Hadley Cell Dynamics in a Primitive Equation Model. Part II: Nonaxisymmetric Flow

This paper investigates the effect of baroclinic eddies on the structure of the Hadley cell. Self-consistent calculations of both axisymmetric and nonaxisymmetric circulations allow an unambiguous estimate of baroclinic eddy effects on the structure of the Hadley cell. Furthermore, a diagnostic analysis allows us to partition the influence of baroclinic eddies into “direct” and “indirect” responses. The former refers to the meridional circulation attributable to the explicit eddy fluxes while the latter refers to the meridional circulation attributable to part of other processes, such as surface friction and diabatic heating changes, which are in fact induced by the baroclinic eddies. For a realistic parameter range, it is found that these indirect responses are comparable to the direct response. While the direct response of the eddies is always found to be a strengthening of the Hadley cell, the indirect response can either strengthen or dampen the Hadley cell. When the thermal driving of the atmosphere is moderate, baroclinic eddies always amplify and broaden the Hadley cells. On the other hand, if the thermal driving over the Tropics and subtropics becomes sufficiently strong, the net effect of baroclinic eddies is to dampen (strengthen) the Hadley cell above (below) the height level of maximum diabatic heating. An explanation for this behavior is given in terms of competition between the Hadley cell driving by the eddy fluxes (both direct and indirect) and damping of the Hadley cell by potential temperature mixing.