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South Pacific Seasonal Outlook for Tropical Cyclones
A seasonal assessment of tropical cyclone risk for the South Pacific Ocean.
Summary: Average tropical cyclone activity expected in the South Pacific
Based on the current climate status in tropical areas, the South Pacific region tropical cyclone (TC) outlook for 2009/10 suggests average to below average tropical cyclone activity in the coming cyclone season in the Western Region (142.5°E to 165°E), and average to above average tropical cyclone activity in the Eastern Region (165°E to 120°W).
Tropical Cyclone Climatology
In the South Pacific Ocean, the tropical cyclone season usually extends from November to April. The season reaches its peak during January, February and March. Cyclones can also occur during the other months although the risk is lower. On average about 10 tropical cyclones form in the South Pacific Ocean each year. Tropical cyclone year is defined as the 12 month period from July to the next June inclusive.
The El Niño-Southern Oscillation (ENSO) is a significant contributor to the year-to-year variability in tropical cyclone activity in the South Pacific Ocean. Maps of average annual tropical cyclone distribution based on past tropical cyclone data have been prepared for the South Pacific Ocean and are reproduced below. They provide a conditional probability forecast for the tropical cyclone season ahead based on the phase of ENSO.
The geographical distribution of tropical cyclone activity varies significantly depending on the ENSO phases. In the South Pacific Ocean, tropical cyclone occurrences in El Niño years in the area east of around 165°E are higher than in La Niña years. In La Niña years, tropical cyclone occurrences in the area between around 140°E and 165°E are higher than in El Niño years. In neutral years, tropical cyclone activity is close to the all years average over the South Pacific Ocean.
Figure 1. Average annual TC number in the South Pacific region.
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 2) suggests there is limited scope for these El Niño conditions to develop further.
Figure 2. POAMA forecast for Nino3.4 sea surface temperature anomaly3.
Seasonal Outlook for Tropical Cyclones
The unusual nature of the current El Niño event means that there is some uncertainty attached to tropical cyclone predictions for the coming season. Oceanic indicators of the ENSO suggest a moderate strength El Niño event is underway, whereas atmospheric indicators are less clear and more indicative of near neutral conditions.
The following outlook is a product of the operational seasonal outlook model developed at the National Climate Centre, based on the statistical relationships between tropical cyclones and the SOI and Nino3.44 indices, which measure the atmospheric and oceanic states of ENSO, respectively. In the South Pacific Ocean, the area affected by tropical cyclones extends from the Australian coast (east from about 142.5°E) to 120°W. There are two areas with distinctly different response in tropical cyclone activity to ENSO: east and west of about 165°E. In this tropical cyclone outlook, boundaries for these two regions are defined as 142.5°E to 165°E for the Western Region and 165°E to 120°W for the Eastern Region (Figure 2).
Figure 3. The regions defined for this tropical cyclone outlook.
This outlook is for the whole 2009/10 tropical cyclone year (typically the tropical cyclone season is from November to April, but sometimes early or late season cyclones occur). The forecast chances of above average tropical cyclone numbers are presented in Table 1.
The different information contained from the SOI and Nino3.4 indices is reflected in the rather different forecasts for the coming season. In terms of the total number of tropical cyclones for the coming season for the Western Region, the forecast range is 4 to 5 tropical cyclones (5 on average). For the Eastern Region, 7 or 8 tropical cyclones are forecast. However, the current forecast model has low skill in this region, so this prediction should be viewed with caution (LEPS5 scores are around 1% for the predictors Nino3.4 and SOI). On average, this region experiences seven tropical cyclones each season.
| Outlook Region | Chances of above average tropical cyclone numbers, % | Forecasted tropical cyclones (Average tropical cyclones) | Confidence (LEPS) |
|---|---|---|---|
| Western region (142.5°E to 165°E) | 29% based on Nino3.4 (52% based on the SOI) | 4~5 (5) | Moderate (13% for Nino3.4; 17% for SOI) |
| Eastern region (165°E to 120°W) | 61% based on Nino3.4 (48% based on the SOI) | 7~8 (7) | Low (2% for Nino3.4; 2% for SOI) |
Table 1. Chances of above average tropical cyclone numbers in the South Pacific Ocean regions.
For the Western Region (142.5°E to 165°E), chances that tropical cyclone numbers will be above average during the 2009/10 season are 29% based on Nino3.4 (52% based on the SOI). For the Eastern Region (165°E to 120°W), chances that tropical cyclone numbers will be above average are 61% based on Nino3.4 (48% based on the SOI).
In general, the outlook for tropical cyclones highlights that the Western region show a shift towards average to below average TC activity and the Eastern region average to above average TC activity, while it is noticed that the forecast skills are higher for the Western Region than those for the Eastern Region.
Further Information
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.
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