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Technical ENSO Update

17 December 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-December 2009, SSTs are above-average throughout the central and eastern equatorial Pacific, indicative of El Niño conditions. Between mid-June and mid-July, SSTs in the east-central tropical Pacific warmed to levels indicative of weak El Niño conditions. Starting in October, strong and persistent westerly wind anomalies in the western Pacific, extending into the central Pacific, substantially increased the magnitude of the warm SST anomalies. The traditional Southern Oscillation Index (SOI) became negative during October, as did the equatorial SOI. The traditional SOI has remained negative since then, although the equatorial SOI has weakened. Positive convection anomalies were observed intermittently near and just west of the dateline between June and September, and became somewhat stronger and more persistent since October. On the oceanic side, equatorial heat content had been above-average since early in the year, and became stronger since October. The SST anomalies may strengthen still further in response to recent westerly wind anomalies that are in the process of deepening the thermocline in the central and eastern part of the basin.

For November 2009, the SST anomaly in the NINO3.4 region was 1.67 C, sufficient to be classified as moderate(+) El Niño conditions for this time of year. For the Sep-Oct-Nov season the anomaly was 1.18 degrees C. 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 NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Dec-Jan-Feb and the Jan-Feb-Mar seasons are approximately (-0.65C, 0.65) and (-0.55, 0.50), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 1.8 C, indicating moderate-to-strong El Niño conditions in the tropical Pacific. What is the outlook for the ENSO status going forward? The strengthened westerly wind anomalies along the equator in the western Pacific in October, and again since late November, provided substantial positive depth anomalies to the central and eastern thermocline. Since October, the spatial pattern of SST anomalies, and of the subsurface temperature anomalies, has become much more structured and reminiscent of El Niño events. It is still not clear, however, to what extent this pattern of SST anomalies will induce a strong, multi-month atmospheric response that will lead to air-sea couping of the type observed in strong El Niño events. Significant positive convection anomalies currently exist near and just west of the dateline, and while this is already representative of air-sea coupling, eastward expansion of the anomalous convection would allow for still greater coupling.

December is the time of year when existing ENSO events are near their mature phase, and typically persist for several subsequent months while slowly weakening. For this event, it seems most likely that El Niño conditions will persist at least into February 2010, and given the current subsurface anomalies, may still strengthen slightly more before beginning to weaken. A premature dissipation, such as that observed in early 2007, seems unlikely for this event.

Presently, the models and observations taken together indicate probabilities of about 98% for maintaining El Niño conditions and about 2% for dissipation to ENSO-neutral conditions for the Dec-Jan-Feb season in progress. Going forward, probabilities for El Niño stay at or above 90% until Feb-Mar-Apr, after which they decrease to approximately 55% by Apr-May-Jun 2010, falling to climatological probabilities of 25% by Jul-Aug-Sep. Probabilities for La Niña conditions are predicted to be negligible for the next several months, not reaching 10% until Apr-May-Jun 2010.

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 boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal 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 in general agreement in their ENSO forecasts through the 10-month forecast period. The statistical and dynamical models agree in that they predict El Niño conditions through the end of this year. The details of their predictions differ, however. The warmest NINO3.4 forecasts come from the dynamical models. For the current Dec-Jan-Feb season, 100% of the models are predicting El Niño conditions; none predict ENSO-neutral conditions. At lead times of 4 or more 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, 11 of 14 (79%) indicate El Niño conditions for the Apr-May-Jun season, and 3 of 14 (21%) predict ENSO-neutral SSTs. (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. This method shows probabilities for El Niño at about 100% and 99% for Dec-Jan-Feb and Jan-Feb-Mar, respectively, declining to near 65% by Apr-May-Jun 2010 and 50% by May-Jun-Jul. 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 98% probability for El Niño conditions in the Dec-Jan-Feb season in progress, slowly decreasing to near 80% for Mar-Apr-May 2010 and decreasing rapidly thereafter.