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.
Average to below average tropical cyclone activity expected for the Australia region
Based on the current climate status in tropical areas, the Australian region tropical cyclone (TC) outlook for 2009/10 suggests average to below average TC activity. Likewise, average to below average TC numbers are also likely in each of the Western, Eastern and Northern Australian TC subregions.
Seasonal Conditions in the Pacific
The latest assessment of El Niño-Southern Oscillation (ENSO) conditions by the Bureau of Meteorology1 states that Pacific Ocean temperatures remain at levels typical of an El Niño event, and that most leading climate models predict that these warm conditions will persist until early 2010.
Unlike previous El Niño events, surface conditions have also been warmer than average in the Coral Sea, off Australia's northern coasts and in the far western Tropical Pacific. These unusual oceanic conditions have impacted upon atmospheric conditions in the tropics, with cloud and wind patterns yet to show a consistent El Niño signature. This is highlighted by the Southern Oscillation Index (SOI2), which has generally remained at neutral levels since March 2009.
The majority of international climate models surveyed by the Bureau of Meteorology predict that warm Pacific sea surface temperatures are likely to remain until early 2010. However the Bureau's POAMA model (Figure 1) suggests there is limited scope for these El Niño conditions to develop further.
Outlook influences
A level of inconsistency between the atmosphere and ocean conditions thus far in the El Niño event means that there is some uncertainty attached to tropical cyclone predictions for the coming season. However despite these mixed signals, the recent climate pattern over Australia has been broadly consistent with an El Niño event, with below average rainfall across large parts of eastern Australia, and above normal temperatures observed in many areas, though exceeding levels observed in previous El Niño events.
The following outlook is from the National Climate Centre's operational seasonal outlook model, based on statistical relationships between tropical cyclones and the SOI and Nino3.44. These two indicators provide a measure of the atmospheric and ocean state of ENSO, respectively. Boundaries for the regions in the tropical cyclone outlook are defined as 90°E to 160°E for the Australian region, with sub-regions 90°E to 125°E (Western Region), 125°E to 142.5°E (Northern region) and 142.5°E to 160°E (Eastern region).
This outlook covers the whole TC year from July to the following June, though typically most tropical cyclones occur between the months of November and April. The forecast chances of above average TC numbers are presented in the Table 1. Note that the sum of the average TC numbers in the three regions is larger than the average number for the whole country because a TC can travel across regions and hence be included in more than one region's total.
Regional details
- For the Northern Australian region, 3 or 4 TCs are forecast. However, the current forecast model has low skill in this region, so this prediction should be used with caution (LEPS scores are low, from 3% to 8% for the predictors Nino3.4 and SOI, respectively). On average, this region experiences 4 TCs each season.
- The different information contained from the SOI and Nino3.4 indices is reflected in rather different probability forecasts for the coming season. The outlook for tropical cyclones highlights that the Australian region as a whole and the three sub-regions tend to experience average to below average TC activity. The shift from average is more significant across both the Australian region as a whole and the Western Australian region where the forecast skill is high, rather than in the Eastern and Northern regions.
- The outlook for TCs in the area from 105°E to 130°E6, where TCs can impact upon coastal WA communities, shows a 34% chance of above average cyclone activity based on Nino3.4 (53% based on the SOI) in 2009/10 season. Forecast confidence for this region is low.
Product code: IDCKAUTCSO
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