Technical ENSO Update

15 September 2011

Current Conditions

Expected Conditions

Presently,the models and observations taken together indicate probabilities of approximately 52% for maintaining at least weak La Niña conditions, 48% for returning to ENSO-neutral conditions, and near 0% for developing El Niño conditions, during the Sep-Nov 2011 season in progress. Probabilities for La Niña rise to 53% for the Oct-Dec 2011 season. Probabilities for El Niño conditions remain below 10% through Feb-Apr 2012. La Niña conditions are the most likely scenario during the remainder of 2011 and into very early 2012, but by a fairly small margin over neutral conditions. La Niña probabilities decrease to 48% by Jan-Mar 2012 before returning to the climatological value of 25% by the middle of 2012. During the remainder of 2011, the probability for neutral conditions hovers between 45 and 50%.

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.

A majority of the dynamical and statistical models continue to show neutral conditions for the Sep-Nov season and thereafter through 2011 and into early 2012. For the Sep-Nov season, 71% of the models indicate neutral ENSO conditions, and 29% indicate La Niña conditions. For Nov-Jan, 75% still show neutral conditions and 25% 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, 11 of 17 (65%)  predict ENSO-neutral SSTs for the Dec-Feb season, 5 of 17 (29%) predict La Niña conditions, and 1 of 18 (6%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Jan-Mar require a NINO3.4 SST anomaly of -0.55 or stronger, and El Niño conditions require 0.50 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 22% for Sep-Nov, increasing to 29% for Nov-Jan and hovering between 25 and 32% into early 2012. Model probabilities for El Niño are 1% for Oct-Dec and remain below 10% through Feb-Apr 2012. Probabilities for ENSO-neutral conditions are 78% for Sep-Nov and stay above 65% through Feb-Apr 2012, finally decreasing toward 50% by middle to late northern spring 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. In the case of this forecast, the forecasters have increased the probability of continuation of La Niña above the levels indicated by the models, based on the recent and current La Niña observations and the increasing negative heat content anomaly in the central and eastern tropical Pacific. It indicates a 52% probability for La Niña conditions in the Sep-Nov season in progress, remaining at or just above 50% through Dec-Feb 2011-12. Probabilities for El Niño conditions remain below 10% until Mar-May 2012. Probabilities for ENSO-neutral conditions are between 47 and 49% through Jan-Mar 2012. This indicates that while La Niña is slightly more likely than neutral conditions until the beginning of 2012, a return to neutral conditions during the remainder of 2011 is still a definite possibility with nearly 50% likelihood.

See also: 

• Summary of model predictions