The Role of Monsoon-like Zonally Asymmetric Heating in Interhemispheric Transport

Recent studies have quantified the cross-equatorial transport by the age of tropospheric air since last contact at the Northern hemisphere midlatitude surface using trace gases in the observations and comprehensive models. The interhemispheric transport is often linked to the strength and position of tropical convection or the seasonal migration of the zonally averaged Hadley circulation. While the importance of the seasonal migration of the zonally averaged cross-equatorial flow on the interhemispheric exchange has been recognized, the role of the zonally asymmetric circulation has not been well understood. This study examines the role of monsoon-like zonally asymmetric heating on interhemispheric transport, using an idealized dry atmospheric model. It is found that the interhemispheric transport time scale is unrealistically large under perpetual equinoctial radiative forcing. Both the seasonal migration of the Hadley cell and the monsoon-like zonally asymmetric heating reduce this interhemispheric exchange time to more realistic values compared with the observations, but the zonally asymmetric heating is more effective than the seasonal migration of the zonal mean circulation. This highlights the importance of large-scale eddy circulation for interhemispheric transport.