Additional Details about this Probabilistic ENSO forecast
The categories of La Niña, neutral, and El Niño are defined such that for any 3-month period, the historical frequency of being in the La Niña and the El Niño categories is 25% each over the base period of 1950-2002, and the frequency of being in the neutral category is 50%. This results in a classification in which the years appearing in the non-neutral categories approximately match the years that have been commonly accepted as being La Niña and El Niño years. Because the amount of year-to-year variability of the NINO3.4 region's SST varies with the season of the year, the thresholds for La Niña and El Niño must vary seasonally also to maintain the 25% - 50% - 25% distribution. The resulting SST anomaly thresholds required to be outside of the neutral category (in degrees C) for the NINO3.4 region are shown in the following table. Note that in addition to the thresholds' seasonal variation, they also differ between La Niña and El Niño for most of the seasons.
LN; EN LN EN LN EN
DJF -0.65 0.65
AMJ -0.45 0.45
ASO -0.55 0.50
Note: The most likely ENSO scenario suggested by this probabilistic
assessment may differ from the NCEP SST anomaly prediction which is used
for the tropical Pacific in the two-tiered forecast system to drive the
atmospheric general circulation models. The NCEP dynamical coupled model
is only one of the models considered in this probabilistic ENSO outlook.