The IRI South Pacific Tropical Cyclone Activity Experimental Dynamical Forecasts
November 2003 IRI South Pacific Tropical Cyclone Activity Forecast
There is an enhanced probability (approximately 40%) that the number of tropical cyclones in the South Pacific region (east of 165E) during the 2003/2004 peak season (December to March) will be in the below normal category, which is defined as 2 or less named tropical cyclones. There is also a 35% probability that the number of tropical cyclones in this period will be in the normal category (between 3 and 6 named storms). These probabilities are greater than the long-term average probability of 33%. This forecast is based on the IRI sea surface temperature (SST) forecast that reflects the neutral ENSO conditions, but may be dependent on the details of the SST forecast, such as a large region of above normal SST along the equator centered near the date line.
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. While low-resolution (approximately 2.8 degrees longitude and latitude) AGCMs are not adequate for forecasts of individual tropical cyclones, 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. Model tropical cyclones are weaker and larger than observed, 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.