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November 2009 IRI South Pacific Tropical Cyclone Activity Forecast

There is a slightly enhanced probability (approximately 40%) that the number of tropical cyclones in the South Pacific region (east of 165E) during the 2009/2010 peak season (December to March) will be in the above-normal category, which is defined as between 7 or more named tropical cyclones. There is also a 35% probability that the number of tropical cyclones in this period will be in the normal category (3 to 6 named tropical cyclones). These probabilities are greater than the long-term average probability of 33%. The probability for a below-normal season (2 or less named tropical cyclones) is 25%.

Fig. 1 - Probability Forecasts for Number of Tropical Cyclones

This forecast is based on the IRI sea surface temperature (SST) forecast , with above-normal SSTs conditions in the eastern equatorial Pacific (El Niño event).

Background Information

The mean number of observed South Pacific named tropical cyclones (1971/1972 - 2001/2002) in the peak season (December to March) is 4.8 with a standard deviation of 2.4. The near normal category is between 3 and 6 named tropical storms in the December-March peak season. The lowest number of tropical cyclones in the peak season during this historical period was 0 and the maximum was 11.

This outlook was produced by tracking South Pacific 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 South Pacific 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 temporal distribution of these model tropical cyclones in the South Pacific 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 South Pacific region is strongly influenced by ENSO.