Regionalization of the Hadley Circulation and implications for Australian rainfall

The Hadley Circulation (HC) in the Southern Hemisphere is decomposed into three sectors defined by the equatorial heat sources of Europe-Africa, Asia-Pacific and the Americas. A regional HC is defined by a streamfunction derived from the divergent component of the meridional winds. In the Southern Hemisphere, the variability of the edge of the HC is modulated by those of the regional HC in the Asia-Pacific sector, evident in the annual and seasonal means. This variability is associated with variability of the sea surface temperature (SST) in the tropical Pacific. In particular, an expanded Southern Hemisphere HC both in the zonal mean and in the Asia-Pacific sector is associated with a poleward expansion of the tropical wet zone that occurs primarily in the austral warm seasons, as well as with the positive phase of the Southern Annular Mode (SAM). However, we find that the variation of the edge of the regional HC centered on the Australian sector does not strongly affect that of the intensity of the local subtropical ridge (STR). Variations in the intensity and position of the STR that are strongly associated with southeastern Australian dry conditions appear as a remote response to tropical diabatic heating associated with the Indian Ocean Dipole (IOD) in austral winter and spring. The dry conditions over southeastern Australia are therefore unlikely to be driven by the expansion of the HC but more likely to be affected by a positive phase of the IOD.