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Positive Indian Ocean Dipole fuels bushfires devastating Australia and deadly flooding in Africa

By Irena Ceranic, Wednesday December 11, 2019 - 17:46 EDT
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A hot and parched Australian landscape is fuelling devastating bushfires on the east coast. - ABC

A climatic phenomenon of near-record intensity has contributed to two extremes on opposite sides of the Indian Ocean — torrential rain in East Africa and exceptionally dry conditions fuelling bushfires in Australia.

The bushfire season began much earlier than usual in eastern Australia and has been one of the most devastating on record, based on the area burnt.

Australia's most populous city of Sydney has been , its air quality exceeding hazardous levels on several occasions.

But the parched and hazy landscape in Australia is in stark contrast to , where severe floods, mudslides and landslides have killed nearly 300 people.

The common link is a strong positive Indian Ocean Dipole (IOD), the counterpart to the El Nino weather system that develops in the Pacific Ocean.

The negative effect of a positive IOD

The IOD measures differences in sea surface temperatures between the western and eastern parts of the Indian Ocean.

A positive IOD occurs when waters near the Horn of Africa are warmer than average leading to enhanced rainfall there, while cooler waters develop off Indonesia resulting in less rainfall and high temperatures in Australia.

The Bureau of Meteorology's head of long-range forecasting, Andrew Watkins, said the IOD event peaked in mid-October when the waters around east Africa were about 2 degrees Celsius warmer than those near Australia.

"It's the strongest we've had since 1997 and it's comparable to only three events that we've seen in the past — in 1997, 1994 and 1961," he said.

Dr Watkins said it played a key role in .

"In Australia it has contributed to the very dry and warm conditions we've seen over winter and spring … and that has unfortunately set us up for a bad fire season in parts of eastern Australia," he said.

Dr Watkins said it had also played a role in Perth posting its hottest ever start to summer, and with , it is likely other records will also tumble.

"It does appear to have some impact on southern parts of WA, so you do tend to see it being a bit drier and warmer during IOD events … December so far has been very hot."

Meanwhile, rainfall across the greater Horn of Africa was up to 300 per cent above average from October to mid-November, according to the Famine Early Warning Systems Network.

"The comparable flooding that we've seen in the past in parts of eastern Africa was in 1997 during the positive IOD event," Dr Watkins said.

"It is their wet season, so they do expect to have more rain than normal … but this year has really taken the cake.

"They've also had quite a number of tropical cyclones. You don't normally see that many tropical cyclones in the north-west of the Indian Ocean, typically you might only get one in a season, but we've seen about five in that area — although not all of them have affected eastern Africa."

When will it end?

IOD values have gradually weakened since their peak of 2.1C in mid-October and are down to 0.9C, but that is still well above the positive IOD threshold value of 0.4C.

"Normally a positive Indian Ocean Dipole would die off at the end of November or start of December as the monsoon moves into the southern hemisphere," Dr Watkins said.

"But the monsoon is really taking its time, it's pretty sluggish moving into the southern hemisphere and hence we're waiting to see it move a little further before we become more confident of when the IOD may end.

"But it could be the end of December or even early January before it goes away, so possibly a month later than normal."

Forecasters are also closely monitoring another climate driver, the negative Southern Annular Mode (SAM).

"That basically refers to our weather patterns being further north than normal, and it means that there have been lots of westerly winds blowing the hot and dry air from the interior across New South Wales and southern Queensland," Dr Watkins said.

"So we've had the drying from the IOD and then we've had the hot and dry winds from the SAM bringing about the terrible conditions we've seen over eastern Australia."

suggests the negative SAM may influence the first half of January before returning to neutral levels.

It is predicted that a drier-than-average January to March is likely for eastern Australia, but that the period will be wetter for parts of Western Australia.

But the drier-than-usual pattern in the east is likely to ease as we progress into summer, when the positive IOD and negative SAM patterns break down.



© ABC 2019

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