Zoe Gillett: Research Associate at the Climate Change Research Centre and ARC Centre of Excellence for Climate Extremes, University of New South Wales. She is collaborating with project 2.8 Extreme climate: dry, wet, hot-and-dry to better understand Australia’s climate variability.
Ashley Huang: Research Officer at the Research School of Earth Sciences, Australian National University, Canberra, Australia.
Andréa Taschetto: Associate Professor at the Climate Change Research Centre and ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, Australia. She is working with scientists from Extreme climate: dry, wet, hot-and-dry to better understand Australia’s climate variability.
Figure 1: The El Niño-Southern Oscillation (ENSO) drives rainfall over eastern Australia.
Our research looks at how climate drivers impact Australia’s rainfall. In the latest edition of Geophysical Research Letters, we explored the impacts of single and multi-year El Niño-Southern Oscillation (ENSO) events on Australian rainfall.
Multi-year La Niña events are overall wetter than single-year ones
This work looked at how repeated La Niña events increase eastern Australia’s rainfall.
The recent 2020-2023 ‘triple-dip’ La Niña was associated with record-breaking rainfall and flooding across eastern Australia. Continuous rainfall during a multi-year La Niña event can increase climate risk as increased rain falls on already saturated catchments.
How often do multi-year ENSO events occur?
It is not uncommon for multi-year La Niña events to happen. In fact, half of all La Niña events are multi-year events, that is, La Niña conditions occur over 2 or more consecutive summers back to back. About 30% of all La Niña events between 1900 and 2023 were double-dip events and about 20% were triple-dip events.
Multi-year events are less common for El Niño events. Double El Niño events occur in about 30% of all El Niño events. We only found one rare case of a triple El Niño event since 1900, which happened in 1939-1941 (Figure 2).
Figure 2: The Oceanic Niño Index shows El Niño events in red and La Niña events in blue between 1900 and 2023.
How does Australian rainfall change during single, double and triple La Niña events?
There is an asymmetric relationship between ENSO and Australian rainfall, meaning that La Niña events generally promote a larger rainfall response than El Niño. This means La Niña events are wetter than El Niño are dry. We show that this non-linear relationship persists over multi-year events (Figure 3).
1. There is no difference in single or double El Niño events
There is no significant change in rainfall impacts during single or double El Niño events. In general, the drying impact associated with El Niño is also weaker than the wetting impact during La Niña.
2. But the third year can affect rainfall
We found that rainfall tends to progressively increase throughout a triple-dip La Niña event. The highest rainfall occurred in the third spring-to-summer season in four out of five recorded triple-dip La Niña events (rightmost column of Figure 2). This suggests that multi-year La Niña events have a stronger impact on east Australian rainfall over time.
3. Local processes at play
This rainfall increase in the third La Niña doesn’t correlate with stronger ocean cooling in the tropical Pacific Ocean. Instead, other local processes, like a feedback loop between high soil moisture levels and rainfall, seem to exacerbate the effects of multi-year La Niña events in Australia.
Figure 3: Australia spring to summer rainfall anomalies (mm month-1) in the first and second years of El Niño events and the first, second and third years of La Niña events.
Understanding the impacts of multi-year ENSO events on Australian rainfall can have important implications for improving seasonal forecasting and understanding rainfall predictions and extremes. Our findings provide an important message for managing timely responses to possible record-breaking rainfall and increased flood risk during future triple-dip La Niña events.