along the western coast of Australia— enhancing the transport of warm tropical waters along the WA coast( Figure 4). Local meteorology also likely played a role, with reduced cloud cover off WA exposing the ocean to increased solar radiation. Reduced outgoing longwave radiation and suppressed latent heat loss may also have sustained the marine heatwave( Figure 4).

Although these conditions may have acted independently or together, other triggers are possible. For example, the positive Indian Ocean Dipole onset in September 2023 also increased the likelihood of this marine heatwave. The exact mechanisms, including why it developed in September and lasted through the 2025 summer, require further study.

Grampians( Gariwerd) bushfires

In December 2024 and early 2025, significant bushfires broke through the Grampians( Gariwerd) National Park, destroying homes and killing many native animals and plants as well as hundreds of livestock.

The fires were ignited by dry lightning on 16 December and burned over 76,000 hectares before being contained after three weeks of great efforts from fire agencies, including over Christmas and the New Year period. However, new blazes were sparked by additional dry lightning in late January. These fires took another two weeks to contain and burn significant areas, including one section of over 59,000 hectares.

The fires were further exacerbated by hot and dry conditions in the region— with temperatures exceeding 40 ° C in some areas during the fire period— and gusty winds associated with the passage of cold fronts on 20 December and 4 February.

Some weather conditions during the fires were extremely dangerous, with fire-generated thunderstorms— known as pyrocumulonimbus or cumulonimbus flamagenitus clouds— forming on 20 December 2024 and 4 February 2025. These appear as smoke plumes in Figure 5. Pyrocumulonimbus clouds form when the heat from fires causes air to rise quickly. As the air rises, water vapor condenses, forming tall clouds. Pyrocumulonimbus clouds can be extremely hazardous, with strong, erratic winds pushing fires in unexpected directions and ash carried in updrafts igniting new‘ spot fires’. In rare cases, pyrocumulonimbus clouds can also generate tornadoes or lightning, with the potential for lightning to start new fires.

The occurrence of pyrocumulonimbus clouds is often associated with strong dry winds near the surface, dry vegetation available to burn and large temperature lapse rates. These factors can be estimated by indices such as the Forest Fire Danger Index( FFDI) and Continuous Haines Index( C-Haines). As shown in Figure 6, the December case had more conducive conditions for the occurrence of pyrocumulonimbus than the February case. Although both FFDI and C-Haines can provide useful insights, these indices are not perfect and should be interpreted accordingly.

There has been a long-term trend towards more dangerous weather conditions for fires in the Grampians region. The FFDI values have increased significantly since 1950 due to more frequent hot and dry conditions attributable at least in part to human-caused climate change. Conditions at higher levels of the atmosphere have also become more dangerous for extreme fires in this region, as indicated by increasing C-Haines values. These recent trends are projected to continue and even worsen

Figure 4: Monthly wind vector anomalies and wind speed anomalies at 1000mb pressure( top) and outgoing longwave radiation anomalies( bottom) for December 2024 and January 2025, relative to the 1991 – 2020 baseline. Source: Wind anomalies: IRI / LDEO Climate Data Library; Outgoing longwave radiation anomalies: NOAA PSL