CSIRO scientists are investigating the possibility that a climate shift has brought a long-term decline in rainfall over southwest Western Australia.

"Measurements indicate a slow decline in rainfall since the 1940s-50s, leading to the present drier regime," says Dr Bryson Bates of CSIRO Land and Water.

In some parts of the southwest, average rainfall appears to have settled into a pattern about 20 per cent lower than the norm for the first half of the 20th century - but, as the thirsty landscape soaks up more moisture, this has led to a 40 per cent reduction in inflow to Perth's dams. At the start of this summer, the dams were less than a third of capacity.

The immediate cause, says Dr Bates, is a clearly discernible climate shift that took place in the mid 1970s.

"At that time the tropical Pacific warmed abruptly and stayed warm, and there was a sudden warming in sea surface temperatures in the Indian Ocean. Since then there have been unusually frequent, persistent and intense El Ni�os, and fewer La Ni�as."

Working from global atmospheric data, the researchers have been trying to work out what this all means for local climate and rainfall across the southern part of WA.

The most obvious fact to emerge is that there are now more dry days than before due to an increase in the presence of high pressure cells to southeast of the region, causing moisture-bearing air streams to miss the continent. The encouraging news is that the system appears to have stabilized somewhat, although there is a large amount of year-to-year variability.

The main change lies in the absence of particularly wet winters, which once recharged the dams. Since 1975 there has been only one winter of above average inflow to the dams, compared with 13 in the period from 1950 to 1975.

As to what is "forcing" the new climate pattern, the team is exploring apparent links with changes in the behaviour of El Ni�o and the Antarctic Oscillation Index. Prior to the 1970s - when times were wetter - this index was negative. However since the mid-1970s it has swung into the positive, with zones of higher than usual air pressure forming over the southern Indian and Pacific Oceans.

"To be frank, we don't know really what's behind it yet. To begin with, we blamed it all on changes in the Indian Ocean, but now we've realised we may be dealing with something that affects the entire southern hemisphere.

"So we've started to explore this idea that shifts in pressure patterns off the Antarctic can have an influence across southern, and particular southwestern Australia."

Dr Bates says that present indications are that such a prolonged dry spell is fairly rare, and that it is likely to be due to the earth's natural climatic fluctuations, rather than man-made changes to the atmosphere.

"However, the present experience matches what climate projections are indicating may happen over the next 100 years.

"So the experience of southwestern WA may foreshadow the sorts of impacts we will start to see in southern Australia under greenhouse-induced climate change."

Dr Bates said that the team hoped to extend the implications of the work to other states, such as SA and Victoria, but funds were not yet available to do this.

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