Eddy-mediated Regime Transitions in the Seasonal Cycle of a Hadley Circulation and Implications for Monsoon 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 center of the summer and equinox Hadley cell lies in a latitude zone of upper-level westerlies and significant eddy momentum flux divergence; the influence of eddy momentum fluxes on the strength of the cell is strong. The center of the cross-equatorial winter Hadley cell lies in a latitude zone of upper-level easterlies and is shielded from the energy-containing midlatitude eddies; the influence of eddy momentum fluxes on the strength of the cell is weak. Mediated by feedbacks between eddy fluxes, mean zonal winds at upper levels, and the mean meridional circulation, the dominant balance in the 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. At the transitions, a feedback involving changes in the strength of the lower-level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross-equatorial winter cell. The transitions resemble the onset and end of monsoons, for example, in the shift in the dominant zonal momentum balance, rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels, rapid changes in strength of the upward mass flux, and changes in direction and strength of the zonal wind at upper and lower levels. 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 as at the regime transitions in the simulation-mechanisms that can act irrespective of land-sea contrasts and other inhomogeneities in lower boundary conditions-may be implicated in large-scale monsoon dynamics in Earth's atmosphere.