The Scientific Reports paper "Rainfall reductions over Southern Hemisphere semi-arid regions: the role of subtropical dry zone expansion" was published on October 3, 2012, by Wenju Cai, Tim Cowan, and Marcus Thatcher, all from CSIRO Marine and Atmospheric Research, in Aspendale, Victoria, Australia.
The three researchers state within the paper "Since the late 1970s, Southern Hemisphere semi-arid regions such as southern-coastal Chile, southern Africa, and southeastern Australia have experienced a drying trend in austral autumn, predominantly during April and May." The picture, above, shows bones lying in the dry ground of southern Australia.
The reason is also reported by the group: "The rainfall reduction coincides with a poleward expansion of the tropical belt and subtropical dry zone by around 2°–3° [degrees] in the same season."
The October 3, 2012 CSIRO article "Southern Hemisphere becoming drier" details the work on the Australian research team.
"However, the extent to which these regional rainfall reductions are attributable to the poleward expansion of the subtropical dry-zone has not been clarified before now."
Tim Cowan, one of the researchers on the study, said the drier weather is attributed to what is called the "Hadley cell", which is a large circulatory pattern that takes heat from the tropics and moves it to the sub-tropics.
Cowan states, “There has been a southward expansion of the edge of the Hadley cell – also called subtropical dry-zone – over the past 30 years, with the strongest expansion occurring in mid-late autumn, or April to May, ranging from 200 to 400 kilometres."
And, the drier weather caused by this weather pattern is greatest over southeastern Australia.
The three researchers conclude in their Scientific Reports paper, "Here we show that the impact of the poleward subtropical dry-zone shift is not longitudinally uniform: a clear shift occurs south of Africa and across southern Australia, but there is no evidence of a poleward shift in the southern Chilean sector. As such, a poleward shift of climatological April-May rainfall can explain most of the southeastern Australia rainfall decline, a small portion of the southern Africa rainfall trend, but not the autumn drying over southern Chile."