Extreme wind speeds induced by tropical cyclones (TCs) can have considerable impacts on coastal communities and infrastructure situated in tropical Australia. In light of anthropogenic-induced climate change, it is important to investigate the potential evolution of TC-wind hazards for the benefit of future planning and disaster risk management in Australia.
The expected average recurrence interval (ARI) of extreme wind speeds is a metric widely used by engineers and climate scientists alike to gauge the wind hazard for specific regions. In Australia, this is described by the “structural design actions” for winds1 herein referred to as “the Standard”. Different regions of Australia are assigned various designations (A, B, C or D, in ascending order of strength) based on the historical wind strength (ARI) observed in that region. For example, Australia’s tropical and subtropical coastlines have much higher designations (B–D) than the southern and inland regions (A) due to the increased exposure to TCs. Historically, appropriate regional wind speed designations have been determined by weather station surface anemometer measurements (corrected for terrain, topography and gust duration) and to a lesser extent damage surveys from past events (including in some cases back calculations of maximum wind speeds from failed simple structures such as road signs) and these methods remain very useful.
In the latest edition of the AS/NZS structural design Standard (1170.2:2021), for the first time, a climate change multiplier (1.05) was included as mandatory to consider in some regions of Australia. This is consistent with a broad review and synthesis of available lines of evidence that indicate an increase in TC intensity with projected future warming is more likely than a decrease2,3. For local context, the East coast of Australia has experienced a recent influx of severe Australian Category 4/5 TCs4 (Table 1) while the West coast of Australia has experienced fewer Category 4/5 TCs recently5. This leads to the question of what role can climate model experiments play in informing us of likely future changes in fine-scale metrics such as wind speed ARIs that have traditionally come from observed ground truth data. Climate models, despite having their own limitations such as biases and inaccurate representations of some small- and large-scale physical processes, still provide homogenous simulations that can be useful for examining the potential influence of climate change now and into the future, as a complementary line of evidence to observations-based data (which also have limitations including due to evolving operational practices for wind observations in Australia with a relatively short time period of high-quality data being available).
Table 1
The Australian TC wind scale as defined by wind gust speed (3-second).
Category
|
Gust speed
|
1
|
24.5–34 m s− 1
|
2
|
34–45.5 m s− 1
|
3
|
45.5–61.6 m s− 1
|
4
|
61.6–77.4 m s− 1
|
5
|
> 77.4 m s− 1
|
It is timely that with the recent release of new, finer resolution reanalysis products, including the Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia (BARRA6) and the global ERA5 reanalysis7 produced by the ECMWF; the feasibility of determining wind speed ARIs in climate model type products can be tested. This can be achieved by direct comparison with the ground truth determined wind speed ARIs from AS/NZS 1170.2. The two reanalysis datasets can also provide additional lines of evidence in relation to current historical TC trends and extreme wind speeds affecting Australia.
The simulation of peak wind gust speeds from TCs in reanalysis products poses many challenges, and the products used here do not completely resolve the absolute wind gust speeds of the more intense TCs2,8. However, by calibrating the reanalysis data to the observations, such as from Automatic Weather Stations (AWS) and from the best track estimates of TC wind speeds9,10,11 useful information on absolute gust speeds in reanalysis data can be obtained, though with some obvious limitations12. That is, we are not suggesting that the ARIs calculated here should replace the current structural AS/NZS wind speed Standard, but rather to serve as an additional line of evidence contributing to a more comprehensive understanding of potential long-term changes in this metric and in the risk of TC-wind hazards for coastal Australia. In particular, several data sets are used here for the first time to examine TCs and associated extreme wind speeds in these regions, thereby providing new insight on this important topic, which is intended to help provide useful guidance complementary to existing information.
Here, we evaluate ARIs of TC-related daily-maximum wind gusts over eight Australian coastal regions (Fig. 1). These regions closely correspond with those designated by AS/NZS 1170.2:2021, allowing for a comparison of these wind speeds to the Standard. Two subtropical regions on either side of the Australian continent, where weakening TCs and extratropical cyclones (ETCs) can sometimes occur, are additionally examined. Examination of these two regions may provide some guidance on a key scientific question around the potential poleward movement of TCs and their wind hazards. The following section provides details on the Data and Methods used. Results are shown in Section 3, and a Summary and Conclusion is provided in Section 4.