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Technical ENSO Update

20 November 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-October 2008 SSTs in the central equatorial Pacific are near their long-term average, being very slightly below-average across the central Pacific. In the eastern Pacific, the SSTs had been above-average over the past several months, but they did not draw the cooperation of the atmosphere to evolve towards El Niño conditions. These warm anomalies have largely subsided, with temperatures returning to average conditions and just a few localized areas of above-average SST. The equatorial Southern Oscillation index (SOI) has remained positive, but has been weakening since July. The traditional Tahiti-Darwin SOI increased over the last month, which may be related to intraseasonal variability and associated high pressure center moving over the region of Tahiti, rather than an indication of La Nina development. Taken together, the current anomalous ocean-atmosphere conditions indicate continuation of ENSO-neutral conditions.

For September 2008, the SSTs in the NINO3.4 region were weakly below average (-0.16 degrees C anomaly). For the Jul-Aug-Sep season they were 0.03 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 Oct-Nov-Dec and Nov-Dec-Jan seasons are approximately 0.75C.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.3 C, 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 a La Niña in the coming months? Currently there is little to no slowly evolving structure of the upper tropical Pacific Ocean, which is the main element of the tropical Pacific system imparting ENSO predictability. Low-level wind anomalies, mainly easterly anomalies, have come and gone over the last several months due to a combination of intraseasonal weather fluctuations and the anomalous zonal temperature gradients between the central and western Pacific. However, they have not produced a large enough or persistent enough effect on the subsurface ocean structure that could translate into large-scale SST anomalies in the eastern and central equatorial Pacific. A succession of downwelling and upwelling Kelvin waves are propagating across the equatorial Pacific, which will likely have little impact on the surface. The tropical Pacific ocean-atmosphere system appears to be largely open to the influence of atmospheric weather disturbances at this time, the timing of which is difficult to predict months in advance. If intraseasonal wind anomalies were to happen to constructively interfer with a propagating Kelvin wave, it may be possible to trigger an either warm or cool ENSO event that is not currently foreseen.

Most of the ocean-atmosphere indicators suggest that ENSO-neutral conditions are likely to continue not only during the Oct-Nov-Dec season in progress, but through the remainder of 2008 and early 2009.

Currently, the models see very little possbility of developing either El Niño or La Niña conditions, although the model probabilities for La Niña are slightly higher. The tiny sub-set of the models that do see the possibility of developing La Niña conditions are primarily the dynamical models, rather than the statistical models. All things considered, during the Sep-Oct-Nov-Dec season there is 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 Oct-Nov-Dec season, the majority of the predictions indicate ENSO-neutral conditions. The majority of models remain ENSO-neutral through mid-2009, while a couple models develop either weak El Niño or La Niña conditions. For the NDJ 2008/09 season, only 1 of 21 models (5%) predict La Niña conditions to develop, and none predict El Niño 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 2 of 14 (14%) indicates the existence of La Niña and similarly 2 of 14 (14%) for El Niño conditions for the Feb-Mar-Apr season; 10 of 14 (72%) 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 slightly higher probabilities for La Niña conditions than for El Niño conditions for the next several seasons, although probabilities for either one are low. For example, for DJF seasons, probabilities are about 1% for El Niño and about 10% for La Niña. This method shows probabilities for ENSO-neutral consistently being the most likely possibility at over 90% probability through the end of the year, and still the most likely outcome through mid-2009. 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.