Summary: Tropical cyclone activity for the South Pacific is expected to be higher than average in the western region and lower than average in the eastern region.
The coming tropical cyclone season is likely to have
The confidence in this forecast is higher for the Western region than for the Eastern region.
Figure 1 The regions defined for this tropical cyclone outlook
For the Western region, the forecast chance that the total number of tropical cyclones will be higher than average is 79%. In terms of the total number of tropical cyclones for the coming season, the forecast range for the Western region is 7-8 tropical cyclones. This is 2-3 more than the long-term average value of 5 tropical cyclones.
For the Eastern region, the forecast chance that the total number of tropical cyclones will be higher than average is 33%. In terms of the total number of tropical cyclones for the coming season, the forecast range for the Eastern region is 5-6 tropical cyclones (1-2 less than the average value of 7 tropical cyclones). However, as the computer model used to produce this forecast has relatively low skill in the Eastern region, this prediction should be used with some caution.
This outlook covers the period from July 2010 to June 2011. Most tropical cyclones in the Southern Hemisphere typically occur from November until April.
Table 1 Forecast values for the 2010/11 Seasonal Outlook for Tropical Cyclones (TCs) for the South Pacific regions
|Region||Chance of more TCs than average||Likely number of TCs (average number)||Confidence (LEPS skill1)|
(142.5°E to 165°E)
|79%||7-8 (5)||Moderate (13%)|
(165°E to 120°W)
|33%||5-6 (7)||Low (2%)|
This outlook is produced based on statistical relationships between tropical cyclone numbers and two indicators: the Southern Oscillation Index2 (SOI) and the Niño3.4 sea surface temperature anomaly3. These two indicators provide a measure of the atmospheric and oceanic state, respectively, of El Niño-Southern Oscillation (ENSO).
The early October assessment of ENSO conditions by the Bureau of Meteorology4 states that Pacific Ocean temperatures are at levels typical of a La Niña event and that these conditions are likely to continue until at least early 2011. Surface conditions are warmer than average in the Coral Sea, off Australia's northern coasts and in the far western Tropical Pacific. The La Niña is highlighted by the SOI, which has been showing strong positive values since early 2010 (Figure 2). The majority of international climate models surveyed by the Bureau of Meteorology predict that La Niña conditions are likely to persist into 2011, as indicated by the Bureau's climate model (Figure 3).
Figure 2 The 30-day Southern Oscillation Index
Figure 3 Bureau climate forecasts for the Niño3.4 sea surface temperature anomaly5
showing continuing La Niña conditions
1 Linear Error in Probability Space (LEPS) is used to measure forecast skills, with higher values suggesting better skill: http://www.bom.gov.au/bmrc/wefor/staff/eee/verif/LEPS.html
2 SOI data: http://www.bom.gov.au/climate/current/soi2.shtml
3 Niño3.4 data: ftp://ftp.cpc.ncep.noaa.gov/wd52dg/data/indices/sstoi.indices
4 ENSO wrap-up: http://www.bom.gov.au/climate/enso/
5 POAMA forecast for Niño3.4 sea surface temperature anomaly: http://www.bom.gov.au/climate/coupled_model/poama.shtml