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Technical ENSO Update

19 August 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-August 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 have been above average over the past several months, but have not drawn the cooperation of the atmosphere to evolve towards El Niño conditions. The deep thermocline anomalies in the western Pacific that were created during the 2007/08 La Niña event and then subsequently adjusted onto the equator, led to the warm SST anomalies in the eastern Pacific over the past several months. The heat content associated with the deep thermocline anomalies has been above average primarily in the western and eastern Pacific, but has been dissipating over the last couple months. The equatorial Southern Oscillation index (SOI) has remained slightly positive, even though the traditional Tahiti-Darwin SOI has been close to zero since May. Related to the anomalous pattern of surface pressure, the low-level wind anomalies continue to be easterly, but weak, since May. Taken together, the current anomalous ocean-atmosphere conditions indicate ENSO-neutral conditions as of early June.

For July 2008, the SSTs in the NINO3.4 region shifted to above average (0.12 degrees C anomaly). For the Apr-May-Jun season they were -0.26 degrees C from average, weaker than the previous recent running 3-month periods and indicating that the La Niña event has ended. 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 Jul-Aug-Sep and Aug-Sep-Oct seasons are approximately 0.5C, but will exceed 0.5C in Sep-Oct-Nov and later seasons.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.3 C, this index being positive since early July 2008. However, it is still weak and only indicates that neutral conditions have returned to 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 have been near average for more than two months. The positive heat content anomalies, associated with deep thermocline anomalies generated during the 2007/08 La Niña, eventuslly appeared in the eastern Pacific leading to positive SST anomalies since April 2008. However, those temperatures anomalies were not reinforced by atmospheric feedbacks, and so the heat content has been declining. There is some possibility that the remaining positive heat content anomalies, although not very strong , could still initiate El Niño conditions. However, most of the ocean-atmosphere indicators suggest that ENSO-neutral conditions are likely to continue not only during the Aug-Sep-Oct season in progress, but through most of the remainder of 2008 and early 2009.

Currently, the models see the development of El Niño conditions slightly more likely than the re-emergence of La Niña conditions, although the probabilities for either reach only about 20% 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 La Niña conditions to re-emerge during the Aug-Sep-Oct season, a 15% likelihood for El Niño conditions to develop, and a 80% 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.

We are well through the [boreal] "Spring Barrier" to ENSO prediction. Therefore, the models are showing fair agreement in their ENSO forecasts through the 10-month forecast period. For the current Aug-Sep-Oct 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 SON 2008 season, only 3 of 23 models (13%) predict El Niño conditions to develop, and none predict La Niña to re-emerge. 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, none indicate the existence of La Niña conditions for the Dec-Jan-Feb season; 12 of 16 (75%) predict that SSTs will be ENSO-neutral, and 4 of 16 (25%) 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 SON and OND seasons, probabilities are about 17% for El Niño and about 6% for La Niña. This method shows probabilities for ENSO-neutral consistently being the most likely possibility at just over 85% probability, and holding at about 75% likely through the end of 2008. 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 80% probability for ENSO-neutral conditions in the coming season.