Drought alert as El Nino works up double whammy
Asa Wahlquist, Rural writer | July 07, 2009
EASTERN Australia is increasingly likely to be hit by the drought-inducing double whammy of an El Nino and a positive Indian Ocean Dipole.
The Bureau of Meteorology is putting the odds of an El Nino at more than 50 per cent.
Bureau meteorologist Andrew Watkins said there were several signs of a looming El Nino in the Pacific Ocean. “We have trade winds that are weaker than normal,” he said.
“We have warmer than normal sea-surface temperatures; they are about one degree warmer than normal in the equatorial Pacific. The water below the surface of the ocean is up to three degrees, or even a bit more, above normal. The SOI (southern oscillation index) has been negative, as well.”
However, there is not the cloudiness along the equatorial Pacific usually associated with El Nino events. Nor are the ocean and atmospheric systems reinforcing each other, which happens in an El Nino.
“The climate models are pretty much in agreement there will be a continued warming,” Dr Watkins said. “Most of them are saying there will be an El Nino later in the year.” El Nino events are linked with reduced rainfall in eastern, northern and parts of southern Australia in the second half of the year, and higher daytime temperatures. The last El Nino events occurred in 2002 and 2006, when, according to the bureau, “rainfall deficiencies were widespread and severe”.
There is also growing interest in the influence the Indian Ocean has on Australia’s rainfall.
The Indian Ocean Dipole, like El Nino, is a coupled ocean-and-atmosphere phenomenon. When it is in its positive phase, the Indian Ocean is cooler near Australia. Dr Watkins said scientists believed that a positive IOD reduced rainfall over southeastern Australia.
CSIRO scientist James Risbey said that although most of the models pointed to a looming El Nino, there was less consistency about the IOD.
“The models may be leaning towards a positive IOD, but some of them have flip-flopped, which lowers our confidence,” he said.
“And when we look at the observations, it is not classically set up for a positive IOD. We don’t see cold water where we expect it.”
Some scientists thought the Indian Ocean was “in part a slave to the Pacific Ocean”, Dr Risbey said. “They believe the IOD is just a reflection of what the Pacific Ocean does to the Indian Ocean.”
There have been 11 positive-IOD events since 1958. Seven of them coincided with El Ninos in 1963, 1972, 1977, 1982, 1994, 1997 and 2006. Dr Risbey said ocean temperature was critical to forecasting because it changed slowly: “Once you get an anomaly in the ocean, it tends to persist for some months.”
In El Nino years, as was happening this year, Dr Risbey said a big pool of warm water came to the surface in the Pacific Ocean. That not only led to changes in rainfall patterns, it also pushed up global temperatures. “There is a chance we could set a record for global mean temperatures this year with the El Nino,” he said.