Technical ENSO Update

16 June 2011

Current Conditions

Expected Conditions

Presently, the models and observations taken together indicate probabilities of approximately 84% for maintaining ENSO-neutral conditions, 9% for returning to La Niña conditions, and near 7% for developing El Niño conditions, during the Jun-Aug 2011 season in progress. Probabilities for La Niña remain near 15% from Jul-Sep through the remainder of 2011. Similarly, probabilities for El Niño conditions increase to 15% for Jul-Sep and remain close to that level throughout the rest of the calendar year. Hence, neutral conditions are clearly the most likely scenario during the second half of 2011 and into early 2012, with probabilities hovering near 70%.

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 Jun-Aug season and thereafter through the remainder of 2011. For the Jun-Aug season, 87% of the models indicate neutral ENSO conditions, and 9% indicate weak El Niño conditions. By Jul-Sep, 78% show neutral conditions, 9% 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, 11 of 16 (69%)  predict ENSO-neutral SSTs for the Oct-Dec season, 2 of 16 (12%) predict La Niña conditions, and 3 of 16 (19%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Oct-Dec require a NINO3.4 SST anomaly of -0.75 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 9% for Jun-Aug, increasing to 12% for Jul-Sep and hovering near 14% throughout the remainder of 2011. Model probabilities for El Niño are 5% for Jun-Aug, rising to 15% for Jul-Sep and hovering near 17% through the rest of 2011. Probabilities for ENSO-neutral conditions are 85% for Jun-Aug, settling to near 70% throughout the remainder of 2011 and into early 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 9% probability for La Niña conditions in the Jun-Aug season in progress, increasing slightly to near 15% thereafter, into early 2012. Probabilities for El Niño conditions rise from 7% in Jun-Aug to near 15% thereafter through 2011 and into early 2012. Probabilities for ENSO-neutral conditions are considerably higher than those of the other two categories for all seasons, starting at 84% for Jun-Aug and settling to near 70% thereafter, dropping just slightly by northern spring 2012.

See also: 

• Summary of model predictions