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Technical ENSO Update

21 July 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-July 2011, SST anomalies remain close to 0, reflecting ENSO-neutral conditions in the central and eastern equatorial Pacific. For June the SST anomaly in the NINO3.4 region was -0.18 C, indicative of neutral conditions, and for the April-June season the anomaly was -0.46 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 Jul-Aug-Sep and the Aug-Sep-Oct seasons are approximately (-0.50C, 0.45) and (-0.55, 0.50), 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 close to the -0.18 C level observed in June. A moderate to strong La Niña episode ended during early May as the SST anomaly warmed past the -0.5C level and the Southern Oscillation Index (SOI) dropped from near-record levels in April to near neutral by early May. What is the outlook for the ENSO status going forward? July and August often represent a time of the year of possible development of a new ENSO state. The currently near-average subsurface temperatures across the equatorial Pacific imply fairly low probabilities of returning to La Niña conditions in the short term, and also only a slight possibility for El Niño development during the coming few months. Now that the "northern spring predictability barrier" is nearly completed, and the ENSO condition and evolutionary trajectory do not show any obvious anomaly or movement, the continuation of an ENSO-neutral state appears most likely for the remainder of 2011. However, some uncertainty nonetheless remains, and one cannot rule out the possibility for re-emergence of weak La Niña or development of weak El Niño in the September to November time frame. Between those two minor possibilities, re-emergence of weak La Niña appears relatively more likely, as a pocket of weakly below normal subsurface sea temperature has developed between 110W and 140W longitude, overlain by weakly above average water. This pocket of cooler water must be monitored carefully in the coming weeks.

Presently, the models and observations taken together indicate probabilities of approximately 82% for maintaining ENSO-neutral conditions, 10% for returning to La Niña conditions, and near 8% for developing El Niño conditions, during the Jul-Sep 2011 season in progress. Probabilities for La Niña rise to 25% or slightly higher from Oct-Dec into 2012. Probabilities for El Niño conditions increase only to about 14% from Sep-Nov into early 2012. Hence, neutral conditions are the most likely scenario during the second half of 2011 and into 2012, with probabilities hovering near 60% during the final seasons of 2011 and early 2012.

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 Jul-Sep season and thereafter through 2011 and into early 2012. For the Jul-Sep season, 96% of the models indicate neutral ENSO conditions, and 4% indicate weak La Niña conditions. For Sep-Nov, 96% still show neutral conditions and 4% show La Niña 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, 14 of 17 (82%) predict ENSO-neutral SSTs for the Nov-Jan season, 3 of 17 (18%) predict La Niña conditions, and 0 of 17 (0%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Nov-Jan require a NINO3.4 SST anomaly of -0.70 or stronger, and El Niño conditions require 0.75 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 10% for Jul-Sep, increasing to 13% for Aug-Oct and hovering between 14% and 20% for the rest of 2011 and into 2012. Model probabilities for El Niño are 6% for Jul-Sep, rising to 12% for Aug-Oct and hovering mainly between 10% and 12% through the rest of 2011 and into 2012. Probabilities for ENSO-neutral conditions are 84% for Jul-Sep, settling to near 70% throughout the remainder of 2011 and into 2012. 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 10% probability for La Niña conditions in the Jul-Sep season in progress, increasing to near 26% from Sep-Nov through early 2012. Probabilities for El Niño conditions rise from 8% in Jul-Sep to near 14% from Sep-Nov through early 2012. Probabilities for ENSO-neutral conditions are higher than those of the other two categories for all seasons, starting at 82% for Jul-Sep and settling to near 60% thereafter, dropping toward 50% by middle of northern spring 2012.