Australia’s southern winters are drying out. Here’s why
“It’s actually quite unusual for us to get such a widespread dry through the winter without having an El Nino,” Bureau of Meteorology senior climatologist Blair Trewin said.
“It’s almost more about what hasn’t been happening,” Dr Trewin said.
Normally in winter, storms come up from the southern Indian Ocean and clip the bottom of Western Australia, delivering rain to the south of the country.
But until mid-July, the storms largely missed the continent.
What kept the storms south is a little-known climate driver called SAM, or Southern Annular Mode.
When SAM is positive, as it was in early winter, storms stay south and southern Australia experiences clear, calm days and cold, frosty nights.
SAM went negative in late July and the southern fringe of Australia was hit by a series of rain-bearing fronts.
GIF: Australia Rainfall Deciles May to August 2017
Climate driver getting stronger in winter
The August rains saved crops in far southern areas, but did not reach up far enough to help farmers in central New South Wales or WA’s northern wheatbelt.
Instead, an area of high pressure called the Sub Tropical Ridge dominated the continent, confining rain to the south.
The Sub Tropical Ridge is a climate driver worth paying attention to because over the past few decades, it has been getting stronger in winter.
Ben Hawken says the NSW central-west had 50 to 55 days of below-zero days this winter. ABC News: Robert Virtue, file
As a result, winters in the south of the country are drying out.
Agronomist Ben Hawken said he had never seen crops in the central-west of New South Wales damaged by frosts like he had this winter.
“We had 50 to 55 below-zero days this season. Normally we’d have half a dozen,” he said.
Frosts combined with low rainfall across all the southern states have jeopardised many farmers’ entire winter crops.
“Yesterday I looked at one farmer’s five paddocks. We wrote off three of them,” Mr Hawken said.
Rainfall continues to decline in southern Australia
According to the Bureau of Meteorology, May to July rainfall has reduced by about 19 per cent since 1970 in the south-west of Australia.
There has been a decline of about 11 per cent since the mid-1990s, in the April–October growing season rainfall in the continental south-east.
CSIRO Agriculture and Food senior principal research scientist Zvi Hochman said winter rain in Australia’s southern wheatbelt had declined by a whopping 28 per cent since 1990.
“I was surprised as anyone to find the extent to which that trend, across the 50 weather stations, is there,” Dr Hochman said.
“Yes, it varies geographically, but it is still a very strong trend. “A period of 26 years is not sufficient to say without any doubt that it is climate change. But it is long enough to say without any doubt that it’s not just climate variability”.
“There is a one in a hundred billion chance that this trend is by chance alone.”
Farmers well versed in adapting to changing climate
Dr Hochman said Australian grain growers had been very good at adapting to the reduced rainfall.
“The farmers have been able to actually cope with that by adopting new techniques or just tightening their management around their farming practices, and there is still room to go in that regard,” he said.
Dr Trewin said farmers may get some relief this spring.
“The seasonal outlook from October to December is actually quite encouraging for New South Wales,” he said.
“The outlook is going for about a 60 percent chance of above-average rainfall on the coast and 55 to 60 percent chance of above average for a lot of the inland areas.”
Original article posted on ABC NEWS, 20 Sep 2017 (link)