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Technical ENSO Update

15 April 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-April 2010, SSTs are still somewhat above-average throughout the central and most of the eastern equatorial Pacific, indicative of weak El Niño conditions. Moderate(+) El Niño conditions were observed from late-October until mid-February, due to the accumulated effects of intermittently strong westerly wind anomalies in the western and/or central Pacific. Both the traditional and the equatorial Southern Oscillation Index (SOI) were negative during that period, with the equatorial SOI most negative in January and February. Positive convection anomalies were observed in the vicinity of the dateline, most prominently between October and February. Between late January and early March convection anomalies became stronger, located near and just east of the dateline so as to efficiently influence the zonal and meridional atmospheric circulation patterns as observed during previous moderate to strong El Nino episodes. Equatorial Pacific oceanic heat content had been above-average since early in 2009, but became more strongly so beginning late October. During January and early February 2010 the heat content anomaly somewhat decreased, but remained well above average into mid-March due to a late-occurring westerly wind anomaly in the western and central equatorial Pacific. That relative pause in the depletion of anomalous sub-surface heat allowed for a continuation of positive SST anomalies through the end of March, resulting in only a slow decline in the strength of the El Niño with the progession of northern spring.

For March 2009, the SST anomaly in the NINO3.4 region was 1.14 C, sufficient to be classified as moderate El Niño conditions for this time of year. For the Dec-Jan-Feb season the anomaly was 1.32 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 Apr-May-Jun and the May-Jun-Jul seasons are approximately (-0.45C, 0.45) and (-0.50, 0.45), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.8 C, indicating weak El Niño conditions in the tropical Pacific, somewhat lower than the 1.14 C level observed in March. What is the outlook for the ENSO status going forward? Strongly positive convection anomalies near and just east of the dateline from late January to early March produced many of the historically observed teleconnected climate effects. However, the convection anomalies have decreased markedly over the last several weeks as both the oceanic and atmospheric indicators of El Niño weakened. April is the time of year when existing ENSO events are often in their final waning phase, if dissipation has not already ocurred. It seems most likely that this El Niño will persist at least through the remainder of April. However, a double-year event (such as what occurred in 1986-87-88) does not appear likely, as negative subsurface sea temperature anomalies, initially residing in the western tropical Pacific in February and March, have been expanding at depth into the central and east-central Pacific in the most recent weeks. Meanwhile, the downwelling Kelvin wave associated with the above-mentioned zonal wind anomalies in January and February is beginning to manifest itself in the far eastern tropical Pacific SSTs. Hence, the SST component of this El Niño may endure longest in the NINO1+2 index region, of greatest relevance to the near-coastal waters of northern Peru and Ecuador, in late April and early May.

Presently, the models and observations taken together indicate probabilities of about 38% for maintaining El Niño conditions and about 57% for dissipation to ENSO-neutral conditions for the Apr-May-June season in progress. Going forward, probabilities for El Niño decrease to approximately 28% by May-Jun-Jul, falling to 20% (just below the climatological probability of 25%) by Jun-Jul-Aug. Probabilities for La Niña conditions are predicted to be negligible until May-Jun-Jul, when they rise to 10%. By Jul-Aug-Sep, the probability for La Niña rises to 25% and during the later portion of 2010 the La Niña and El Niño probabilities become 35% and 10%, respectively.

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 rough agreement in their ENSO forecasts for the first one or two seasons of the 10-month forecast period, but diverage considerably beginning in northern summer. The statistical and dynamical models agree in predicting a weakening and demise of El Niño conditions during the next 1 to 3 months, but the details of the rate of dissipation, and what happens following the dissipation, differ among models. For northern summer onward, some models predict warm-neutral ENSO, some neutral ENSO, some cool-neutral ENSO, and some La Nina. For the current Apr-May-Jun season, 45% of the models are predicting El Niño 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, 0 of 11 (0%) indicate El Niño conditions for the Sep-Oct-Nov season, 10 of 11 (91%) predict ENSO-neutral SSTs, and 1 of 11 (9%) predict La Niña conditions. (Note 1). (Note that La Niño conditions for Sep-Oct-Nov require a NINO3.4 SST anomaly of -0.75 or stronger.) 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 35% and 20% for Apr-May-Jun and May-Jun-Jul, respectively, declining to 12% by Jun-Jul-Aug and 10% from Jul-Aug-Sep onward into early 2011. Probabilities for La Niña increase to 30-35% from Jul-Aug-Sep onward through early 2011. 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 38% probability for El Niño conditions in the Apr-May-Jun season in progress, decreasing to near 20% for Jun-Jul-Aug and to 10% by Oct-Nov-Dec and into early 2011, while La Niña probabilities rise to 30% beginning Aug-Sep-Oct, and to 35% from Oct-Nov-Dec through early 2011. Neutral ENSO conditions have a 60-65% likelihood from May-Jun-Jul through Jul-Aug-Sep, decreasing to approximately 55% from Oct-Nov-Dec onward into early 2011.