Monsoons transitions as regime transitions of a Hadley cell: II. Moist dynamics in an aquaplanet GCM

In simulations with an idealized aquaplanet GCM, the tropical mean meridional circulation undergoes rapid regime transitions in the course of a seasonal cycle. A transition in early summer is characterized by strengthening and broadening of the cross-equatorial winter Hadley cell, rapid relocation into the subtropics and intensification of precipitation, and reversals in the upper-level and lower-level winds. A reverse transition occurs in late summer. The simulated transitions resemble the onset (and end, at the reversed transition) of Earth's monsoons. Consistent with similar dry simulations (Schneider & Bordoni 2007), the rapid rearrangements of the circulation mark shifts in the leading balance of the vertically averaged zonal momentum equation, from regimes in which the eddy momentum flux divergence dominates near the center of the Hadley cell, to regimes in which the eddy momentum flux divergence is negligible and the mean momentum flux dominates. The summertime convergence zones form in the subtropics just equatorward of the lower-level moist static energy maximum, which is colocated with the boundary between the winter and summer cell. Factors controlling the exact location of these precipitation zones are examined and discussed.