2011-12 Australian Tropical Cyclone Season Outlook

Summary: Above average tropical cyclone activity likely for the Australian region this season

The outlook favours the following scenarios for the coming season:

• The whole Australian Region has an 80% chance of having more than the long-term average number of cyclones. The long-term average is twelve.
• The Western Region has a 65% chance of observing above average number of tropical cyclones, where the long-term average is seven.
• The North-western Sub-region has a 60% chance of above average number of tropical cyclones. The long-term average is five.
• The Northern Region has a 60% chance of above average number of tropical cyclones, where the long-term average is two or three cyclones.
• The Eastern Region has a 65% chance of observing above average number of tropical cyclones, where the long-term average is three or four.

Figure 1 The regions defined for this tropical cyclone outlook with the average number of cyclones per season for each region and the forecast chance of that region having above average TC activity in the 2011-12 season.

Seasonal Outlook for Tropical Cyclones

Tropical cyclone activity in the Australian region (5°S-40°S, 90°E-160°E) is likely to be above average this coming season. The outlook suggests a 80% chance of having more than the long-term average number of cyclones in the Australian region during the 2011-12 season. (For more info about average tropical cyclone activity and La Niña events see info about Tropical cyclone average conditions.)

This outlook is based upon the status of the El Niño - Southern Oscillation (ENSO) over the preceding July to September period. During this period in 2011, neutral to borderline La Niña conditions were present. Historically, these conditions have favoured an above average number of cyclones in the Australian region.

Table 1 Forecast values for the 2011/12 Seasonal Outlook for Tropical Cyclones (TCs) for the four main tropical cyclone regions and the north western sub-region.

Region	Chance of above average tropical cyclone activity	Long-term average	Forecast skill
Australian region	80%	12	Very high
Western region	65%	7	Moderate
North-western sub-region	60%	5	Moderate
Eastern region	65%	4	Moderate
Northern region	60%	3	Low

Regional features

• The outlook for the Western Region suggests a 65% chance of the total number of tropical cyclones being above average (35% chance of being below average). In the past, the skill level for forecasts in the Western Region is moderate. On average, around 30% of tropical cyclones in the western region will have an impact on the coast at some stage in their life.
• The North-western Sub-region (the area from 105°E to 130°E, where tropical cyclones can impact upon coastal WA communities) has around a 60% chance of experiencing more tropical cyclones (40% chance of receiving fewer) than its long-term average. On average, around 40% of tropical cyclones in the north-western sub-region will have an impact on the coast at some stage in their life.
• For the Northern Region, the outlook is a 60% chance of an above average number of tropical cyclones, although the skill level for this outlook is low. In an average year the Northern region sees two or three named storms and one or two tropical low pressure systems that become cyclones after moving into the Western or Eastern regions. A relatively high number (75%) of tropical cyclones in the northern region cross the coast at some stage in their life.
• The Eastern Region has a 65% chance that it will experience a higher than average number of tropical cyclones (35% chance of fewer than average), with a moderate confidence level based upon historical skill. Around 25% of the tropical cyclones in the Eastern region cross the coast.

This outlook covers the Australian tropical cyclone season which is considered to be between 1 November and 30 April.

Coastal Impacts

Before a tropical cyclone forms it is difficult to predict its exact strength and path, including whether it will make landfall. Along the east and west coasts fewer than half of the cyclones affect the coast, with most staying out to sea. Conversely, along the north coast more than half of the cyclones impact the coast. Tropical cyclones which remain out to sea can still cause storm surges, gales and areas of intense rain over land. During the cyclone season, ensure you are well informed of any warnings from the Bureau of Meteorology and instructions from local Emergency Services authorities.

Method

This outlook is produced based on statistical relationships between tropical cyclone numbers and two indicators: the Southern Oscillation Index (SOI) and the Niño3.4 Sea surface temperature index. These two indicators provide a measure of the strength of the atmospheric and oceanic state, respectively, of the El Niño - Southern Oscillation (ENSO).

The July, August and September SOI and NINO3.4 values were used in making the Australian tropical cyclone season outlook. The SOI values were +10.7, +2.1 and +11.7 respectively, while the NINO3.4 values were -0.1, -0.5 and -0.6.

The current status of ENSO can be viewed via the Bureau's ENSO Wrap-up. As of mid-October, this indicated trends consistent with a developing La Niña event. The sea surface temperatures around northern Australia, including the Timor and Arafura Seas, the Gulf of Carpentaria, and the Coral Sea, were near their long-term average at the time this outlook was produced. This contrasts with the 2010-11 season when all these areas were warmer than average.

Further Information

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 2). However outlooks may be considered generally indicative of each area.

Figure 2 The regional boundaries used by the three Tropical Cyclone Warning Centres:
Perth, Darwin and Brisbane

Past Tropical Cyclone Season Outlooks: 2010-11