Abstract
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. Here, an idealized model of a dry, three-dimensional atmosphere is used to compare the response to zonally symmetric and asymmetric off-equatorial thermal forcings. For 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.
