Written by Laure Poncet
The recent 2020-2023 triple-dip La Niña brought record-breaking rainfall and flooding across Eastern Australia, resulting in billions of dollars worth of damage.
Triple-dip La Niña events – where La Niña occurs successively over three consecutive years – can be particularly impactful, as persistent wet conditions increase the risk of flooding. Understanding the rainfall patterns of such multi-year events is vital for limiting damage.
In a recent study, researchers from the ARC Centre of Excellence for Climate Extremes investigated the evolution of rainfall in Australia during the recent triple-dip La Niña.
They show that rainfall increased in the third year of the event compared to the first and second years. Surprisingly, the increase in rainfall occurred despite no strengthening of La Niña over that period. So, what exactly happened?
La Nina’s impact on Australian weather
La Niña is part of the El Niño Southern Oscillation (ENSO), a natural phenomenon associated with changes in sea surface temperatures in the central and eastern tropical Pacific.
During la Niña, temperatures in those regions are cooler than average, while in an El Niño, they’re warmer than average.
This variability can shift weather patterns, bringing droughts to some regions and floods to others.
In Australia, La Niña is typically associated with wet conditions in winter and spring, while El Niño increases the chance of dry and warm weather in southern and eastern Australia, particularly in spring.
La Niña events can happen in a row
It is not uncommon for two La Niña events to occur successively, what we call a double-dip La Nina. Triple-dip La Niña events are less common.
In fact, half of La Niña events that happened between 1900 and 2023 are multi-years events – 30% are double-dip and about 20% are triple-dip events.
Multi-year La Niña events can be associated with an increased risk of flooding, especially when rain falls on already saturated catchments.
Multi-year El Niño events are less common: about 30% of all El Niño events that occurred between 1900 and 2023 were double-dip, while only one triple-dip event occurred (1939-1941).
Figure 1: The Oceanic Niño Index that demonstrated the occurrence of El Niño and La Niña events between 1900 and 2023. The red colour represents El Niño events and the blue colour La Niña.
The third year effect
To better understand the impacts of multi-year ENSO events, researchers at the Centre have analysed how rainfall evolved during triple-dip La Niña.
Their findings reveal that rainfall progressively increased throughout the event, with the highest rainfall occurring in the austral summer of the third year.
The same pattern was observed in four of the five previous triple-dip La Niña, suggesting that rainfall impacts are stronger in the later years of a multi-year La Niña event.
Interestingly, the increase in rainfall observed during those events occurred despite no strengthening of the La Niña signal in the tropical Pacific. Instead, other local processes may have played a role in modulating the rainfall response.
One plausible mechanism is a feedback loop, where soil moisture acts to intensify rainfall. How would this have worked?
During the events, prolonged rainfall increased soil moisture. Wet soils, in turn, released that moisture to the atmosphere through evaporation, boosting humidity levels and leading to more rainfall. More rainfall further increased soil moisture, perpetuating the cycle.
In the third year of the events, this feedback loop occuring in the austral spring may have led to the observed increase in rainfall in the austral summer.
Figure 2: Australia spring to summer rainfall anomalies (mm month-1) in the first, second and third years of La Niña events. Rainfall increases in the third year of La Niña.
What’s next?
Understanding the impacts of multi-year La Niña events on Australian rainfall is crucial for improving seasonal forecasting and informing stakeholders and decision-makers on rainfall predictions and extremes.
These study’s findings provide an important message for managing timely responses to possible heavy rainfall and increased flood risk during future triple-dip La Niña events.
While the study focuses on a limited sample size of events, it lays a foundation for future research. Further work could build upon these findings and provide deeper insights into the dynamics of multi-year ENSO events using the outputs of multiple climate models.