Technical ENSO Update

15 July 2010

Current Conditions

Expected Conditions

Presently, the models and observations taken together indicate probabilities of about 80% for developing La Niña conditions, 20% for returning to ENSO-neutral conditions, and near 0% for developing El Niño conditions during the July-August-September season in progress. Going forward, probabilities continue at approximately these same levels for the three- month periods extending through November-December-January 2010/11. In early 2011 the tilt of the odds toward La Niña begins to weaken, reaching about 52% by March-April-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.

The models show general agreement regarding the high probability for La Niña conditions over the remainder of 2010 and into early 2011, but nonetheless indicate some divergence throughout most of the period. The statistical models tend to call for weak La Niña strength (weaker than -1C), while the dynamical models tend to predict moderate strength (-1 to -1.5C). Collectively the average of the model predictions is for weak to moderate La Niña conditions. For the current Jul-Aug-Sep season, 82% of the models are predicting La Niña conditions, while 18% predict ENSO-neutral conditions. This tilt of the odds is maintained in the model predictions throughout the remainder of 2010. Several of the dynamical models predict the maximum La Niña strength to occur in the coming two to three months, in contrast to the usual peak strength near the end of the year. 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, none indicate El Niño conditions for the Nov-Dec-Jan season, 4 of 14 (29%)  predict ENSO-neutral SSTs, and 10 of 14 (71%) predict La Niña conditions. (Note 1). (Note that La Niña conditions for Nov-Dec-Jan require a NINO3.4 SST anomaly of -0.70 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 about 90% and 82% for Jul-Aug-Sep and Aug-Sep-Oct, respectively, and approximately 65% from Sep-Oct-Nov through Dec-Jan-Feb. (Note that the threshold for La Niña increases from approximately -0.50C to -0.75C between Jul-Aug-Sep and Oct-Nov-Dec, and some of the model predictions for La Niña do not increase in strength in parallel with this seasonality of the interannual variance). 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 an 80% probability for La Niña conditions in the Jul-Aug-Sep season in progress, and approximately the same probability through Nov-Dec-Jan 2010/11, while probabilities for ENSO-neutral conditions hover near 20% during this same period. The probability for a return to El Niño conditions remain at very low levels from the present until early 2011.

See also: 

• Summary of model predictions