2010–2011 Australian tropical cyclone season outlook

Above average tropical cyclone activity expected for the Australian region

The outlook suggests that the coming tropical cyclone season is likely to have:

• a higher than average number of tropical cyclones over the full Australian region,
• a higher than average number of tropical cyclones in the Western region,
• an average to above average number of tropical cyclones in the Northern region,
• a higher than average number of tropical cyclones in the Eastern region.

Figure 1 The regions defined for this tropical cyclone outlook

Seasonal outlook for tropical cyclones

For the full Australian region, there is a high degree of confidence that the total number of tropical cyclones will be above average (see Table 1). The forecast values from the two models (20-22) are significantly higher than the long-term average value of 12.

The likely number of tropical cyclones is indicative only. It is expected that the total number of tropical cyclones will be in the vicinity of the values listed, and not necessarily within the given range. The values are the most likely number of tropical cyclones forecast by two models.

Regional features

• The largest increase in tropical cyclone numbers is expected to occur in the Western region, where 11-12 tropical cyclones are forecast (4-5 more than the average value of 7 tropical cyclones).
• For the Northern region, an average to higher than average number of tropical cyclones is expected with 5 tropical cyclones being forecast (the average value is 4 tropical cyclones). However, as the statistical model used to produce this forecast has low skill in this region, this prediction should be used with some caution.
• The Eastern region is expected to experience a higher than average number of tropical cyclones, with about 6-7 tropical cyclones being forecast (the average value is 4 tropical cyclones).
• The outlook for tropical cyclones in the area from 105°E to 130°E, where tropical cyclones can impact upon coastal WA communities, shows a 75% chance of above average tropical cyclone numbers for the 2010/11 season. However, forecast confidence for this region is relatively low.

This outlook covers the period from July 2010 to June 2011. Most tropical cyclones in the Southern Hemisphere occur from November until April.

Production of the outlook

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 strength of the atmospheric and oceanic state, respectively, of the El Niño-Southern Oscillation (ENSO).

The mid-October assessment of ENSO conditions by the Bureau of Meteorology⁴ states that the La Niña in the Pacific Ocean is a moderate to strong event, and that La Niña conditions are likely to continue until at least early 2011. Surface conditions are also warmer than average in the Coral Sea, off Australia's northern coasts and in the far western Tropical Pacific. The current La Niña is highlighted by the SOI, which has been showing very high positive values since August (Figure 2). The majority of international climate models surveyed by the Bureau of Meteorology predict that La Niña conditions are likely to persist into at least the first quarter of 2011, as also 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