It seems that every second Tuesday when Australia’s Bureau of Meteorology issues its fortnightly climate driver update, people are waiting for El Niño to be rubber-stamped.
While the BOM is confident a declaration of the climate phenomenon known for reducing rainfall across the south-east of Australia will be made, a few things need to fall into place.
First – a sustained low in the Southern Oscillation Index. This index is a calculation of the difference in air pressure in Darwin and Tahiti. Sustained levels below -7 on the index generally indicate El Niño is underway. It was -18.5 in May but flipped positive (+0.2) in June and dropped to -4.3 in July.
Factors like sea surface temperatures, trade wind strength and Pacific clouds are also monitored. SSTs have cleared thresholds required for El Niño, but the other two factors haven’t.
Still, the BOM is confident Australia will be in the grip of an El Niño towards the end of the year.
But these Pacific events are not the only climate phenomena that Australians should pay attention to.
The Indian Ocean Dipole (IOD) is a similar climate system that plays out on the other side of Australia. Like the El Niño-Southern Oscillation (ENSO), the IOD influences rainfall and temperature patterns on Australia too.
“The IOD is defined by the difference in sea surface temperatures between the eastern and western tropical Indian Ocean,” says Dr Milton Speer, a meteorologist at the University of Technology Sydney.
“A negative phase, when temperatures are warmer in the east, near Australia, typically sees above average winter-spring rainfall in Australia, while a positive phase with colder sea surface temperatures near Australia – which is now occurring – brings drier than average winter-spring rain across the southern part of the continent.”
During a positive IOD, it’s typical for only the far west and east Australian coastlines to avoid lower-than-average rainfall during winter and spring. In a neutral IOD, winds push warm water towards the northwestern part of the Australian continent. But as Speer points out, the positive IOD results in this water being dragged towards Africa as westerly winds reduce.
With less atmospheric moisture above Australia, much of the country could expect to see less rain and much warmer conditions than usual – only Australia’s top end and coastline north of Byron Bay tend to experience normal winter-spring temperatures.
Right now, the IOD is at +1.05°C – well above the +0.4°C threshold to push within positive territory. Taken together, one could expect a positive IOD and a to-be-confirmed El Niño to bring warm conditions and little rain, as occurred in 1982 when the southeast part of the nation experienced its driest year since federation.
“We should expect more extensive effects if both of those are in play,” Speer says.
‘Compound’ events aren’t always the same
“It has varied from event to event,” says Zhi-Weng Chua, a senior climatologist at the Bureau.
Compound years where both a positive IOD and El Niño have occurred, are few and far between. Most recently, confirmed dual events fell in 2015, 2006, 1997, 1994, 1972 and 1963. While logic suggests two drying phenomena at the same time would result in big rainfall reductions, joint IOD-El Niño years don’t always play out this way. Chua points to the most recent compound event – 2015.
“In 2015, we did see drier than average conditions over parts of the country,” he says.
“But then, in 1997… there wasn’t as much widespread dry conditions.”
Throw 2006 into the mix and you can see how rainfall can vary substantially during winter and spring months:
To top it off, 2019 – historically Australia’s driest year recorded – fell during a very positive IOD, but not during a declared El Niño:
It highlights the complex and unpredictable effects of climate systems affecting Australia. In the case of 2019, a rare stratospheric warming event – caused by a sudden weakening in polar winds in the high atmosphere above Antarctica – exacerbated the effects of the positive IOD. “A stratospheric warming event enhances the effects of El Niño [and] a positive IOD,” says Speer.
For its part, the BOM is also yet to formally declare a positive IOD is underway, but expects to do so.
“We do rely on looking at a sustained signal from either driver,” says Chua.
“With the Indian Ocean Dipole, the length of which we need to see sustained changes tends to be a bit shorter than El Niño, because the Indian Ocean Dipole tends to be a shorter event as well.
“What I think the models are saying now is that a compound event would be likely since both events are being forecast or modelled that way. In the compound event, we see most of the eastern two-thirds of Australia being below average rainfall because it kind of combined three different reasons.”