After dominating the last three consecutive years, La Niña weakened in the tropical Pacific Ocean at the beginning of 2023 and officially ended in March. Neutral conditions returned; however, there were early signs that an El Niño could form later in the year. El Niño is characterised by unusually warm ocean temperatures in the central and eastern equatorial Pacific Ocean. An El Niño was announced at the beginning of July by the World Meteorological Organization after sea surface temperatures in the tropical Pacific warmed and exceeded El Niño thresholds.

By mid-September, the onset of El Niño was declared by the Australian Bureau of Meteorology (BoM) (Figure 1). The BoM uses different thresholds and metrics to define an El Niño, including the atmospheric components of wind and pressure. El Niño is typically associated with drier and warmer conditions over much of Australia in spring and early summer, and increases the risk of heatwaves, droughts, and bushfires.

Figure 1: (top) Map showing the sea surface temperature anomalies in October 2023 relative to 1971-2000. Dark orange shows higher than normal sea surface temperature while dark blue shows lower than normal. The map shows El Niño conditions with warmer sea surface temperatures in the central-eastern Pacific and positive IOD with colder sea surface temperatures around northern Australia. Source: National Oceanic and Atmospheric Administration (NOAA). (bottom) Timeline of the El Niño-Southern Oscillation and IOD in 2023.

The positive Indian Ocean Dipole (IOD), a phenomenon similar to El Niño but located in the tropical Indian Ocean, was announced during this time. The positive IOD gained strength quickly and became the second strongest positive IOD event since BoM began recording IOD events in 2001, approaching the record-breaking positive IOD event of 2019. A positive IOD typically promotes below-average rain and increased daytime temperatures over central and south-eastern Australia during winter and spring.

The combined influence of El Niño and the positive IOD is associated with a higher chance of a warm and dry Australia, particularly during spring. Spring 2023 was Australia’s fifth warmest spring on record since observations began in 1900. Overall, Australia’s rainfall was below average in spring 2023. September was the driest on record, and October was the fifth driest on record. This is in clear contrast to spring 2022, which was Australia’s second wettest on record. However, late spring thunderstorms and showers meant that Australia’s November rainfall was above-average. This is an important reminder: El Niño and the positive IOD do not guarantee that eastern Australia will be hot and dry, rather it increases the likelihood of hotter and drier conditions.

In addition to the impacts of these climate drivers, the impact of global warming is also notable. Australia’s climate has warmed by about 1.48 ± 0.23°C since national records began in 1910. There has been a tendency towards more extreme rainfall, especially in northern and eastern Australia, and a reduction in cool-season rainfall in southern Australia.

Given the probability of warm and dry weather is influenced by the natural climate drivers and temperature changes are affected by an increase in greenhouse gas emissions, the increased impact on fire danger metrics was evident across most states and territories as spring turned into summer.

Research contacts:

Dr Hien Bui,
Dr Zoe Gillett,