Matthew Ward Agius
Matthew Agius is a science writer for Cosmos Magazine.
After months of waiting for key climate factors to align, Australia’s Bureau of Meteorology has declared two climate phenomena – El Niño and a positive Indian Ocean Dipole (IOD) – are underway and will influence the nation’s temperatures and rainfall across the rest of 2023.
It’s the agency’s first El Niño declaration since 2015. The last positive IOD event occurred in 2019.
“Today we’re declaring an event,” said Dr Karl Braganza, the BOM’s climate services manager.
Despite other weather bureaux declaring the climate phenomenon was in effect months ago, the BOM waited for the alignment of at least three of four climate criteria, which include:
- A clear 3-6 month sea surface warming trend was observed in key regions of the Pacific Ocean.
- Weaker-than-average trade winds blowing across the west and central Pacific for 2 in the last 3 months.
- A sustained strongly negative Southern Oscillation Index (SOI).
- Climate models show sustained warming above 0.8°C in spring.
Until today, only the sea surface temperatures and climate modelling had met the BOM’s El Niño criteria, with the sustained decline in the SOI only being registered in the last week. The IOD has been trending positive for many weeks.
“We have been waiting for the atmosphere to ‘couple’ with the ocean,” Braganza said of the BOM’s El Niño monitoring.
“The oceans have been in an El Niño pattern for a couple of months. In the last two weeks, we have seen the atmosphere over the tropical Pacific respond to that pattern and lock in a coupling of the ocean atmosphere.
“That’s the sort of thing that sustains an El Niño event out until autumn.”
Only recently had the 60-day average of the SOI dropped beneath the threshold to meet the BOM’s requirement for an El Niño event. The SOI indexes differences in surface air pressures taken between Tahiti and Darwin. This is the so-called ‘atmospheric’ component of El Niño, which BOM climatologists were awaiting to couple to the other marine criteria.
El Niño and a positive IOD: What Australia can expect
A compound El Niño and positive IOD event can produce a distinct pattern of climate effects, but the double-whammy declaration does not necessarily result in identical temperature and rainfall patterns.
The most recent compound events include 2015, 2006 and 1997.
In general, southern Australia can expect warmer temperatures than average in spring and summer. Australia’s hottest year – 2019 coincided with a positive IOD. The previous record in 2015 occurred during a compound event.
Reduced rainfall is typically experienced during El Niño and a positive IOD, as warm ocean waters shift away from Australia. Cooler oceans near Australia mean reduced evaporation and condensation of seawater in the atmosphere falls as rain over land. This will be a big change for parts of Australia’s eastern seaboard in particular, which recently experienced an uncommon ‘triple La Niña’ where warmer waters push against the coast and increase rainfall.
A positive IOD typically drives down spring rainfall across Australia, though this is not always uniform: during the 2012 event, the east of the continent saw below-average rainfall, while WA’s rainfall was above expectation.
Together, warmer temperatures and reduced rainfall can prove challenging for the southern part of the continent, with already hot and dry summer conditions amplified by these events.
The BOM forecasts more extreme temperatures – including single days and heatwaves – overnight frosts and heightened fire dangers across New South Wales, the ACT, Victoria and South Australia are expected.
“The strength of an El Nino by those measures doesn’t necessarily correspond to the severity of the rainfall deficiencies over Australia,” Braganza said.
“We have had weak events that have caused quite significant drought. And we’ve had strong events that haven’t caused such severe conditions in the past.”
Natural events amid an unnatural background
“This year’s El Niño is developing during some of the warmest global average temperatures in history, meaning that scientists are still learning how El Niño and its impacts are changing as this event unfolds in a warmer world,” says CSIRO senior research scientist, Dr Nandini Ramesh.
El Niño and positive IOD events form part of natural climactic cycles in the ocean and atmosphere near Australia.
However in the background to these events is the hidden influence of climate change.
Some research has suggested global warming could shift the swings between El Niño and La Niña cycles, while the overall increase in temperatures due to anthropomorphic greenhouse gas emissions combined with rainfall-lowering natural events means Australians should expect a hot end to the year.
“Eastern Australia is already experiencing some of the impacts that an El Niño and positive IOD event can bring,” says Dr Linden Ashcroft, a climate lecturer at the University of Melbourne.
“These climate drivers — on top of background warming due to climate change — means we need to prepare for a hot and dry end to 2023.”
Although soil moisture and high reservoir levels will potentially buffer against catastrophic fire amid hotter and drier conditions, fire experts say a compound event declaration should prompt bushfire readiness.
“We can take time now, before the busy summer season, and prepare our homes and properties,” says Professor Delene Weber, a bushfire researcher at the University of South Australia.
“We can take time to make sure our Bushfire Survival Plans are up to date, and that everyone in the house is aware of the plan and knows what they can do if they are together or alone in the home if an incident occurs.”
