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Technical ENSO Update

19 May 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-May 2011, SST anomalies have weakened to the point of reflecting cool ENSO-neutral conditions in the central and eastern equatorial Pacific. For April the SST anomaly in the NINO3.4 region was -0.77 C, indicative of weak La Niña conditions, and for the January-March season the anomaly was -1.01 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 -0.4 C, indicating cool ENSO-neutral conditions in the tropical Pacific; this is slightly weaker than the -0.77 C level observed in April. The moderate to strong La Niña episode came to an end during early to middle May as the SST anomaly finally warmed beyond the -0.5C level, and the Southern Oscillation Index (SOI) suddenly dropped from near-record levels in April to near average by early May. What is the outlook for the ENSO status going forward? May and June often represent a time of the year of transition of the ENSO state, when ENSO episodes are ending or new episodes are just beginning to grow. The currently warmer than average subsurface temperatures across the equatorial Pacific imply fairly low probabilities of returning to La Niña conditions in the coming few months, and even a possibility of El Niño development. However, because of the strong and long-lasting atmospheric component of the now decayed La Niña, some of the climate effects associated with La Niña may continue to linger through early June. Whether the somewhat positive subsurface sea temperature anomalies will surface in the east-central tropical Pacific and induce El Niño conditions in the coming few months is currently uncertain, since this is still the time of year when future ENSO evolution is most difficult to predict.

Presently, the models and observations taken together indicate probabilities of approximately 63% for maintaining ENSO-neutral conditions, 24% for returning to La Niña conditions, and near 13% for developing El Niño conditions, during the May-Jul 2011 season in progress. Probabilities for La Niña decrease to 22% for Jun-Aug, and to 21% for Jul-Sep. Meanwhile, probabilities for El Niño conditions increase to 21% for Jun-Aug, and to 23% for Jul-Sep. From Jun-Aug onward, probabilities for La Niña, neutral and El Niño conditions remain at 21%, 56% and 23%, respectively, making neutral conditions the most likely scenario during the second half of 2011.

The above assessment was made in part on the basis of an examination of the current predictions 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 majority of the dynamical and statistical models show neutral conditions for the May-Jul season and thereafter through the remainder of 2011. For the May-Jul season, 87% of the models indicate neutral ENSO conditions, and 13% indicate weak La Niña conditions. By Jun-Aug, 83% show neutral conditions, 4% show La Niña conditions, and 13% show 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 subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 9 of 16 (56%) predict ENSO-neutral SSTs for the Sep-Nov season, 2 of 16 (12%) predict La Niña conditions, and 5 of 16 (31%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Sep-Nov require a NINO3.4 SST anomaly of -0.75 or stronger, and El Niño conditions require 0.70 or stronger.) Caution is advised in interpreting the distribution of model predictions 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 predictions 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 Niña at 20% for May-Jul, declining to 15% for Jun-Aug and 14% for Jul-Sep. Model probabilities for El Niño increase to 24% for Jul-Sep and to 27% for Aug-Oct, remaining between 22 and 28% thereafter. Probabilities for ENSO-neutral conditions are close to 60% for Jul-Sep and beyond. The same cautions mentioned above for the distribution of model predictions 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 prediction 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 24% probability for La Niña conditions in the May-Jul season in progress, decreasing to 22% for Jul-Sep. Probabilities for El Niño conditions rise from 13% in May-Jul to 21% in Jun-Aug, and 23% by Jul-Sep and later seasons. Probabilities for ENSO-neutral conditions are higher than those of the other two categories for all seasons, and settle to 56% from Jul-Sep through the rest of 2011.