* La Nina increases cyclone risk in north-east Australia
* Increasing dry periods seen in key WA grain region
* Greenhouse gases linked to increasing dryness
By Bruce Hextall
SYDNEY, Oct 1 (Reuters) - Australia's north-eastern sugar growing regions and coal ports face a higher risk of cyclone activity during the coming summer months, the country's weather bureau said on Friday.
The bureau also warned of increasing dry periods in south-western Australia, a key wheat producing region.
It said greenhouse gases were linked to less rain in the region where crops are currently under stress following the driest year to date since rainfall records have been kept 110 years ago.
"We would expect to see with increases in levels of greenhouse gases a decline in long-term rainfall in south-west Western Australia and, with that, an increased frequency of extreme dry years.. there is a relationship with climate change," said Blair Trewin, climatologist at the Bureau of Meteorology at a briefing. For a statement/graphics from bureau (here .pdf)
Chances of increased cyclone activity in the country's north-east were associated with a La Nina weather event that has developed in the central Pacific.
"Historically, La Nina years do tend to see more cyclone activity closer to Australia and less cyclone activity out in the central Pacific," said Trewin.
"One would expect an elevated risk of cyclone activity," he said.
The bureau is yet to issue is official cyclone activity forecast report but cyclones in the north-east state of Queensland can damage sugarcane crops and disrupt shipping from coal ports.
Queensland produces nearly all of the sugarcane grown in Australia, the world's third-largest sugar exporter, while the state is also one of the world's largest coal exporting regions.
Trewin said Australia, as a whole, was on track to record its sixth wettest year since rainfall on record despite dryness in south-western Australia .
"The broad-scale climate situation is essentially perfect for getting rains in Australia with a well developed La Nina in the central Pacific," said Trewin.
"The other thing we have..is a negative Indian Ocean dipole..that is very favourable for good rain into a lot of Australia at this time of the year," he said.
La Nina weather events, which usually bring wet weather to northern and eastern Australia, typically start to breakdown in the early months of the following year.
"It would be a bit of surprise if La Nina conditions held on through 2011.. but we do have a very long-term trend towards reduced rainfall across the southern fringe Australia," he said. Western Australia is now facing the spectre of its wheat harvest being halved following a dry winter and spring across the state's south-west.
Estimates for state's 2010/11 harvest, that starts later this month, range as low as 4.25 million tonnes compared with 8.2 million tonnes in 2009/10. A long-term trend of declining rainfall in the region has been seen since the late 1960s.
"The really wet years have more or less disappeared," said Trewin.
A sub-tropical high pressure ridge, which is a normal feature of Australian climate has been shifting further south in recent decades, reducing the number of rain bearing systems reaching Australia's southern fringes.
Trewin said the shift south was linked to climate change.
Editing by Ed Davies