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Technical ENSO Update

18 June 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-June 2008 SSTs in the central equatorial Pacific remain below average, but have been trending towards average conditions since February 2008. Over the past months, the cold anomalies have lessened in the eastern equatorial Pacific to the point where the SSTs are no longer indicative of La Niña conditions. The equatorial Southern Oscillation index (SOI) continues to be positive but has weakened considerably over the last couple months, while the traditional Tahiti-Darwin SOI became negative in May. The deep thermocline anomalies in the western Pacific have been adjusting onto the equator and have contributed to the reduction in the La Niña conditions in the far eastern Pacific. The low-level wind anomalies related to the La Niña conditions have also weakened in May. The equatorial heat content is now above average throughout the equatorial Pacific. Taken together, these anomalous ocean-atmosphere conditions indicate the return towards neutral conditions.

For May 2008, the SSTs in the NINO3.4 region remained below average (-0.59 degrees C anomaly), although the weekly values have been less than -0.5 since mid-May. For the Mar-Apr-May season they were -0.84 degrees C from average, indicating that the La Niña event steadily weakened since February. 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 Jun-Jul-Aug and Jul-Aug-Sep seasons are approximately 0.5C.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at -0.4 C, down from -1.9C in February and -0.6C a month ago, suggests the continued weakening of the La Niña conditions. It is clear that neutral conditions are returning to the tropical Pacific. The question now is how long will conditions remain neutral, and what are the probabilities for developing an El Niño or re-developing 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 are currently near average. The low-level easterly wind anomalies from several months ago associated with the La Niña event, however, left behind a build-up of warm water in the western equatorial Pacific that has been adjusting onto the equator. As the deep thermocline anomalies adjust they have been bringing the heat content up, which is now positive along most of the equatorial Pacific. It is possible that the positive heat content anomalies may even help initiate El Niño conditions. Most of the ocean-atmosphere indicators indicate the near-demise of La Niña conditions, and most of the models suggest that ENSO-neutral conditions are likely to be in place during the Jun-Jul-Aug season.

At this time the models see the development of El Niño conditions about as likely as the re-emergence of La Niña conditions, although the probabilities for either reach only above 15% through the forecast period. For the most part, the sub-set of the models that do see the possibility of developing El Niño conditions are the dynamical models, rather than the statistical models. All things considered, there is an approximately 15% likelihood for La Niña conditions to continue during the Jun-Jul-Aug season, and a 75% 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 passing out of the [boreal] "Spring Barrier" to ENSO prediction. Therefore, the models are again showing greater agreement in their ENSO forecasts through the 10-month forecast period. Starting with the current Jun-Jul-Aug season, the majority of the predictions indicate ENSO-neutral conditions. The majority of models then remain ENSO-neutral through 2008, while a couple models develop weak El Niño conditions and a couple re-develop weak La Niña conditions. For the JAS 2008 season, only 2 of 23 models (9%) predict La Niña conditions to exist. 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 Oct-Nov-Dec season, and 14 of 15 (93%) predict that SSTs will be ENSO-neutral (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 alightly higher probabilities for La Niña for the next several seasons. For those probabilities, La Niña is 16% likely for the Jul-Aug-Sep season. The probabilities for La Niña then remain steady through 2008. This method shows probabilities for ENSO-neutral beginning as the most likely possibility at 73% probability, and holding around 70% likely through the end of 2008. El Niño likelihood is estimated at between 10-15% likely 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. 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 75% probability for ENSO-neutral conditions in the coming season.