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. The latitude
ϕ_0 of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time
to study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are
compared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations
for all ϕ_0 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 ϕ_0 is increased, from an equinox regime
(small ϕ_0) in which eddy momentumfluxes strongly influence both Hadley cells to a solstice regime (large ϕ_0)
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 transitions between a linear
equinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddypermitting
simulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in
the axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddypermitting
simulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum
budget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the
eddy-permitting simulations, 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—makes it possible for
the circulation fields to change at the transition more rapidly than can be explained by the steady-state response
to the thermal forcing. 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
