Technical ENSO Update

18 October 2005

Current Conditions

Since mid-May, there have been no coherent large-scale anomalies observed in the ocean's subsurface structure. However, the slow evolution the near equatorial ocean shows a slow, weak shoaling of the thermocline developing over the course of 2005 . Overall, the equatorial heat content is near average. There also is no anomalous large-scale ocean-atmosphere coupling occurring over the tropical Pacific Ocean. The zonal winds have remained near their climatological average throughout of the tropical Pacific over the last several months. There are no substantial or large-scale SST anomalies developing in the tropical Pacific as of mid-October.

Expected Conditions

Presently in mid-October the potential for maintaining neutral SST conditions through the Nov-Dec-Jan period carries a probability of 97%, which represents a significant enhancement of the odds relative to the historical climatological likelihood of 50%. There is a mere 1% probability for La Niña SST conditions, leaving approximately 2% probability for El Niño SST conditions. 

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.45 degrees C in northern spring season and as high as 0.75 degrees C in late northern autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.

There is considerable variation among ENSO model forecasts for the coming several seasons. No models are forecasting El Niño conditions to occur in the Nov-Dec-Jan period. The number of models that predict La Niña conditions is 1 out of 19 (5%). At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, no models call for SSTs of at least a minimum El Niño level for the Nov-Dec-Jan period, and 1 out of 14 (7%) predicts SSTs cold enough to be considered a La Niña. (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will continue to be near-normal over the next several seasons. For the longer lead forecast for Feb-Mar-Apr 2006, 6 of the 17 models (35%) forecast El Niño conditions, 11 models (65%) forecast neutral conditions, and no models (0%) forecast La Niña conditions. If only the 12 models that use subsurface ocean temperature data are included, these figures become 5 (42%), 7 (58%), and 0 (0%).  Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the coming several months. The expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the above probability estimate from the true probability. 

An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. When this method is applied to the current model forecasts, results indicate a probability of El Niño that is much lower than the climatological value (25%)--beginning at about 1% for Oct-Nov-Dec and Nov-Dec-Jan, rising to approximately 30% by mid-2006. For all periods the probability for La Niña is well below its climatological value of 25%. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills.

The IRI's probabilistic ENSO forecast takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates much higher than average probabilities for neutral conditions, and lower than average probabilities for La Niña throughout all of the outlook periods. Similarly, throughout most of the forecast period, chances for El Niño are below their climatological average of 25%, but increase to that level in northern spring 2006. Neutral conditions are clearly indicated as being most likely over the remainder of 2005 and into 2006.
See also: 

• Summary of model forecasts