21 June 2007Cosmos MagazineHilary Jones
ADELAIDE: Hope is fading of a La Niña weather pattern bringing higher than usual rainfall to the east and south of Australia, thus breaking the continent's drought this year.
In February the El Niño weather pattern, a major contributor to the drought, was declared over by the Australian Bureau of Meteorology (see, El Nino ends, heralding rain Down Under, Cosmos Online). The Bureau announced cautious optimism of wetter-than-normal conditions associated with a developing La Niña in 2007.
A La Niña weather pattern, influenced by winds and currents in the Pacific Ocean, usually results in relatively wet conditions in Australia - the reverse of an El Niño pattern, which generally brings reduced rainfall. These phenomena are called the El Niño Southern Oscillation, or ENSO.
During a La Niña, the Pacific Ocean's easterly current and trade winds strengthen. The winds carry warm moist air that rises over eastern Australia and Indonesia, producing rain.
Northwest cloudbands
According to Grant Beard, a senior meteorologist at the Bureau of Meteorology in Melbourne, the chances of a La Niña developing have now fallen. "A month ago I would have said yes, but the process has stalled recently," he said. "A La Niña event would be the most beneficial climate situation we could hope for, particularly if it is accompanied by frequent 'northwest cloudbands'."
Northwest cloudbands are cloud masses that extend south from the tropical oceans northwest of Australia, and bring significant rainfall to southeastern Australia in late autumn to mid-spring, Beard said.
The bureau's climatic computer models predicted that a La Niña was likely in 2007, based on climate and ocean observations of previous La Niñas. The most important precursor of a La Niña event observed by the bureau was cooler than normal conditions in the equatorial Pacific Ocean, especially in the eastern Pacific.
However, in the past fortnight there has been some warming observed in the eastern Pacific, which they suggest has at least temporarily stalled the development of a La Niña event.
"There is still time for the development of a La Niña to re-gain momentum, as the ENSO phase [El Niño, La Niña or neutral] usually becomes fixed after about July, or in rarer instances, August," said Beard – but hope is fading.
Even if a La Niña does not develop, the end of the El Niño should still herald higher rainfalls. "Using history as a guide, the years following an El Niño usually bring moderate to good falls of rain in winter and spring across eastern and southern Australia," said Beard. "[Though] higher falls are more likely if a La Niña develops."
"In fact in this month of June we've already seen significant rain along the New South Wales coast and in northwest Queensland," added Beard.
Dustbowl Australia
It is uncertain what effect climate change will have on ENSO and rainfall in the future, according to meteorologist Neville Nicholls, a professor at Monash University in Melbourne.
"However, the models and physical reasoning and empirical studies all indicate that future droughts will be warmer even if they are no drier - we have already started to see this happening," said Nicholls. "The warmer conditions will presumably lead to a greater demand for water."
A drier Australia is also the picture painted in a new study by Queensland climatologists who analysed peat samples from North Stradbroke Island off the Queensland coast. The peat samples contained layers of dust blown from locations around Australia, allowing the researchers to reconstruct weather patterns over the past 40,000 years.
"People talk about Australia being in the worst drought in 100 years," said Hamish McGowan, who led the research at the University of Queensland. "But what the evidence is showing us is that in the last 5,000 years South-East Queensland has been much drier than at present."
He also warned that the research shows the Australian climate is not isolated, and in the past was influenced by events far away, such as in the North Atlantic Ocean. "It is likely that should similar events occur due to global warming and melting of the Greenland and Antarctic ice sheets, then our weather in Queensland would be affected," he said.
According to McGowan, the most surprising finding was that rather than climate changes taking place over centuries, the peat samples showed that rapid environmental changes could take place in as little as 30 to 60 years.
