Indian Ocean, not El Nino, may be cause of Aussie drought

SYDNEY: An Indian Ocean weather system may be causing, and prolonging, severe droughts in southeast Australia, according to a new study.

Scientists from the University of New South Wales (UNSW), in Sydney, have found evidence that El Niño, which is associated with bringing warm and dry conditions, and La Niña, with bringing cooler, wetter conditions, actually have little influence on the ongoing drought.

The current drought, which began in 1992, is the longest and more severe known since records began in the 19th century. The same study has linked the World War II drought (1937-1945) and the Federation Drought (1895-1902) to the weather system, known as the Indian Ocean Dipole.

Dry winds

That system determines if moisture-bearing winds or dry winds are carried across the southern half of Australia, and a continuous period of dry winds could be the cause of our current drought, say the scientists in a study to be published in the journal Geophysical Research Letters.

The Indian Ocean Dipole (IOD), is an oscillating weather system where warm and cool surface waters at either side of the Indian Ocean periodically swap places

Using growth bands of corals, scientists at the Australian National University (ANU) in Canberra constructed a record of climate change and weather conditions dating back to 1846.

They found that tropical weather patterns in the Indian Ocean have become more variable over the last century. This strongly suggests that we are in for even more extreme weather patterns in the future...

Intense and frequent

The coral growth records point to an increase in the intensity and frequency of the Indian Ocean Dipole (IOD), an oscillating weather system similar to the El Niño/El Niña system found in the Pacific Ocean.

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"Grim picture"

In a similar way to how tree rings are used to reconstruct historic climate on land, annual growth bands of corals correspond to environmental factors such as water temperature, salinity and wave action.

The ANU team compared two different verities of oxygen – isotopes known as oxygen-18 and oxygen-16 – that were locked into the bands as the corals grew over time.

The ratio of these isotopes changes with differing ocean temperatures, giving a picture of the water temperature in different ocean regions. Using coral sample from different bands dating back to 1846 allowed the scientists to piece together a detailed climate history for the region.
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The results suggest that "greenhouse warming will lead to strengthened Asian monsoon rainfall and more persistent El Niño-like conditions during the 21st century," said Nerilie Abram, who led the research team. "This means the picture is grim for southern Australia in particular."

Australia and Indonesia could experience more frequent droughts, and East Africa and Southern India more frequent rain, as tropical weather patterns intensify, she said.

"We don't have much information on how these weather patterns change with time. Observations are limited," commented Malcolm McCulloch, an earth scientist from the ANU who was not involved in the study.

"Studies like this enable us to go back in time to get a better feeling for what happened, in order to understand the processes better, and to help make future climate models more accurate," he said.

This cycle involves a negative phase, a positive phase and a neutral position. In the negative phase, the Indian Ocean waters are cool, with warmer waters to the north in the Timor Sea. The westerly winds pick up moisture from the cool water and carry it across the land, so southeast Australia experiences moist, warm air and rainfall.

In the positive phase, the patterns are reversed. Less moisture is picked up from the warmer waters, which brings weak, dry winds and much less rainfall.

Losing streak

The new study reports that the IOD has only seen positive and neutral phases now for over 15 years.

"There hasn't been a single negative phase since 1992, and this is the longest no negative phase event we have on record" said Caroline Ummenhofer, a climate change scientist and lead author of the study. "All you're left with is dry events."

The last three years haven't seen a single neutral phase either, with a record breaking three positive phases in a row.

"If the Indian Ocean Dipole events do follow the trend of seeing more positive events and less negative ones, this is a terrible piece of information for the Murray Darling Basin," said co-author Matthew England. "This Basin is under a lot of stress at the moment with drought, and it needs replenishment with more negative Indian Ocean Dipole events."

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