Climate Index: Record for extreme weather reflects a season of severe flooding
The Australian Actuaries Climate Index reached a record level across Australia during autumn, driven by extreme rainfall across the east coast and high temperatures in northern Australia.
The East Coast South region, which covers Sydney and most of the NSW coast, set a record for extreme rainfall. The Wet Tropics region, which includes the northern most parts of Queensland, recorded the second highest index value for extreme rainfall since the index began. This was influenced by the ongoing La Niña weather system that stayed in place throughout the season . The rainfall was enhanced by an east coast low that affected the region in early March (see Figure 1).
On the ground there was a continuation of the severe flooding that started in the previous season. Schools were closed, evacuations took place and supply chains were disrupted . The combination of saturated soil conditions and continued heavy rainfall throughout the season meant flooding persisted throughout the period for much of QLD and NSW. This continues in winter.
The Climate Index is calculated at the end of each season by Finity following the release of data from the BoM and is updated quarterly. It draws on six component indices measuring changes in the frequency of extreme high and low temperatures, heavy precipitation (rainfall), dry days, strong winds and changes in sea levels across 12 Australian regions that are climatically similar. The Index reached 1.01 in autumn, the highest since the index began. Each season is compared to the same season in previous years, and against a reference period from 1981-2010.
Figure 2 depicts the ongoing nature of the extreme rainfall experienced by this part of the country. It compares how often the regions experienced extreme rainfall in 2022 to the base period average (where extreme is defined as exceeding the 99th percentile of the base period of 1981-2010). By the end of March, the East Coast South region had experienced approximately six times as many observations of extreme rainfall than during the base period. By the end of May, extreme rainfall in both the East Coast South and East Coast North regions had been about four times more frequent in 2022 than during the base period.
Notes: The cumulative nature of the calculations means that, for example, May = January through May. Extreme rainfall is measured in the Index as a day in which the reading is at, or above, the 99th percentile of observations from the base period.
Along with extreme rainfall, Australia experienced extreme high temperatures in autumn. This was primarily seen in the northern parts of the country, as is shown in Figure 3. The Wet Tropics and Monsoonal North regions both saw the second highest extreme high temperature index value, while the East Coast North region recorded the third highest value since the index began.
The Bureau of Meteorology observed similar trends, noting that rainfall totals were more than double their seasonal average for several parts of NSW , and that the national mean temperature was the third highest on record .
The La Niña event that contributed to the wet year so far ended in June, but may re-form in spring and bring back with it above average rainfall. Also, a negative Indian Ocean Dipole event is predicted for the coming months . This increases the chances of above average winter and spring rainfall for much of the country.
According to Rade Musulin, Chair of the Actuaries Institute Climate Risk Working Group and Principal at Finity, “this extreme weather is consistent with the warnings of climate scientists, recently summarised in a joint paper by the International Actuarial Association and the Intergovernmental Panel on Climate Change, Climate Science: A Summary for Actuaries. It is important that we invest in improving the resilience of our communities in light of trends in extreme weather.” Mr. Musulin was a lead author of that paper.
The detailed AACI Design Documentation is available at https://www.actuaries.asn.au/microsites/climate-index/about/development-and-design