Technical ENSO Update

20 September 2006

Current Conditions

Expected Conditions

Indications, suggesting the potential for El Niño development, exist in the surface and sub-surface conditions. These include persistent westerly wind anomalies in the western Pacific during July and August, increasing equatorial heat content and appearance of warm SST anomalies, particularly in the eastern equatorial Pacific. However, the sub-surface indicators, such as large-scale deep perturbations to the equatorial thermocline, whose adjustment could provide reinenforcement of surface warming, are modest. Given the apparent inability for the tropical Pacific environment to support coupled ocean-atmosphere growth over the last several years, even when provided with stronger perturbations to the ocean state than what is currently observed, it is unclear whether the current state can initiate and sustain an El Niño event. If that situation remains, then the future variability of the tropical Pacific air-sea system will be dominated by less deteministic atmospheric perturbations, the effects of which are much more difficult to foretell. It is possible that the atmosphere may respond to the recent warming at the surface to persist or amplify the current conditions. To date, precipitation anomalies have not responded to the SST anomalies, and no air-sea coupling has been observed. Similarly, the southern oscillation index (SOI), while quite negative a couple weeks ago, is currently trending back to neutral. Still, the tropical Pacific ocean-atmosphere system has been slowly evolving towards warmer conditions over the last several months, and the model predictions of El Niño to have been suggesting progressively warmer outlooks. As a result, the development of El Niño conditions has become the most likely outcome for the remainder of 2006. There is an estimated 55-60% likelihood for El Niño, approximately 2% probability for La Niña conditions to return, leaving approximately 40% probability that ENSO-neutral conditions will remain in place.

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/early-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 but warm to moderate El Niño conditions occuring near the end of the year. No model is forecasting La Niña conditions to occur through the forecast period. For the SON 2006 season,  11 out of 20 models (55%) 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, 11 of 13 models (85%) call for El Niño conditions by the Jan-Feb-Mar period; none 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 exhibit El Niño conditions throughout the forecast period: SON 2006 (55%) to MJJ 2007 (50%). 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 also indicate a probability of El Niño that is higher than the climatological value (25%)-- fluctuating between approximately 50-60% from Sep-Oct-Nov 2006 through the end of the year. For La Niña the probabilities do not exceed 10% throughout the forecast period. 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 a more than doubling of the odds for El Niño conditions, lower than expected probabilities for ENSO-neutral, and negligible probability of La Niña conditions through the end of 2006 and into early 2007.
See also: 

• Summary of model forecasts