ENSO Quick Look
IRI ENSO Update / Forecast
IRI Probabilistic ENSO Forecast
Technical ENSO Update
ENSO Prediction Plume
Monthly archive

Technical ENSO Update

20 May 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-May 2010, SSTs have returned to ENSO-neutral levels throughout the central and eastern equatorial Pacific. Moderate(+) El Niño conditions prevailed from late-October until mid-February, during which the traditional and the equatorial Southern Oscillation Index (SOI) were mainly negative. Positive convection anomalies were observed in the vicinity of the dateline, most prominently between October and March, becoming particularly strong from late January to early March. Equatorial Pacific oceanic heat content was above-average during most of 2009, becoming strongest from late October through early March. A slow decline in the strength of the El Niño allowed it to endure through all of April, finally dissipating rapidly during the first half of May. For April 2009, the SST anomaly in the NINO3.4 region was 0.70 C, sufficient to be classified as weak El Niño conditions for this time of year. For the Feb-Mar-Apr season the anomaly was 1.03 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 May-Jun-Jul and the Jun-Jul-Aug seasons are approximately (-0.50C, 0.45) and (-0.50, 0.45), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.1 C, indicating ENSO-neutral conditions in the tropical Pacific; this is considerably lower than the 0.70 C level observed in April, indicating that we are in a state of transition. What is the outlook for the ENSO status going forward? The strongly positive convection anomalies that were observed near and just east of the dateline early in 2010, that produced many of the historically observed teleconnected climate effects of El Niño, have now been replaced by near-zero and some negative convection anomalies along the immediate equator. May is the month of the year historically most indicative of transitions in the ENSO state. With the demise of the moderate(+) El Niño, the question now is whether the current ENSO-neutral condition will persist over the coming several months, or rather represents an intermediate point within a transition toward a La Niña state. A clue to the answer resides in the fact that negative subsurface sea temperature anomalies, initially residing in the western tropical Pacific in February and March, have expanded at depth into the central and eastern Pacific during the last two months, overlain by an increasingly weak and shallow layer of residual postive sea temperature anomalies near the surface. Sea temperature anomalies below the surface often precede sea surface temperature anomalies to be expected in the following months, particularly as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies indicate slightly enhanced trades across most of the tropical Pacific, and positive traditional and equatorial SOI indices. These atmospheric features imply average to somewhat above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely find their way to the surface during the coming months, providing the possibility for developing a cool/neutral ENSO state and possibly La Niña conditions.

Presently, the models and observations taken together indicate probabilities of about 80% for maintaining ENSO-neutral conditions, 13% for developing La Niña conditions, and 7% for returning to El Niño conditions during the Apr-May-June season in progress. Going forward, probabilities for La Niña increase to approximately 40% by Jul-Aug-Sep, while probabilities for neutral and El Niño conditions become 57% and 3%, respectively. During the seasons of Aug-Sep-Oct and through early 2011, the probabilities for La Niña, neutral and El Niño conditions remain at 42%, 55% and 3%, 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 show general agreement regarding the likely lack of El Niño conditions over the remainder of 2010 and into 2011, but nonetheless indicate considerable divergence throughout most of the period. Most of the statistical models predict neutral ENSO conditions through the period, while the dynamical models tend to predict either low-neutral or some magnitude of La Niña condition. Collectively the models average in the low-neutral range, closer to the Niña threshold than to zero anomaly. For the current May-Jun-Jul season, 83% of the models are predicting ENSO-neutral conditions, while 9% apiece predict El Niño and La Niña conditions, respectively. At lead times of 4 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 0 of 11 (0%) indicate El Niño conditions for the Oct-Nov-Dec season, 8 of 12 (67%) predict ENSO-neutral SSTs, and 4 of 12 (33%) predict La Niña conditions. (Note 1). (Note that La Niño conditions for Oct-Nov-Dec 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 La Niño at about 8% and 23% for May-Jun-Jul and Jun-Jul-Aug, respectively, increasing to 38% by Jul-Aug-Sep and about 35-40% from Jul-Aug-Sep onward through early 2011. It should be noted, however, that several statistical models do not use the subsurface temperature anomalies as predictors, and are thus missing the explicit information that the subsurface is now considerably cooler than average. 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 13% probability for La Niña conditions in the May-Jun-Jul season in progress, increasing to near 40% for Jul-Aug-Sep and to 42% from Aug-Sep-Oct forward into early 2011, while ENSO-neutral probabilities begin at 80% for May-Jun-Jul and decrease to 55% from Aug-Sep-Oct onward. Probability for a return to El Niño conditions begin at just 7% for May-Jun-Jul and remain at 5% or less from Jun-Jul-Aug onward into early 2011.