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  September 2007 IRI Australian Tropical Cyclone Activity Forecast

There is a slightly enhanced probability (approximately 45%) that the number of tropical cyclones in the Australian region (105E to 165E) during the 2007 peak season (January to March) will be in the above normal category, which is defined as 9 or more named tropical cyclones. There is also a 35% probability that the number of named tropical cyclones in this period will be in the normal category (5 to 8 named tropical cyclones). These probabilities are slightly greater than the long-term average probability of 33%. There is a 20% probability that the number of named tropical cyclones in this period will be in the below normal category (4 or less named tropical cyclones). This forecast is based on the IRI sea surface temperature (SST) forecast with colder than normal SSTs conditions in the equatorial Pacific (weak La Niña).

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 2007 Australian tropical cyclone activity forecast differs from the Tropical Storm Risk September 2007 statistical forecasts of Australian tropical cyclones for the 2007/2008 season (November 2007 to April 2008). The Tropical Storm Risk forecast called for a near normal number of named tropical cyclones. It is important to notice that the definition of the Australian region is slightly different in this forecast.