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  September 2011 IRI Australian Tropical Cyclone Activity Forecast

The forecast the number of tropical cyclones in the Australian region (105E to 165E) during the 2011 peak season (January to March) has a 55% probability of being in the above-normal category (9 or more tropical cyclones). This probability is higher than the long-term average probability of 33%. The probability for a near-normal season (5-8 tropical cyclones) is 30%, and for a below-normal season (4 or fewer tropical cyclones) is only 15%. This forecast is based on the IRI sea surface temperature (SST) forecast with above-normal SSTs conditions in the western equatorial Pacific (La Niña event).

Fig. 1 - Probability Forecasts for Number of Tropical Cyclones

Background Information

The mean number of observed Autralian named tropical cyclones (1971 - 2002) in the peak season (January to March) is 7.1 with a standard deviation of 3.2. The near normal category is between 5 and 8 named tropical storms in the January-March peak season. The lowest number of tropical cyclones in the peak season during this historical period was 1 and the maximum was 13.

This outlook was produced by tracking Australian tropical cyclone-like systems in one of our operational atmospheric general circulation models (AGCMs), ECHAM4.5, forced with IRI's predicted sea surface temperatures. AGCMs are not adequate for forecasts of individual tropical cyclones. However, they can have significant skill in predicting the amount of tropical cyclone activity over specific basins, as is the case for the ECHAM4.5 over the Australian region. Due to the low-resolution (approximately 2.8 degrees longitude and latitude) model tropical cyclones are weaker and larger than observed tropical cyclones, but have an identifiable signature with many observed tropical cyclone characteristics. The model skill is due to the variability of the tropical cyclone activity being mainly determined by large-scale variables that affect that activity, such as sea surface temperatures and vertical wind shear, which can be predicted using AGCMs. The seasonal distribution of these model tropical cyclones in the Australian region is similar to that of observed tropical cyclones in the region. The amount of tropical cyclone activity of both model and observations in the Australian region is strongly influenced by ENSO.

The IRI September 2011 Australian tropical cyclone activity forecast qualitatively agrees with the Tropical Storm Risk September 2011 statistical forecasts of Australian tropical cyclones for the 2011/2012 season (November 2011 to April 2012). The Tropical Storm Risk forecast calls for a slightly above-normal number of named tropical cyclones. It is important to note that the definition of the Australian region is slightly different in this forecast.