Monday 14 November, 2005

MEDIA RELEASE

Less winter rain in southern Australia

If you're in the southern half of Australia, you could be in for more of the same – water shortages and less winter rain, according to the latest climate research from the Bureau of Meteorology.

Over the past 40 years or so, we’ve seen a decline in rainfall across southern Australia in early winter (May to July), particularly in south-west Western Australia and the western and southern part of Victoria. In some locations the reduction has been up to 25 per cent (Katanning and Dwellingup in WA, and Kaniva and Natimuk in Victoria). Climate model projections point to more of the same: less rain, and not just in the first part of winter, but affecting the entire "wet season", from May to October.

Dr Pandora Hope, a climate scientist with the Bureau of Meteorology Research Centre, says: “In the late 1960s, the area around Perth saw a drop in rainfall of around 15 per cent, and to date we have not seen a recovery.” The research comes out of the Indian Ocean Climate Initiative, a collaborative project between the bureau, CSIRO and the WA Government.

Colleague Dr Bertrand Timbal says there were similar drops in rainfall in the western and southern part of Victoria in the mid 1990s. Dr Timbal, who will present these results at the Greenhouse2005 conference on Thursday (11.50am to 12.10pm, Carlton Crest, St Kilda), says: “We know that local rainfall is linked to large scale changes in air pressure and to the amount of moisture in the air. So we looked at all existing data on rainfall, pressure and air moisture for the past 50 years. This information was used to analyse computer model simulations of our current climate as well as our future climate.

“In WA, for example, rainfall changes are difficult to simulate with a climate model if only natural variations are considered,” Dr Timbal says. “However if you take into account human activities – through the emissions of greenhouse gasses and other pollutants – climate models can start to represent the drop in rainfall observed over the last 40 years. The picture is even clearer if you look at the modelled changes in the air pressure and amount of moisture in the air.”

The southern half of Australia is normally under the influence of a region of high surface pressure, referred to as the subtropical ridge. Analysis of the recent rainfall decline and projections of future climate change suggests that both the subtropical ridge is strengthening, and the low pressure systems which develop further south over the Southern Ocean - within the "roaring 40s" - are less frequently penetrating north.

Rain-bearing frontal systems provide the majority of winter rainfall in southern Australia. Researchers associated with the Indian Ocean Climate Initiative have found that since the mid 1970s, fewer rain-bearing frontal systems are crossing the south-west, and the potential for storm development has decreased. "If we project forward, not only do we expect to see less rain in early winter in the future, but our models tell us that the entire wet season - from May to October - will very likely be reduced," Dr Timbal says.

While some of this research work is on-going, most has been published in scientific journals including Journal of Climate, Climate Research, Climate Dynamics, Geophysical Research Letters and Water Resources Research.

Further information:
Bertrand Timbal, Bureau of Meteorology Research Centre, tel: (03) 9669 4697 or 0425 857 635
Pandora Hope, Bureau of Meteorology Research Centre, tel: (03) 9669 4774
David Jones, Climate Analysis Section, National Climate Centre, Bureau of Meteorology, tel: (03) 9669 4085

Observed and projected change in the southwest: http://www.ioci.org.au

Greenhouse2005 conference: http://www.greenhouse2005.com
Simon Torok mob: 0409 844 302 or Craig Macaulay mob: 0419 966 465

Ends