Australasian Science Magazine

By Stephen Luntz

Cyclones on the opposite side of the continent can increase the intensity of heat waves over South Australia, Victoria and Tasmania, but only when they occur in a particular zone, a paper in Geophysical Research Letters has revealed.

Research by Ms Tess Parker from the ARC Centre of Excellence for Climate System Science identified that the extreme weather preceding the Black Saturday bushfires of 2009 was given a catastrophic boost by Tropical Cyclone Dominic just off the north-west coast of Western Australia.

“Previous studies have shown that the intensity of the tropical cyclone has very little bearing on the strength of heat waves. The relative location of the cyclone is far more important,” Parker said.

“TC Dominic only reached Category 2 status at its peak, but it had powerful amplifying effects for the record-breaking heat wave in Victoria that led to the fatal Black Saturday bushfires,” Parker said. “The cyclone was right in the heart of a 1.5 million km2 area that we have identified as a key area for magnifying the impact of high pressure systems that generate Victoria’s intense heat waves.

“All heat waves in Victoria are associated with upper level anticyclones but, while the essential cause of the heat wave is the same, the impact of cyclones on their formation means not all heat waves are created equal,” said Parker.

Two mechanisms help cyclones off the north-west coast of Western Australia amplify heat waves in south-eastern Australian.

The first occurs when the outflow from the cyclone nudges the jet stream in the upper levels of the atmosphere, producing a disturbance that makes the jet stream wavier. Correctly positioned, this waviness can strengthen an anticyclone to the south.

Air flowing out from the cyclone at upper levels can also fuel the intensity of an anticyclone nearby, making it move more slowly along the jet stream and persist over one area for a longer period of time. Parker compares anticyclones to pebbles in a stream of water, with larger ones being harder to shift.

Similar observations have been made for some weather systems in the Northern Hemisphere, and Parker says it is possible that cyclones in the Coral Sea could have a similar impact on heat waves over New Zealand.

“These results will help forecasters to better predict the intensity of heat waves as they start to develop,” Parker says.