Technical ENSO Update

17 November 2011

Current Conditions

Expected Conditions

The dissipation of a moderate to strong La Niña in early May 2010 was followed by neutral ENSO conditions between May and early August, and then a return to weak La Niña conditions since that time. What is the outlook for the ENSO status going forward? The currently weak La Niña SSTs and continued below average subsurface temperatures across the equatorial Pacific imply a strong likelihood of maintaining at least weak La Niña conditions in the short term, with some chance for further strengthening, possibly into the moderate strength range, during the coming month before beginning to weaken in early 2012.

Presently, the models and observations taken together indicate probabilities of approximately 74% for maintaining at least weak La Niña conditions, 26% for returning to ENSO-neutral conditions, and near 0% for developing El Niño conditions during the Nov-Jan 2011-12 season in progress. Probabilities for La Niña stay at 60% or higher through the Jan-Mar 2012 season. Probabilities for El Niño conditions remain below 10% through Feb-Apr 2012. Although La Niña conditions are the most likely scenario during the remainder of 2011 and into early 2012, a return to neutral conditions during this time period also has a slight possibility. La Niña probabilities decrease to 51% by Feb-Apr 2012 before returning to the climatological value of 25% by the May-Jul season 2012. The probability for neutral conditions rises to 44% by the Feb-Apr season, and equals or exceeds 50% beginning in Mar-May.

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.

As of mid-November, most of the dynamical and statistical models predict La Niña conditions for the Nov-Jan season and for the subsequent seasons in early 2012. For the Nov-Jan season, 67% of the models indicate La Niña conditions, and 33% indicate neutral conditions. For Dec-Feb, these figures become 62% and 38%, 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, 61%  predict ENSO-neutral SSTs for the Mar-May 2012 season, 39% predict La Niña conditions, and none predict El Niño conditions. (Note 1). (Note that La Niña conditions for Mar-May require a NINO3.4 SST anomaly of -0.40 or stronger, and El Niño conditions require 0.40 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 76% for Nov-Jan, slowly decreasing to 73% for Dec-Feb, 71% for Jan-Mar, and to 0.46% by Mar-May. Model probabilities for El Niño are near 0% for Nov-Jan and remain below 10% through Apr-Jun 2012. Probabilities for ENSO-neutral conditions are 24% for Nov-Jan and rise slowly to 29% by Jan-Mar and more quickly thereafter, reaching or exceeding 50% beginning in Mar-May. 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 74% probability for La Niña conditions in the Nov-Jan season in progress, dropping to 69% by Dec-Feb 2011-12 and to only 37% by Mar-May 2012. Probabilities for ENSO-neutral conditions remain less than 40% through Jan-Mar 2012, rising to 50% or higher from Mar-May 2012 onward. These probabilities indicate that continuation of La Niña is more likely than neutral conditions through Feb-Apr 2012, and that a return to neutral conditions during the remainder of 2011 or very early 2012, while possible, is relatively less likely than persistence of at least weak La Niña conditions. Probabilities for El Niño conditions do not rise to 5% until Feb-Apr 2012, and remain below 20% until Apr-Jun 2012.

See also: 

• Summary of model predictions