Subtropical Dry-Zone Is Expanding South

Lee Rannals for — Your Universe Online

New research reported in the journal Scientific Reports claims that the southern hemisphere is becoming drier.

The researchers wrote that a decline in April-May rainfall over southeast Australia is associated with a southward expansion of the subtropical dry-zone.

The scientists were looking to explore why autumn rainfall has been in decline across southeastern Australia since the 1970s, which is a period that included the Millennium drought from 1997 to 2009.

Previous research pointed at a southward shift in the storm tracks and weather systems during the late 20th century. However, the extent to which these regional rainfall reductions are attributable to the pole-ward expansion of the subtropical dry-zone had not been clarified until now.

“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,” Tim Cowan, a Commonwealth Scientific and Industrial Research Organization (CSIRO) scientist who worked on the project, said in a statement.

The team found the autumn southward expansion of the subtropical dry-zone is greatest over southeastern Australia, as well as 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,” CSIRO’s Dr. Wenju Cai said in the statement.

They tested the hypothesis that the dry-zone expansion would give rise to the southward shift in the average rainfall during April and May. They also questioned how rainfall might be affected across semi-rigid regions, including southern coastal Chile and southern Africa.

“During April and May, when the dry-zone expansion is strong, rainfall over south-eastern Africa, south-eastern Australia and southern-coastal Chile is higher than over regions immediately to their north,” Dr. Cai said.

The team found that for southeastern Australia, up to 85% of rainfall reduction can be accounted for by replacing southeastern Australian rainfall with rainfall 250 miles to the north. This southward shift of rainfall can explain only a small portion of the southern African rainfall trend.

“For south-east Australia, autumn is an important wetting season,” Dr. Cai said in a statement. “Good autumn rainfall wets the soil and effectively allows for vital runoff from follow-on winter and spring rain to flow into catchments.”

The study said an important issue remains as to why the poleward expansion is largest in the fall and there is still uncertainty about the role of external forces, like greenhouse gases.