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Autumn 2025 Sees Highest-Ever Climate Index
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The Australian Actuaries Climate Index (AACI) recorded its highest ever composite index value in the autumn of 2025, driven primarily by sea levels.
The extreme sea level index also reached its highest national value on record, with the east and south coasts playing a key role in this record.
It was the second-highest value ever recorded for the East Coast South region which covers the New South Wales coast, including Sydney, and a record high for Southern Slopes (Victoria), which includes Melbourne and surrounding coastline.
A key contributor to these elevated levels was ex-Tropical Cyclone Alfred, which made landfall off the Queensland coast in early March. The system caused large sea level anomalies (SLA) of up to 0.6 metres above normal, especially along the northern New South Wales and southern Queensland coastlines, as shown in the OceanCurrent heatmap below [1] .
Figure 1: Adjusted SLA off Australia's East Coast on 6 March 2025. Atmospheric isobars are shown in cyan (low pressure) and white (high pressure).
These anomalies are partly due to the cyclone's low atmospheric pressure, which creates a “bulge” in sea level. Lower pressure reduces the weight of the air pressing down on the ocean, allowing water to rise.
Cyclones also influence wave energy. As they pass over the ocean, strong winds transfer energy to the surface, generating waves. In this case, waves up to 8 metres high were observed beneath ex-Tropical Cyclone Alfred. [2] According to the Bureau of Meteorology, swell waves – which are longer-period waves generated by distant systems – can travel vast distances. This helps explain how a cyclone in Queensland may have affected sea levels as far south as New South Wales and Victoria.
Cyclones have previously been linked to peaks in the sea level index. The autumn of 1990 remains the highest sea level index value for the East Coast South cluster, coinciding with ex-Tropical Cyclone Hilda, which passed along the southeast Queensland and northern New South Wales coast.
In addition to cyclone activity, sea surface temperatures were another likely contributor to the high sea level index values in the Southern Slopes (Victoria) cluster. The Bureau of Meteorology reported sea surface temperatures of up to 3°C above average during April and May in parts of Australia's southwest, south and east. [3] Warm ocean waters expand, causing sea levels to rise.
The extreme rainfall index showed positive values across the eastern regions, notably the East Coast South and East Coast North. These areas experienced heavy rainfall and flooding, driven by Cyclone Alfred and other persistent low-pressure troughs.
Queensland recorded its third wettest March since 1900, [4] which was reflected in local water storage systems. The Hinze Dam on the Gold Coast saw water levels surge from 100% to 124% from 6-9 March – a sharp increase not seen since severe storms impacted the region in late 2023. [5]
Note: Dams can be reported at over 100% capacity because "100%" typically refers to the full supply level used for normal water storage. Many dams are designed with additional flood storage above this level, allowing them to temporarily hold excess water during heavy rainfall events.
The extreme high temperature index recorded positive values across all regions, though no records were set.
This contrasts with the Bureau of Meteorology's observation that Victoria experienced its warmest autumn on record. [6] The difference highlights how average temperatures and temperature extremes can tell different stories.
In early July 2025, the Southern Annular Mode (SAM) reached its highest positive value in nearly two years. [7] The SAM is a key climate driver in the Southern Hemisphere, describing the north-south movement of the westerly wind belt that circles Antarctica.
During winter, this wind belt typically shifts northward, exposing Australia to more frequent and stronger low-pressure systems and cold fronts. When the SAM is positive, the westerly winds contract closer to Antarctica. This results in less wind, snow and rain in southern Australia but increased rainfall over the eastern states due to strengthened moisture-laden easterly winds.
Looking ahead, the Bureau of Meteorology predicts above-average rainfall from August to October across much of eastern Australia, alongside warmer-than-average nights nationwide. [8] Meanwhile, the El Niño Southern Oscillation (ENSO) is expected to remain in a neutral state until at least December 2025. [9]
The Index measures the frequency of extreme weather conditions and sea levels across Australia and how these vary over time. Components cover extreme high and low temperatures, extreme rainfall, consecutive dry days, extreme wind and sea level. The Index was launched in November 2018 by the Actuaries Institute and is updated four times a year by Finity Consulting for the Actuaries Institute.
Unlike many other measures, the AACI focuses on changes in the extremes. This is a more relevant metric for the insurance industry (and others) than averages, as it correlates more closely with damage. This is done by measuring how often we observe the current quarter's observations exceeding the 99th percentile of the corresponding observations in the reference period of 1981-2010. More details and full results can be found here .
The purpose of the AACI is to provide a publicly available and objective measure of extreme weather conditions and is one way in which the actuarial profession can contribute to the assessment of climate risk.
Alongside the North American Actuaries Climate Index, [10] it is one of two climate indices produced globally on behalf of actuarial associations and has been referenced extensively in the media and other reports. [11]
References
[1] Pilo, G. S. (2025, March 31). TC Alfred's imprint in the deep ocean. AODN Ocean Current. https://oceancurrent.aodn.org.au/news.php#TC_Alfred_rsquo_s_imprint_in_the_deep_ocean
[2] IMOS-OceanCurrent. (n.d.). Surface waves. Australian Ocean Data Network. https://oceancurrent.aodn.org.au/waves/waves3.php?date=20250301120000
[3] Australian Bureau of Meteorology. (2025, May 6). ENSO wrap-up. Bureau of Meteorology. http://www.bom.gov.au/climate/enso/wrap-up/archive/20250506.archive.shtml
[4] Australian Bureau of Meteorology. (2025, March). Climate summary. Bureau of Meteorology. http://www.bom.gov.au/clim_data/IDCKGC1AR0/202503.summary.shtml
[5] ABC News. (2023, December 15). Severe storms lash Brisbane and south-east Queensland bringing hail and downing trees. ABC News. https://www.abc.net.au/news/2023-12-15/dangerous-storm-brisbane-south-east-queensland-hail-trees-down/103236452
[6] Australian Bureau of Meteorology. (2025, June). Seasonal climate summary for Australia. Bureau of Meteorology. http://www.bom.gov.au/climate/current/season/aus/summary.shtml
[7] Domensino, B. (2025, July). Strongest positive SAM event in two years – here's what it means for Australia. Weather Zone. https://www.weatherzone.com.au/news/strongest-positive-sam-event-in-two-years--heres-what-it-means-for-australia/1890719
[8] Australian Bureau of Meteorology. (n.d.). Climate outlooks. Bureau of Meteorology. http://www.bom.gov.au/climate/ahead/outlooks/
[9] Australian Bureau of Meteorology. (n.d.). El Niño-Southern Oscillation (ENSO) current status. Bureau of Meteorology. http://www.bom.gov.au/climate/enso/?ninoIndex=nino3.4&index=nino34&period=weekly
[10] Actuaries Climate Index. (2025, May 27). Actuaries Climate Index. https://actuariesclimateindex.org/home/
[11] Australian Government Royal Commission into National Natural Disaster Arrangements. (n.d.). Australian actuaries climate index: Some comments on extremes [Exhibit 1.006.003]. https://www.royalcommission.gov.au/natural-disasters/report
