A northwest cloud band from the Indian Ocean affects rainfall in southern and central Australia. (Supplied: Bureau of Meteorology)
Cool seas off WA’s north-west could kick off a climactic phenomenon that may exacerbate a winter drought across central and southern Australia.
- There are three phases of the Indian Ocean Dipole: positive, negative a neutral
- A positive IOD is developing and that causes lower winter rainfall
- But the threat of an El Nino system impacting the east coast is decreasing
This climate driver is called a positive Indian Ocean Dipole (IOD) and affects rainfall patterns across the country, particularly during the cooler months of the year.
“There are three phases of the IOD being positive, negative and neutral,” Bureau of Meteorology (BOM) spokesman Neil Bennett said.
“And the phase of the IOD can have an impact, either good or bad, on rainfall through the Australian region during the winter months.”
The IOD is currently neutral, but five of the six forecast models which the BOM uses to predict the weather suggest a positive IOD event is likely to develop in winter and persist into mid-spring.
The negatives of a positive IOD
A positive phase, ironically, tends to have a negative impact on winter rainfall.
A positive phase of the Indian Ocean Dipole can lead to reduced rainfall across Australia.
(Supplied: Bureau of Meteorology)
It occurs when sea surface temperatures in the Indian Ocean near Africa’s east coast are warmer than usual, while waters to the north-west of Australia are relatively cooler.
That reduces the amount of moisture available in that region to be dragged across Australia by rain-bearing clouds associated with cold fronts and low pressure systems.
The opposite happens with a negative IOD — cooler water near Africa and warmer water on Australia’s side of the Indian Ocean can enhance winter and spring rainfall in parts of the country.
A negative phase of the IOD can enhance rainfall across Australia.
(Supplied: Bureau of Meteorology)
“The stream of moisture coming down from the north-west into eastern Australia, across from the Indian Ocean, is a critical element in the production of rainfall,” Mr Bennett said.
“In WA, a good rainfall event is one where we have a cold front moving up from the Southern Ocean interacting with a feed of moisture coming down from the Indian Ocean and producing widespread cloud and heavy rainfall.
“If you have a positive Indian Ocean Dipole and you have cooler than normal ocean temperatures, then the amount of moisture available is reduced compared to if you have above average temperatures as you would get in a negative IOD.”
El Nino threat decreases for east coast
The BOM’s outlook for June through to August appears grim for the already parched agricultural areas across Australia.
It shows winter is likely to be drier than average for much of Queensland, New South Wales, Victoria, south-eastern parts of South Australia, northern parts of Tasmania, the Northern Territory, and northern and far south-western parts of WA.
The BOM’s winter outlook shows large parts of Australia are likely to be drier than average. (Supplied: Bureau of Meteorology)
But while the outlook suggests an increased chance of a positive IOD event forming during winter, it also shows a gradual weakening of the El Nino-like pattern in the tropical Pacific Ocean, which typically brings drier than average conditions for eastern Australia.
The Bureau has slashed the chances of El Nino occurring from 70 per cent (an alert level) to 50 per cent (a watch level), which is still double the normal likelihood.
Mr Bennett said unlike El Nino, the IOD tended to have a widespread impact across Australia.
“With an El Nino being over in the Pacific Ocean, there’s less of an impact in Western Australia than the eastern states, whereas the IOD can have an impact right across the whole of southern and central Australia,” he said.
The IOD usually impacts Australia from May or June onwards, peaking in about July or August, before decreasing from October as the southern hemisphere monsoon kicks in.
But a positive IOD does not guarantee dry weather.
Just like El Nino and La Nina in the Pacific Ocean, the IOD is a background influence on the weather and individual heavy downpours can still occur.
When the Indian and Pacific oceans conspire
Australians can count their blessings that they aren’t dealing with both an El Nino and a positive IOD at the same time.
A positive IOD and El Nino can work among other climate drivers to complement each other. (ABC Weather: Kate Doyle)
When an El Nino event coincides with a positive IOD, the two phenomena can have a dire consequences on the climate by reinforcing their dry impacts.
Likewise, when La Nina — which usually brings wetter and cooler conditions — coincides with a negative IOD, there’s a greater chance of above average winter and spring rainfall.
“All of these things are working together, so a positive or negative IOD on its own is not going to be the full story,” Mr Bennett said.