Where El Niño or La Niña forms in the Pacific is crucial to Australian impacts

When it comes to rainfall impacts on Australia, not all El Niño events affect us in the same way. Some leave large parts of eastern Australia dry, while others have a stronger influence on the Northern Territory. Scientists have recognised these different types of El Niño for years, but understanding exactly why they produce distinctly different rainfall patterns has remained a challenge.

Researchers from 21st Century Weather, based at UNSW and Monash University, have now traced those differences to changes in the large-scale atmospheric circulation across the Southern Hemisphere, helping explain how different types of El Niño and La Niña shape Australia’s spring rainfall. 

The team analysed rainfall observations and atmospheric data from 1950 to 2024, to compare El Niño and La Niña events that develop in either the eastern or central Pacific Ocean. They found that Eastern Pacific events tend to produce localised rainfall changes, with significant effects in the Northern Territory, the Murray-Darling Basin and southwest Western Australia. Central Pacific events, by comparison, influence rainfall across a much broader area of northern and eastern Australia. 

The difference comes down to how each type of ENSO changes the movement of air and moisture through the atmosphere.

Eastern Pacific El Niño events influence temperatures in the Indian Ocean, which in turn helps to establish a large, high-pressure system south of Australia. This changes wind patterns in ways that reduce rainfall across parts of southern Australia. During a Central Pacific El Niño, the Indian Ocean plays a much smaller role. Instead, changes in the tropical Pacific shift the position of a large low-pressure system closer to Australia’s east coast, reducing the flow of moisture onto the continent and producing drier conditions across much of northern and eastern Australia. 

The researchers also found that not all La Niña events bring the same rainfall response, with the impacts of two La Niña types not simply mirroring their El Niño counterparts. Central Pacific La Niña events produced more consistent and widespread increases in rainfall than Eastern Pacific events, reinforcing the idea that the location of warming or cooling in the Pacific Ocean matters for Australia’s climate. 

Associate Professor Andrea Taschetto of UNSW, a Chief Investigator with 21st Century Weather and the paper’s senior author, said: “It’s interesting to consider the relationship between phases of ENSO and rainfall in the major Australian cities. 

“Spring rainfall in Brisbane, Canberra, Melbourne, Adelaide and Hobart have a consistent relationship with Central Pacific ENSO. Perth spring rainfall is influenced more consistently by Eastern Pacific El Niño, due to the indirect influence of El Niño on sea surface temperatures in the Indian Ocean.”

The findings provide a better physical explanation for ENSO-related Australian rainfall patterns that have been observed for many years. By showing how different tropical ocean conditions shape large-scale atmospheric circulation, the study offers a process-based foundation for evaluating climate models and predicting Australian seasonal rainfall during ENSO events.

The lead author, 21st Century Weather Research Fellow Dr Linyuan Sun, said: “Seasonal forecasts don’t just depend on whether there’s an El Niño or La Niña. They also depend on what type of event develops. By understanding the atmospheric processes behind these different ENSO patterns, we can better interpret what they might mean for rainfall across Australia.”

The research has been published in the American Meteorological Society’s Journal of Climate.