Regime transitions of cross-equatorial Hadley circulations with zonally asymmetric thermal forcings

Observed nonlinearities in the seasonal evolution of monsoons have been previously explained using theories for Hadley circulations driven by zonally symmetric thermal forcings, even though monsoonal forcings deviate strongly from the assumption of zonal symmetry. To understand the effects of zonal asymmetry, we employed an idealized model of a dry, three-dimensional atmosphere to compare the response to zonally symmetric and zonally asymmetric off-equatorial thermal forcings. For zonally symmetric forcings, the zonal mean, cross-equatorial mass flux increases more rapidly with the amplitude of the forcing once the forcing becomes strong enough to reduce the upper-tropospheric absolute vorticity to near zero, consistent with previous studies of the transition to angular momentum-conserving flow. For zonally asymmetric forcings, the zonal mean cross-equatorial flow exhibits a similar dependence on forcing strength and a similar reduction of the zonal mean upper-level vorticity, but asymmetric forcings also produce strong zonal overturnings with subsidence west of the heating, as in the well-known linear response to off-equatorial heatings. The mass flux in these zonal overturnings increases linearly with forcing strength until its rate of increase tapers off for the strongest forcings; the total upward mass flux (i.e. the zonal mean plus zonally asymmetric components) increases linearly with the strength of zonally asymmetric forcings and exhibits no abrupt or nonlinear dependence on forcing amplitude. These results indicate the importance of considering the zonally asymmetric part of the divergent response to off-equatorial forcings, and suggest that theories based on zonally symmetric forcings need further examination before they can be assumed to describe observed monsoons.