Expanding subtropic zone the clue to dry autumns
The Southern Hemisphere is becoming measurably drier, a new internationally published CSIRO study has found.
In a report published in Scientific Reports, a sister publication to the Nature journal, researchers have linked a decline in April-May rainfall over south-east Australia – taking in much of the grainbelt – with a polewards expansion of the planet’s subtropical dry zone.
CSIRO scientists Dr Wenju Cai, Tim Cowan and Marcus Thatcher set out to explore why autumn rainfall has been declining across south-eastern Australia (and southern Africa) since the 1970s, a period that included the devastating Millennium Drought from 1997 to 2009. This rainfall decline has coincided with a southward (in the Southern Hemisphere) expansion of the subtropical dry zone by two to three degrees of latitude.
Previous research into causes of the decline in autumn rainfall across regions such as southern Australia had implicated a southward shift in storm tracks (tropical cyclone belt) and weather systems during the late 20th century. Now the focus has sharpened.
Researchers have been able to clarify the extent to which regional rainfall reductions in Australia, southern Africa and, to a lesser extent, Chile are attributable to the subtropical dry zone also expanding southwards. This zone is estimated to have moved towards the poles by up to 400 kilometres.
Mr Cowan explains that rainfall patterns in the subtropics are known to be influenced by the Hadley cell, the large-scale atmospheric circulation that carries heat from the tropics to the subtropics. Most of the Earth’s arid regions are located underneath descending branches of the Hadley circulation, at around 30 degrees latitude. These latitudes traverse areas such as the Sahara Desert in the Northern Hemisphere, and the Great Victoria Desert in Western Australia in the Southern Hemisphere.
“There has been a polewards 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-to-late autumn, or April to May,” Mr Cowan says.
The expansion itself has already been linked to global warming; solar heating being the main driver of atmospheric circulation.
The CSIRO researchers found that the autumn southward expansion of the subtropical dry zone is greatest over south-eastern Australia, and to a lesser extent, over the Southern Ocean to the south of Africa.
“The Hadley cell is comprised of a number of individual branches, so the impact of a southward shift of the subtropical dry zone on rainfall is not the same across the different semi-arid regions of the Southern Hemisphere,” Dr Cai says.
The researchers tested how the southwards dry zone expansion in the Southern Hemisphere would affect rainfall over areas into which it expanded. Using high-quality observations and an atmospheric model, the CSIRO team found that for south-eastern Australia up to 85 per cent of recent rainfall reduction could be accounted for by directly replacing south-eastern Australia rainfall with rainfall that was previously typical 400km to the north.
The findings have implications for both agriculture and catchment management.
“For south-east Australia, autumn is an important wetting season,” Dr Cai says.
Aside from its role in establishing crops, good autumn rainfall wets the soil to allow follow-on winter and spring rain to provide run-off flow into catchments.
The researchers say the next question to probe is why the poleward expansion is greatest in autumn. There is also still uncertainty about the role of factors such as increasing greenhouse gas levels because climate models have been underestimating the southward expansion of the Hadley cell edge.
Dr Wenju Cai
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