Regime transitions of steady and time-dependent Hadley circulations: Comparison of axisymmetric and eddy-permitting simulations

Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which thermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state and the latitude φ0 of maximum radiative-equilibrium temperature is progressively displaced off the equator. We study how the Hadley circulation responds to seasonally varying forcing and we compare axisymmetric simulations with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations for all φ0 approach the nearly inviscid, angular momentum--conserving limit, despite the presence of finite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting simulations, the Hadley circulations undergo a regime transition as φ0 is increased, from an equinox regime in which eddy momentum fluxes strongly influence both Hadley cells to a solstice regime in which the cross-equatorial winter Hadley cell more closely approaches the angular momentum--conserving limit. In axisymmetric time-dependent simulations, the Hadley cells undergo a similar transition from a linear, viscous regime to a nonlinear, angular momentum--conserving regime because viscosity can act more effectively in the weak equinox cell than in the strong cross-equatorial winter cell. As in the eddy-permitting simulations, the regime transitions in the axisymmetric time-dependent simulations are rendered rapid, compared with the timescale of thermal forcing changes, by a mean-flow feedback involving the upper-level zonal winds, the lower-level temperature gradient, and the poleward boundary of the cross-equatorial Hadley cell. However, the regime transitions in the axisymmetric simulations are less sharp than those in the eddy-permitting simulations because eddy--mean flow feedbacks in the eddy-permitting simulations additionally sharpen the transitions. Implications of the simulated regime transitions for the existence and rapidity of monsoon transitions in the Earth's atmosphere will be discussed.