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Rain-blocking weather pattern extends dry spell
By Peter Hannam
The chances of relief for the country's farmers or firefighters are low as dry climate patterns in the Indian and Southern oceans combine.
To the country's north-west, relatively cool waters in the eastern Indian Ocean mean there is less cloud formation, with a reduced prospect for moisture streams across the continent, the Bureau of Meteorology's latest climate update shows.
What climate analysts call the Indian Ocean Dipole (IOD) - the temperature differences between the western and eastern Indian Ocean - has been in that rain-suppressing or "positive mode" since May. And while it has lately eased back, the levels remain near their highest in at least two decades.
"International climate models surveyed by the bureau indicate the positive IOD is so strong that its decline is likely to be a slow one and could persist into mid-summer, the bureau said, adding events of this kind "are often associated with a more severe fire season for south-east Australia".
Andrew Watkins, head of the bureau's long-range forecasting, said the pattern's strength "puts it in the top handful of events" on record. Instead of breaking down by December it could well linger until January, he said.
The better-known El Nino pattern in the Pacific did not always mean south-eastern Australia is hit by drought and fire. Typically with IODs, however, "the stronger the event, the bigger the impact", Dr Watkins said.
"It sets up high-pressure anomalies over south-eastern Australia," he said.
Eventually, the seasonal pattern will break up with the arrival of the monsoon trough in the southern hemisphere.
"While the IOD continues to show signs it will persist later than usual this year, it remains unlikely that the positive IOD will persist into the second half of summer," the bureau said.
Another influence compounding NSW's drought woes, though, is coming from the south.
The belt of westerly winds blowing in the mid to high latitudes of the southern hemisphere shift south or north on shorter time ranges than the IOD.
Lately that belt of winds - known as the Southern Annular Mode - has shifted northwards. That negative mode usually means reduced chance of summer rain for most of south-eastern Australia.
Melbourne and much of southern Victoria has been relatively cool and dry as those westerlies sweep over the Great Australian Bight and Bass Strait. For Sydney and eastern NSW, however, those winds cross inland Australia and arrive hot and dry by the time they reach the coast, Dr Watkins said.
The negative SAM conditions are forecast to remain until the end of the month at least.
The odds are above 80 per cent in favour of warmer than average conditions for NSW for the rest of November, Dr Watkins said. "And in December, things won't change that much."
If patterns persist, conditions for the state could remain relatively warm "right through summer", he said.
And while Victoria had largely dodged the bushfire emergencies, "it doesn't take a lot of heat to dry it out", he said.