Monsoons transitions as regime transitions of a Hadley cell: I. Dry dynamics

In a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions, the Hadley cells undergo transitions between two regimes distinguishable according to whether large-scale eddy momentum fluxes strongly or weakly influence the strength of a cell. The strength of the summer and equinox Hadley cell is strongly influenced by eddy momentum fluxes. The strength of the cross-equatorial winter Hadley cell is weakly influenced by eddy momentum fluxes. Mediated by feedbacks between eddy fluxes and upper-level mean zonal winds, the dominant balance in the vertically integrated zonal momentum equation at the center of a Hadley cell shifts at the transitions between the regimes, from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell. The transitions resemble the onset and end of monsoons, for example, in rapid shifts in the latitudes of maximum lower-level meridional mass flux, rapid changes in strength of the upward mass flux, and changes in direction and strength of the zonal wind. In the monsoonal regime, the maximum upward mass flux occurs in an off-equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear-frictional to geostrophic. Similar dynamic mechanisms, modified by moist processes (see Bordoni & Schneider, this conference), may be implicated in large-scale monsoon dynamics in Earth's atmosphere.