Technical ENSO Update

18 August 2011

Current Conditions

Expected Conditions

Presently, the models and observations taken together indicate probabilities of approximately 63% for maintaining ENSO-neutral conditions, 36% for returning to La Niña conditions, and near 1% for developing El Niño conditions, during the Aug-Oct 2011 season in progress. Probabilities for La Niña rise to 44% for the Nov-Jan 2011/12 season. Probabilities for El Niño conditions remain below 10% through Feb-Apr 2012. Neutral conditions are the most likely scenario during the second half of 2011 and into 2012, with probabilities of near 55% during the final quarter of 2011, rising slightly in the beginning of 2012 before returning to the climatological value of 50% toward the middle of 2012. Although neutral conditions are most likely throughout the forecast periods, the probability for La Niña conditions reaches or exceeds 40% for four periods (Sep-Nov through Dec-Feb), which is well above its climatological level and must be considered seriously.

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 Aug-Oct season and thereafter through 2011 and into early 2012. For the Aug-Oct season, 92% of the models indicate neutral ENSO conditions, and 8% indicate weak La Niña conditions. For Nov-Jan, 80% still show neutral conditions and 20% 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, 12 of 18 (67%)  predict ENSO-neutral SSTs for the Dec-Feb season, 5 of 18 (28%) predict La Niña conditions, and 1 of 18 (6%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Dec-Feb require a NINO3.4 SST anomaly of -0.65 or stronger, and El Niño conditions require 0.65 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 13% for Aug-Oct, increasing to 18% for Oct-Dec and hovering near 20% for the rest of 2011 and into 2012. Model probabilities for El Niño are 2% for Aug-Oct, hovering near 5 or 6% from Sep-Nov 2011 through Jan-Mar 2012, not exceeding 20% until Apr-Jun 2012. Probabilities for ENSO-neutral conditions are 86% for Aug-Oct, settling to near 75% from Oct-Dec 2011 to Jan-Mar 2012 before decreasing toward 50% in 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 re-emergence of La Niña above the levels indicated by the models, based on the most recent observations. It indicates a 36% probability for La Niña conditions in the Aug-Oct season in progress, increasing to between 40 and 45% from Sep-Nov through Dec-Feb 2011-12. Probabilities for El Niño conditions remain below 10% until Mar-May 2012. Probabilities for ENSO-neutral conditions are higher than those of the other two categories for all seasons, starting at 63% for Aug-Oct, dropping to the middle 50s from Oct-Dec 2011 to Dec-Feb 2011-12, rising to near 60% from Jan-Mar to Feb-Apr 2012, and finally dropping toward 50% by late northern spring 2012.

See also: 

• Summary of model predictions