Rainfall being displaced in critical areas
By Summit Voice
FRISCO — As the northern hemisphere grapples with the impacts of the melting polar ice cap, the southern hemisphere is facing a different climate change issue.
A poleward expansion of the subtropical dry-zone is probably responsible for a significant decline in autumn rainfall over southeastern Australia and may be affecting seasonal precipitation in other areas.
Since most of the world’s landmass and population is in the northern hemisphere, climate change impacts have been recognized and studied more extensively. Much of the southern hemisphere is open ocean, so there’s less good data to work with, but some research has already suggested a southward shift in the storm tracks and weather systems during the late 20th century.
It hasn’t been clear whether that was due to a large-scale shift in hemispheric patterns, but in a new study published in Scientific Reports, CSIRO scientists Wenju Cai, Tim Cowan and Marcus Thatcher explain that rainfall patterns in the subtropics are known to be influenced by the Hadley cell, a large-scale atmospheric circulation that transports heat from the tropics to the sub-tropics.
“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,” Cowan said.
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,” said Cai.
The researchers tested the hypothesis that the dry-zone expansion would give rise to a southward shift in the average rainfall during April and May, and questioned how rainfall across semi-arid regions, including southern-coastal Chile and southern Africa, would be affected.
“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,” Cai said.
Using high-quality observations and an atmospheric model the CSIRO team found that for south-eastern Australia, up to 85 percent of recent rainfall reduction can be accounted for by replacing south-eastern Australia rainfall with rainfall 400 kilometers to the north.
Such a southward shift of rainfall can explain only a small portion of the southern Africa rainfall trend, but none of the autumn drying observed over southern Chile.
“For south-east Australia, autumn is an important wetting season,” Cai explained. “Good autumn rainfall wets the soil and effectively allows for vital runoff from follow-on winter and spring rain to flow into catchments.”
According to the study an important issue remains as to why the poleward expansion is largest in autumn, and there is still uncertainty about the role of external forcings – such as greenhouse gases – as climate models underestimate the southward expansion of the Hadley cell edge.
This research was conducted through CSIRO’s Water for a Healthy Country Flagship, and was funded by the Goyder Institute for Water Research and the Australian Climate Change Science Programme. Wenju Cai, Tim Cowan and Marcus Thatcher are from CSIRO’s Marine and Atmospheric Research division.