La Nina brews destructive cyclones off Australia
By Michael Perry
SYDNEY (Reuters) - A mature La Nina now exists over the Pacific and could produce up to 13 cyclones in Australia's current storm season, along with increased rainfall over the drought-hit nation, weather officials said on Thursday.
Every 30 years Australia's cyclone season, from November to April, reaches a peak and some forecasters say 2008 may become a peak storm season due to La Nina, which produces colder than average seas along the equator forcing warmer waters around Australia's northern coastline and fuelling cyclones.
Australia's northeast coast has been hit by one cyclone and is currently being battered by a severe storm with cyclonic winds, generating huge waves which closed scores of beaches, and caused coastal flooding and the evacuation of tourists.
A second cyclone is expected to form in the next few days in the Gulf of Carpentaria in northern Australia. Only two cyclones developed off the northeast coast last season.
Australia's northwest coast was last week buffeted by a cyclone, temporarily halting offshore oil and gas production.
"Now that La Nina is fully established one would think an above average cyclone season would probably occur," said Jim Davidson, regional director with the government's Bureau of Meteorology in the tropical Australian state of Queensland.
"Probably 12 or 13 (cyclones) in the Australian region," Davidson told Reuters on Thursday.
Australia's National Climate Centre said a weak La Nina over the Pacific had strengthened and was now a "mature event."
"A La Nina event is firmly established in the Pacific, strengthening over the past month and contributing to the enhanced eastern Australian rainfall since November," said the Centre's latest La Nina report released on Wednesday.
Cooler than average sea surface temperatures now extend further west along the equator than at any time since 2000, while warmer than average sea surface temperatures surround northern Australia, said the report.
The December 2007 Southern Oscillation Index (SOI) of 14.4 is the highest monthly SOI since April 2006. The index measures atmospheric pressure differences between Darwin in northern Australia and Tahiti in the central Pacific.
A high positive index indicates wetter than average weather over northern Australia and Southeast Asia.
The Centre said that while some computer models suggested the La Nina was nearing its peak, most continue to indicate a persistence in cool Pacific Ocean temperatures, consistent with a La Nina, until at least autumn in the southern hemisphere.
"The cyclone season goes through until mid to late April, which would mean two or three more bursts of the monsoon, and with the burst of the monsoon there is an increased likelihood that cyclones will develop," said Davidson.
Each year some cyclones in Australia's storm region, from the Indian Ocean to near Fiji in the Pacific, never reach Australia, forming and dissipating in open ocean. But a La Nina heightens the chances of cyclones hitting Australia's northeast coast.
"When we see a La Nina in the Pacific, cyclones are pushed westward towards Queensland," said Blair Trewin from the National Climate Centre. "El Nino pushes cyclones eastward to the central Pacific."
Davidson said some forecasters were suggesting 2007-08 will be a 30-year peak cyclone season, but he said it was still too early to judge.
"In the 1940s and 1970s there was a prevalence of tropical cyclones, but so far there has been a trough in the 1990s and early part of this century," Davidson said.
"Now it's possible we are coming out of a trough and moving up to another peak, but we probably need another season or two to verify that."
The most deadly storm on record in Australia was Cyclone Tracy, which killed 65 people in Darwin in 1974.
(Editing by Jeremy Laurence)