Summary: Above average tropical cyclone activity expected for the Australian region
The outlook suggests that the coming tropical cyclone season is likely to have
Figure 1 The regions defined for this tropical cyclone outlook
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.
Table 1 Forecast values for the 2010/11 Seasonal Outlook for Tropical Cyclones (TCs) for the four main tropical cyclone regions and the northwest WA sub-region (105°E to 130°E).
|Region||Chance of more TCs than average||Likely number of TCs (average number)||Confidence (LEPS skill1)|
|Australian region||98%||20-22 (12)||High (46%)|
|Western region||93%||11-12 (7)||High (31%)|
|Northern region||67%||5 (4)||Low (3%)|
|Eastern region||87%||6-7 (4)||Moderate (21%)|
|Northwest WA||75%||7-8 (6)||Low (7%)|
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.
This outlook covers the period from July 2010 to June 2011. Most tropical cyclones in the Southern Hemisphere occur from November until April.
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
Australia's area of responsibility for tropical cyclone services is divided between three Tropical Cyclone Warning Centres: Perth, Darwin and Brisbane. Please note, for statistical reasons, the regions described in this outlook (Figure 1) differ slightly from the regional boundaries used by the Tropical Cyclone Warning Centres (Figure 4). However outlooks may be considered generally indicative of each area.
Figure 4 The regional boundaries used by the three Tropical Cyclone Warning Centres:
Perth, Darwin and Brisbane
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