Putting a number on it – quantifying ENSO’s impacts on rainfall 

March 21, 2024

Shayne McGregor is the Associate Professor, School of Earth Atmosphere and Environment, Monash University. Together with scientists from Extreme climate: dry, wet, hot-and-dry he is working to better understand Australia’s climate variability.


Most Aussies know that El Niño affects Australia’s rainfall (Figure 1). But with many other factors at play in Australia’s climate, we can’t always quantify by how much. The subtleties of how El Niño, and its counterpart, La Niña, change Australian rainfall can be lost in translation with media reporting predictions of extreme wet or dryness based on El Niño or La Niña forecasts alone. Our new study, published in the March edition of Geophysical Research Letters (Volume 51, Issue 6), seeks to clarify the impact the swings of the El Niño-Southern Oscillation (ENSO), El Niño and La Niña, have on the risk of observing months of high or low rainfall.  

Figure 1: El Niño is linked to increased likelihood of dry conditions across Australia.  In our work we looked at Southeast Australia, East Australia and the Murry Darling Basin.

How does ENSO change our rainfall risk?
We adapted the Fraction of Attributable Risk framework, which is commonly used to understand the impact of human induced climate change on observed extreme events. 

Using this technique, we found that looking at rainfall changes with magnitudes of 20mm/month or larger, El Niño (-20mm/month) and La Niña (+20mm/month) had the following impact on rainfall: 

  East Australia  Southeast Australia  Murry Darling Basin 
El Niño  70% of dry conditions can be attributed to El Niño events.   58% of dry conditions can be attributed to El Niño events.   29% of dry conditions can be attributed to El Niño events. 
La Niña   78% of wet conditions can be attributed to La Niña events.  37% of wet conditions can be attributed to La Niña events.   77% of wet conditions can be attributed to La Niña events. 

On top of these findings, our results also suggested that recent La Niña years are significantly wetter, a result which is consistent with the expected impact of climate change. 


So, what does this mean and how can this information be used? 

This information can be used in two main ways. Firstly, it can be used to understand how the risk of extreme rainfall of a particular magnitude event may change during a El Niño or La Niña event. 

Let’s consider the hypothetical scenario where media are reporting that we are nearing La Niña event thresholds this coming winter (this hypothetical is possible, but certainly not guaranteed).  

The average person will likely think back to last La Niña they experienced in 2022 and think that they will likely experience something similar again. Well, this method allows us to figure out the likelihood of that actually happening.  

In September of 2022, eastern Australia experienced anomalous rainfall of ~30mm/month, so a far bit wetter than normal. Now, our work suggests that a rainfall anomaly of this magnitude or greater is more than 7 times more likely to occur during a La Niña event than they are to occur during neutral conditions, somewhat confirming our expectations. But our work also highlights that other outcomes are possible.  

For instance, it is still possible to experience less rainfall than normal, however this is much less (specifically 55%) likely to occur than they are during neutral conditions.  

Secondly, this method can be used after an even attribute the change in eastern Australian rainfall to the La Niña event that occurred. We can again use September 2022 as an example here. The FAR method suggests that approximately 86% of events of that magnitude (~30mm/month) can be attributed to La Niña events.

Read more in the full article here.

Hub project Extreme climate: dry, wet, hot-and-dry is working to better understand extreme rainfall and drought. By understanding the impact ENSO has on these events we can better prepare in the future. 

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