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Technical ENSO Update

18 December 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-December 2008 SSTs are slightly below-average across the central and eastern Pacific. Although the mean conditions over the previous 1 month (mid-November to mid- December) are regarded as ENSO-neutral, conditions for the latest observed week (December 7-13) are at the borderline of being considered those of La Niña. Although the ocean-atmosphere system appeared to be largely uncoupled in October and November, during the last two weeks the La Niña conditions of the atmosphere have finally begun encompassing the upper ocean as well. More locally above-average SSTs farther east in the Pacific dissipated in late October and recently became slightly below-average, making for a more uniform anomaly pattern from the west-central Pacific to the coast of South America. Since early October 2008, a large fetch of easterly wind anomalies has been observed from the central to the west-central Pacific. Correspondingly both the traditional and equatorial Southern Oscillation Indices (SOI) have been consistently positive. The oceanic response to these atmospheric anomalies had been weak, but has just recently shown non-trivial equatorial cooling. Additionally, the subsurface ocean anomaly structure has recently shown an increase in negative net heat content. However, given the timing of the seasonal cycle, which has passed its coldest months in the eastern Pacific, it appears that if a La Niña event develops in the coming month or two, it would likely be weak and short-lived. Such brief cool conditions occurred during the boreal winters of 2000-2001 and also 2005-2006, but even in those two cases the SST crossed into La Niña conditions earlier than mid-December.

For November 2008, the SSTs in the NINO3.4 region were weakly below average (-0.18 degrees C anomaly), and for the Aug-Sep-Oct season they were -0.21 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 Nov-Dec-Jan and Dec-Jan-Feb seasons are approximately 0.70C and 0.65, respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.7 C, indicating neutral or weak La Niño conditions in the tropical Pacific. What are the probabilities for developing a La Niña in the coming months? Persistent and large-scale easterly wind anomalies along the equator in the western-to-central Pacific since early October have generated some shallow thermocline anomalies in the equatorial central and eastern Pacific, but after two months of relative oceanic unresponsiveness. In the same regions, SSTs have cooled slightly. The thermocline anomalies, which contitute the main element of the tropical Pacific system imparting ENSO predictability, have shoaled lately, particlularly in the eastern part of the Pacific.

Most of the ocean-atmosphere indicators suggest that conditions near the borderline of ENSO-neutral and weak La Niña are likely during the Dec-Jan-Feb season in progress, and into early 2009. The negative anomalies in the east-central Pacific may linger near this borderline, oscillating just above or below it from week to week, or could strengthen to some extent as the recently stronger sub-surface anomaly structure infuences the surface in the coming one to two months.

Currently, the models see very little possbility of developing El Niño conditions, but indicate probabilities of slightly less than 50% for La Niña conditions. The sub-set of the models that see the possibility of developing La Niña conditions are primarily dynamical models, 3 out of 23 of which even indicate moderate La Niña conditions (anomalies stronger than -1.0C). All things considered, during the Dec-Jan-Feb season there is a 48% probability for La Niña conditions, 52% probability for ENSO-neutral conditions, and a near-zero probability for El Niño 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 Dec-Jan-Feb season, slightly more than half of the models are predicting ENSO-neutral conditions, and slightly more than one-third predict weak La Niña conditions. Slightly more than a half of the models remain ENSO-neutral through mid-2009, while those that predict weak La Niña conditions show a return to normal by about February to March 2009. For the JFM 2009 season, 10 of 23 models (43%) predict La Niña conditions and only 1 of 23 (4%) predicts 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 5 of 21 (24%) indicate the existence of La Niña and similarly 1 of 21 (5%) for El Niño conditions for the Apr-May-Jun season; 15 of 21 (71%) 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 lower probabilities for La Niña conditions than for neutral conditions Niño conditions for the next several seasons, although probabilities for La Niña are approximately in the 30s or low 40s for DJF through MAM. Probabilities are less than 1% for El Niño through FMA, rising to climatological probabilities (25%) by boreal summer 2008. This method shows probabilities for ENSO-neutral consistently being the most likely possibility, at more than 60%, through AMJ. 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 nearly 50% probability for La Niña conditions in the coming (DJF) season.