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Seasonal Outlook for Tropical Cyclones
Summary: 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.
Figure 1. POAMA forecast for Nino3.4 sea surface temperature anomaly3.
Seasonal Outlook for Tropical Cyclones
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) These regions are shown in Figure 2. (Please note, that the definitions of the regions here are slightly different from what is used by the Tropical Cyclone Warning Centres; see Appendix for details).
Figure 2. The regions defined for this tropical cyclone outlook.
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.
| Region | Chance of above average no. of TCs, % | Forecast TCs (Average TCs) | Confidence (LEPS skill)5 |
|---|---|---|---|
| Australian region (90°E to 160°E) | 7% based on Nino3.4 (58% based on the SOI) | 7~13 (12) | High (46~48%) |
| Western region (90°E to 125°E) | 16% based on Nino3.4 (56% based on the SOI) | 5~8 (7) | High (28~31%) |
| Northern region (125°E to 142.5°E) | 38% based on Nino3.4 (51% based on the SOI) | 3~4 (4) | Low (3~8%) |
| Eastern region (142.5°E to 160°E) | 25% based on Nino3.4 (55% based on the SOI) | 3~4 (4) | Moderate (~21%) |
Table 1. Chances of above average tropical cyclone numbers in the Australian region (90°E to 160°E) and its three sub-regions.
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.
Further Information
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.
| Nino 3.4 | SOI | |
|---|---|---|
| Chance of above average no. of TCs, % | 34 | 53 |
| Forecast TCs (Average TCs) | 5 (6) | 6 (6) |
| Confidence (LEPS skill) | Low (7%) | Low (9%) |
Table 2. Chance of above average tropical cyclone numbers in the northwest Western Australian region (105°E to 130°E).
Australia's area of responsibility for tropical cyclone services is divided between three Tropical Cyclone Warning Centres: Perth, Darwin and Brisbane. As noted above, the regional boundaries in this map differ slightly from those used in this outlook.
References
1 ENSO Wrap-Up issued in October 2009.
2 SOI data.
3 POAMA forecast for Nino3.4 sea surface temperature anomaly.
4 Nino3.4 data.
5 Linear Error in Probability Space (LEPS) score is to measure forecast skills. High scores suggest good skills.
6 This area is suggested by the Western Australia Regional Office. Note that LEPS score for this area is lower than that for 90°E to 125°E.
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