Technical ENSO Update

14 December 2005

Current Conditions

Since November 2005, below-average SSTs have continued to develop in the eastern equatorial Pacific, and some local air-sea coupling has contributed to that. However, although the slow evolution of the near equatorial ocean shows a slow, weak shoaling of the thermocline developing over the course of 2005, the recent easterly wind anomalies have created deep thermocline anomalies in the central equatorial Pacific that are now acting to erode the shallow eastern equatorial thermocline anomalies. Overall, the equatorial heat content is near average. The zonal winds have recently strengthened in response to the changing SST gradient, but they are still relatively close to average.

Expected Conditions

Presently in mid-December the potential for maintaining neutral SST conditions through the Jan-Feb-Mar period is very high. There is an estimated 10% likelihood for La Niña to develop and less than a 1% probability for El Niño. 

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 some variation among ENSO model forecasts, mainly for the longer-lead seasons. No models are forecasting El Niño conditions to occur in the Jan-Feb-Mar period, but 4 of the 20  models (20%) are suggesting the potential for La Niña SST conditions to develop within that season. 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 El Niño conditions for the Jan-Feb-Mar period; 4 of 15 models (27%) indicate SSTs exceeding the threshold for 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 Apr-May-Jun 2006, 1 of the 18 models (6%) forecasts El Niño conditions, 16 models (88%) forecast neutral conditions, and 1 model (6%) forecasts La Niña conditions. If only the 13 models that use subsurface ocean temperature data are included, these figures become 1 (8%), 11 (85%), and 1 (8%).  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 0% for Dec-Jan-Feb, rising to approximately 25% by mid-2006. For similarly La Niña the probabilities begin at 2% for Dec-Jan-Feb and increase to approximately 25% in mid-2006. 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 climatological probabilities for both El Niño and La Niña throughout most of the outlook periods. Neutral conditions are clearly indicated as being most likely through the first half of 2006.
See also: 

• Summary of model forecasts