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Technical ENSO Update

19 April 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-April 2007 SSTs in the eastern equatorial Pacific are below average. SST anomalies warmer than 0.5C are confined to the western equatorial Pacific, west of the dateline. The pattern of anomalous SSTs is beginning to resemble that of developing La Nina conditions. The cold eastern Pacific anomalies were caused by upwelling across a shallow equatorial thermocline, which resulted from equatorial adjustment following the 2006 El Niño. Anomalously easterly Trade Winds brought 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 -3.0C in March in the far eastern Pacific. In recent weeks the cold anomalies have weakened slightly. However, weather-scale fluctuations in air-sea anomalies are common. What is relevant is the apparent slower changes in the equatorial Pacific suggesting the possibility for a La Nina event to develop over the next few months.

Shallow thermocline anomalies are observed along nearly the entire equatorial Pacific, which have been fairly persistent over the last several months, reinforced by low-level wind anomalies. Peak values of over 25m shallower than average (more than 3C colder than average at the level of the thermocline) exist near 140W. Furthermore, off-equatorial shallow anomalies in the western Pacific north of the equator may provide reinforcement to the existing equatorial shoaling.

For March 2007, the SSTs in the NINO3.4 region were exactly average (0.0 degrees C anomaly), and for the January-February-March season were 0.29 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 Mar-Apr-May and Apr-May-Jun seasons is approximately 0.40 and 0.45C away from average, respectively.

Expected Conditions

The tropical Pacific air-sea system has transitioned out of the 2006/07 El Niño event, leaving shallow equatorial thermocline anomalies in its wake. It is possible that the tropical Pacific 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 emerging pattern of cold SST anomalies in the eastern equatorial Pacific, and some 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 the initiation of La Niña conditions -- shallow thermocline in the east, emergence of cold SST anomalies, and mild easterly wind anomalies -- considerable uncertainty remains over whether such conditions will attain the magnitude of anomalies necessary to be called a La Niña event. In particular, enhanced easterly Trade Winds will be required to draw the subsurface temperature anomalies to the surface and initiate/maintain coupled air-sea growth. The trend over the past several months is 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 5% throughout the forecast period. There is an approximately 52% likelihood for ENSO-neutral conditions to start, with a 45% 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 AMJ 2007 season, no model predicts the development of El Niño conditions, whereas 5 of 21 models (24%) 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, 8 of 13 (62%) indicate SSTs will remain ENSO-neutral through the Aug-Sep-Oct season, and 5 of 13 (38%) predict that SSTs will exceed the threshold for La Niña (Note 1). 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 74% likely in the Apr-May-June season to approximately 50% likely my mid-2007. The probabilities for La Niña remain above the climatologically expected 25% throughout the forecast period, but do not exceed 50%. 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.