Australian tropical cyclone season long-range forecast
About the long-range forecasts
The tropical cyclone season long-range forecast uses the statistical relationships between historical tropical cyclone numbers and two indicators: the Southern Oscillation Index (SOI) and the Niño3.4 sea surface temperature (SST) anomaly (relative to the 1991–2020 average). These two indicators provide a measure of the atmospheric and oceanic state, respectively, of El Niño-Southern Oscillation (ENSO).
The July, August and September SOI and NINO3.4 anomaly values are used in making the Australian Tropical Cyclone Outlook.
Interpreting the long-range forecast
The statistical long-range forecast generates a percentage chance that the upcoming season will have more tropical cyclones than the long-term average. For example, a 40% chance of having more than average tropical cyclones means that for every ten years with similar ENSO patterns to those currently observed, four years would be expected to have an above average number. The percentage chance is converted into a categorical forecast whether each region is expected to have an above average, average, or below average number of tropical cyclones.
The long-term average, since 1969–70, number of tropical cyclones per season in the Australian region is 11, with 4 typically making landfall. In the South Pacific region, the long-term average is 9 tropical cyclones per season.
In recent decades, in a globally warmer climate, the total number of tropical cyclones that have formed in the Australian region has decreased, to an average of 9 per season since 2000–01. In a globally warmer climate, it is generally more difficult for tropical cyclones to form due to less favourable atmospheric conditions. However, due to increased energy provided by warmer oceans, tropical cyclones that do form are likely to be more intense and produce more rainfall.
During El Niño events, there are typically fewer tropical cyclones than average, while more tend to occur during La Niña events.
Australian region long-range forecast accuracy
Historically, over the entire Australian Region, this statistical relationship has had high accuracy, i.e. has skilfully predicted total tropical cyclone activity. However, across the sub-regions this relationship, and thus forecast skill, can vary. The North-western sub-region has moderate to high skill, the Western region has moderate skill, and Eastern and Northern regions both have low skill.
The statistical relationship between historical tropical cyclones numbers and climate indicators used by the long-range forecast does not explicitly include global warming.
Australian tropical cyclone long-range forecast region bounds
| Region | Top | Bottom | Left | Right |
|---|---|---|---|---|
| Australian region | 5° S | 40° S | 90° E | 160° E |
| Western region | 5° S | 40° S | 90° E | 125° E |
| NW sub-region | 5° S | 25° S | 105° E | 130° E |
| Northern region | 5° S | 40° S | 125° E | 142.5° E |
| Eastern region | 5° S | 40° S | 142.5° E | 160° E |
Current information
- Tropical Climate Update
- Tropical (MJO) monitoring
- Southern Oscillation Index (SOI)
- SST indices
- SST analysis maps
- SST outlooks
- Latest Northern wet season summary
- Tropical cyclone climatology maps
Tropical cyclone knowledge centre
Definitions
South Pacific
References
Research documents
Model: Kuleshov, Y., L. Qi, R. Fawcett and D. Jones, 2008: Improving preparedness to natural hazards: Tropical cyclone prediction for the Southern Hemisphere, in Advances in Geosciences, 12 Ocean Science, (Ed. Gan, J.), World Scientific Publishing, Singapore, 127-143.
Data: Kuleshov, Y., R. Fawcett, L. Qi, B. Trewin, D. Jones, J. McBride and H. Ramsay, 2010: Trends in tropical cyclones in the South Indian Ocean and the South Pacific Ocean, Journal of Geophysical Research 115, D01101, doi:10.1029/2009JD012372.
Fewer cyclones than average likely for Australia this season
- Fewer than average numbers of tropical cyclones are expected in the Australian region for the 2019–20 cyclone season (November–April)
- On average, there are 9 to 13 tropical cyclones each season in the Australian region, four of which typically cross the coast.
- At least one tropical cyclone has crossed the Australian coast each season since reliable records began in the 1970s.
- Higher than average pressure over northern Australia and a neutral El Niño–Southern Oscillation (ENSO) have influenced this year's tropical cyclone outlook.
- During ENSO-neutral cyclone seasons, the first cyclone to cross the coast is typically in late December.
- Cyclone formation is rarely spread evenly throughout the season; often quiet periods are followed by bursts of activity.
- Tropical lows that do not intensify into cyclones, or lows that are the remnants of older cyclones, can still cause widespread rainfall and dangerous flooding. These impacts can extend beyond the tropics into southern areas of the country.
- Tropical cyclones that stay well out to sea can still cause damaging winds, large and dangerous waves, and storm surges along the coast.
Outlook influences
This outlook is based on the historical relationships between the status of the El Niño–Southern Oscillation (ENSO) over the preceding July to September and the subsequent tropical cyclone season. Sea surface temperatures in the tropical Pacific Ocean have been ENSO-neutral since April 2019. All eight international climate models surveyed by the Bureau predict tropical Pacific Ocean temperatures will remain within neutral ENSO bounds until at least February 2020. The Southern Oscillation Index (SOI) was close to zero throughout July and August but exceeded El Niño thresholds in September, primarily due to higher than average pressure at Darwin. The corresponding pressure in Tahiti was within normal bounds, suggesting the September SOI value was not related to a developing El Niño. Recent seasonal trends in Darwin may be related to the current record-strong positive Indian Ocean Dipole—another of Australia's major climate drivers, and the cooler waters surrounding northern Australia.
Outlook by region
Fewer than average tropical cyclones are likely in the Australian region and all sub-regions for 2019–20.
- The Australian region has a 35% chance of more tropical cyclones than average, meaning a 65% chance of fewer tropical cyclones than average. Typically, around four tropical cyclones cross the Australian coast in a season. Outlook accuracy for the Australian region is high.
- The Western region is likely to experience a below-average number of tropical cyclones this season, with a 57% likelihood of fewer than average. The likelihood of more cyclones than average is 43%. Typically between about 15% and 40% of tropical cyclones in the Western region affect coastal regions. Outlook accuracy for the Western region is low.
- The Northwestern sub-region has a 39% chance of more tropical cyclones than average and a 61% chance of fewer tropical cyclones than average. Typically, five cyclones form in, or pass through, this area each season. Around 40% of tropical cyclones, or their associated tropical lows, affect coastal areas of the Northwestern sub-region. Outlook accuracy for this region is moderate.
- The outlook for the Northern region suggests a below average number of tropical cyclones, with a 36% chance of more than average and a 64% chance of fewer tropical cyclones than average. In a typical year the Northern region experiences around three cyclones, and one or two tropical lows that later become cyclones after moving into the Western or Eastern regions. About three-quarters of the tropical cyclones in the Northern region impact coastal areas. Outlook accuracy for this region is very low.
- The Eastern region outlook shows a below-average season is most likely, with a 43% chance of more tropical cyclones than average and a 57% chance of fewer. About a quarter of tropical cyclones in the Eastern region make landfall. Outlook accuracy for this region is low.
Product code: IDCKAUTCSO
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