ENSO Quick Look
IRI ENSO Update / Forecast
IRI Probabilistic ENSO Forecast
Technical ENSO Update
ENSO Prediction Plume
Monthly archive

Technical ENSO Update

14 March 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-March 2007 SSTs are near their climatological average in the central and eastern equatorial Pacific. SST anomalies warmer than 0.5C are confined to the western equatorial Pacific, west of the dateline. Following the 2006/07 El Niño event, which peaked in December, warm SST anomalies were largely replaced by cool anomalies within the NINO3 ENSO index region (5S-5N. 150W-90W) between January and February. These resulted from shallow equatorial thermocline, a result of equatorial adjustment following the El Niño. Anomalously easterly Trade Winds helped bring the subsurface cold anomalies to the surface, which were then reinforced by the divergence of local winds away from the cold SST anomaly. Locally some of the cold anomalies reached below -2.0C in late February and early March. In recent weeks, however, the cold anomalies have abated, replaced by average SSTs. This has resulted from local atmospheric forcing. The subsurface ocean still shows shallow thermocline anomalies across the entire equatorial Pacific, with peak values of 30m shallower than average (about 3C colder than average at the level of the thermocline) in the eastern equatorial Pacific.

For February 2007, the SSTs in the NINO3.4 region were 0.11 degrees C above average, and for the December-January-February season were 0.71 degrees C above average. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Feb-Mar-Apr and Mar-Apr-May seasons is approximately 0.4C away from average.

Expected Conditions

The tropical Pacific air-sea system has transitioned out of the 2006/07 El Niño event, with some suggestion that it may transition all the way into La Niña conditions. Relevant aspects of this include the sizable shallow thermocline anomalies extending across the entire equatorial Pacific, the emergence of cold SST anomalies in the eastern equatorial Pacific, and enhancement of the easterly Trade Winds across much of the central and eastern tropical Pacific.

While many of the features in the tropical Pacific air-sea system are consistent with initiation of La Niña conditions -- shallow thermocline in the east, emergence of cold SST anomalies, and easterly wind anomalies -- considerable uncertainty remains over whether such conditions will lead to the growth of a La Niña event. For one thing, the ITCZ is closest to the equator at this time of the year, leading to weaker winds, and thus a weaker connection between subsurface and surface conditions. Still, the trend is currently toward cooling, and the cold/shallow subsurface anomalies necessary for further development of La Niña conditions exist. Thus there is a finite possibility that the system may reach La Niña conditions within the next several months. Probabilities initially favor ENSO-neutral conditions, but by mid-2007 the forecast suggests that weak La Niña conditions are more likely.

A re-emergence of El Niño conditions is very unlikely likely, with probabilities not exceeding 10% throughout the forecast period. There is an approximately 60% likelihood for ENSO-neutral conditions to start, with a 35% probability for La Niña conditions. By mid-2007, the probabilities for La Niña grow to 50%.

The above assessment was made in part on the basis of an examination of the current forecasts 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 0.45 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern 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.

There is some variation among ENSO model forecasts through the 10-month forecast period. Most predictions indicate ENSO-neutral conditions. A few models are forecasting La Niña conditions to develop during the forecast period. For the MAM 2007 season, 4 of 21 models (19%) predict El Niño conditions to re-emerge, and 4 of 21 models (19%) predict La Niña conditions to have developed. At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, 2 of 13 models (15%) suggest that El Niño conditions will re-emerge prior to the Jul-Aug-Sep season; 5 of 13 (38%) indicate SSTs will remain ENSO-neutral. The slight majority of dynamical models, 6 of 13 (46%), predict that SSTs will exceed the threshold for La Niña (Note 1) by the middle of 2007. A recent noted shortcoming of most of the models is that they did not capture the rate at which the tropical Pacific returned to average conditions over the last couple months. Caution is advised in interpreting the distribution of model forecasts 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 forecasts 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. When this method is applied to the current model forecasts, results indicate a greater probability for the ENSO-neutral category, ranging from 95% likely in the Mar-Apr-May season to approximately 55% likely my mid-2007. The probabilities for La Niña climb above the climatologically expected 25% by Jun-Jul-Aug, but do not exceed 35%. The same cautions mentioned above for the distribution of model forecasts 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 not their range, which is considerable in this forecast.

The IRI's probabilistic ENSO forecast 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 that a 50% probability for La Niña development by mid-2007.