portion of this sensitivity is attributable to diabatic heating and surface friction changes, which are ultimately induced by changes in viscosity. Similar behavior is found as the meridional gradient of the radiative-convective equilibrium temperature is increased, except that in this case the indirect response arises through the viscosity and surface friction change. In both cases, the changes in the static stability change are found to be of secondary importance. It is found that the latitudinal extent of the Hadley cell is more sensitive to the meridional temperature gradient than to the static stability. However, when the static stability is decreased (increased) by a sufficient amount, the Hadley cell becomes narrower (broader). Additional analyses indicate that the change in Hadley cell width is a response to the change in Hadley cell strength.