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Technical ENSO Update

18 September 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-September 2008 SSTs in the central equatorial Pacific are near their long-term average, being slightly below-average near the dateline. In the eastern Pacific, extending into the NINO3 region, the SSTs had been above average over the past several months, but did not draw the cooperation of the atmosphere to evolve towards El Niño conditions. These warm anomalies have been subsiding, temperatures returning to average conditions, since mid-August. The deep thermocline anomalies in the western Pacific and the associated heat content anomalies that were created during the 2007/08 La Niña event, and then subsequently adjusted onto the equator, are largely depleted. The equatorial Southern Oscillation index (SOI) has remained slightly positive. The traditional Tahiti-Darwin SOI had been close to zero since May, but increased again in August, which may be related to intraseasonal wind anomalies rather than an indication of La Nina redevelopment. Related to the anomalous pattern of surface pressure, the low-level winds exhibited a large fetch of easterly anomaolies during August that have mostly abated. Taken together, the current anomalous ocean-atmosphere conditions indicate continuation of ENSO-neutral conditions.

For August 2008, the SSTs in the NINO3.4 region stayed weakly above average (0.18 degrees C anomaly). For the Jun-Jul-Aug season they were -0.01 degrees C from average. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Sep-Oct-Nov and Oct-Nov-Dec seasons are approximately 0.75C.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.01C, indicating continuing neutral conditions in the tropical Pacific. How long will conditions remain neutral, and what are the probabilities for developing an El Niño or re-developing a La Niña later in this year? The low-level easterly wind anomalies in the central and western Pacific that were reinforcing the shallow thermocline anomalies reappeared in August. Their main impact was to erode the warm anomalies that had been sitting in the eastern equatorial Pacific. Those easterly wind anomalies were likely associated with an MJO event, as they have subsequently disappeared. Most of the ocean-atmosphere indicators suggest that ENSO-neutral conditions are likely to continue not only during the Sep-Oct-Nov season in progress, but through most of the remainder of 2008 and early 2009.

Currently, the models see the development of El Niño conditions more likely than the re-emergence of La Niña conditions, although the model probabilities for El Nino reach only about 30% through most of the forecast period. The sub-set of the models that do see the possibility of developing El Niño conditions are primarily the dynamical models, rather than the statistical models. All things considered, there is an approximately 5% likelihood for either La Niña or El Nino conditions to develop during the Sep-Oct-Nov season, and a 90% probability for ENSO-neutral conditions.

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 0.40 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.

The models are showing fair agreement in their ENSO forecasts through the 10-month forecast period. For the current Sep-Oct-Nov season, the majority of the predictions indicate ENSO-neutral conditions. The majority of models also remain ENSO-neutral through 2008, while a couple models develop weak El Niño conditions. For the OND 2008 season, only 3 of 23 models (13%) predict El Niño conditions to develop, and none predict La Niña development. 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, only 1 of 16 (6%) indicates the existence of La Niña conditions for the Jan-Feb-Mar season; 10 of 16 (62%) predict that SSTs will be ENSO-neutral, and 5 of 16 (31%) predict development of El Niño conditions (Note 1). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

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 slightly higher probabilities for El Niño than for Niña conditions for the next several seasons, although probabilities for either one are low. For example, for OND and NDJ seasons, probabilities are about 10% for El Niño and about 2% for La Niña. This method shows probabilities for ENSO-neutral consistently being the most likely possibility at just over 85-90% probability. 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. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.

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 90% probability for ENSO-neutral conditions in the coming season.