Technical ENSO Update

13 June 2006

Current Conditions

Expected Conditions

With the lack of strong anomalies in the surface or sub-surface temperature fields, the maintenance of ENSO-neutral conditions remains the most likely outcome for the coming 3-month season. There is an estimated 80% likelihood for neutral conditions, approximately 5% probability for La Niña conditions to return, leaving 15% probability that El Niño conditions will develop during the JJA 2006 season.

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. 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 late-spring/searly-summer 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 ranging from neutral to El Niño conditions for the longer-lead seasons. Only 1 model is forecasting La Niña conditions to occur between Jun-Jul-Aug and Oct-Nov-Dec 2007, and that model begins its prediction with much colder conditions than are currently observed. For the JJA 2006 season,  4 out of 19 models (21%) predict El Niño conditions. 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, 2 of 13 models (15%) call for El Niño conditions by the Oct-Nov-Dec period; 1 out of 13 (8%) 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 remain near-normal throughout the forecast period: JAS 2006 (68%) to FMA 2007 (62%). 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 lower than the climatological value (25%)--beginning at about 14% for Jun-Jul-Aug, rising to 42% by the beginning of 2007. For La Niña the probabilities begin at 3% for Jun-Jul-Aug and peak at 9% in Sep-Oct-Nov 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 ENSO-neutral conditions, much lower than climatological probabilities La Niña through the end of 2006, and slightly enhanced, but near climatological, probabilities for El Niño.
See also: 

• Summary of model forecasts