Bureau of Meteorology Research Centre climate scientists believe they have part of the answer to significant declines in Australian regional rainfall and subsequent stream flow since the 1970s.

Dramatic rainfall declines in early winter in southwest Western Australia became evident from the 1970s. Similar declines were observed in southwest Victoria and southern South Australia from the mid-1990s.

Researchers were puzzled why the impacts were felt two decades earlier in the west. Their latest analyses suggest the changes were driven by an increase in mean sea-level pressure that first weakened rainfall from a long-lasting trough located in winter off the WA’s south west coast. The impact was slower to reach the affected parts in the southeast, which are influenced by a persistent high-pressure ridge.

The impacts are consistent with observed changes of the Southern Annular Mode (variations in the upper level circumpolar westerly jet streams), which affect surface pressure in particular. The researchers are now investigating whether the known human influences on the mode are a plausible cause for the rainfall declines.

For further information: Australia’s Bureau of Meteorology,

Rob Morton, Tel: +61 3 9669 4188,

r.morton@bom.gov.au