Summary: Tropical cyclone activity in the South pacific region appears likely to be shifted westward during the 2011-12 season
The outlook favours the following scenarios for the coming season:
The confidence in this forecast is moderate for the Western region and low in the Eastern region.
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.
The outlook for the 2011-2012 tropical cyclone season calls for a westward shift in tropical cyclone activity. This season there is a 65% chance that the Western Pacific Region will see more than the average number of tropical cyclones (35% chance of fewer than average) and a 40% chance that the Eastern Pacific Region will see above average number (or a 60% chance that fewer cyclones will form). 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 a westward shift in tropical cyclone activity in the South Pacific.
This outlook is for the Southern Hemisphere tropical cyclone season which is usually considered to be between 1 November and 30 April.
Table 1 Forecast values for the 2011/12 Seasonal Outlook for Tropical Cyclones (TCs) for the South Pacific regions
|Region||Chance of more TCs than average||Long-term average||Confidence|
(142.5°E to 165°E)
(165°E to 120°W)
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 South Pacific 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.
Past Tropical Cyclone Seasonal Outlooks: 2010-11