Technical ENSO Update

15 February 2006

Current Conditions

Expected Conditions

With the persistence of below-average SSTs in the eastern equatorial Pacific, due to air-sea coupling, the maintenance of La Niña conditions has become the most likely outcome for the coming 3-month season. There is an estimated 65% likelihood for La Niña to persist and an approximate 1% probability for El Niño leaving 34% probability that neutral conditions will prevail over the FMA 2006 season.

Although La Niña conditions have been observed over the last couple months, there is still uncertainty surrounding the maintenance of those cold SST anomalies. There are shallow thermocline anomalies in the central/eastern Pacific that can perpetuate, and even amplify, the current cold anomalies; however, the models do not indicate much potential for growth of the anomalies. The low-level easterly wind anomalies resulting from the anomalous SST pattern has generated deep thermocline anomalies on and off the equator in the western equatorial Pacific. The off-equatorial thermocline anomalies are beginning to converge towards the equator where they can propagate eastward as Kelvin waves, and nullify the current shallow thermocline/cold SST anomalies in the central/eastern equatorial Pacific. The eastern Pacific is also transitioning toward its warmest climatological temperatures, which occur March; this is a very unusual time of year for a La Niña event to develop. Furthermore, because the projected magnitude of the La Niña event, were it to occur, is weak, destructive interference from westerly wind bursts associated with the MJO could weaken the oceanic cooling enough to place the anomalies back in the neutral category. Such MJO activity could potentially be triggered by the persistent warm anomalies of the western Pacific, but this type of high frequency variability is not well captured by ENSO prediction models.

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 spring 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, mainly for the longer-lead seasons. No models are forecasting El Niño conditions to occur through the Mar-Apr-May period, but 12 of the 20 models (60%) are suggesting the maintenance of La Niña SST conditions to persist within that season. 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, only 1 model calls for El Niño conditions by the May-Jun-Jul period; 3 of 13 models (23%) indicate SSTs still exceeding the threshold for La Niña (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will return to near-normal during the [northern hemisphere] .spring barrier. in AMJ (75%) or MJJ (78%). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the coming several months. The expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the above probability estimate from the true probability.

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 probability of El Niño that is much lower than the climatological value (25%)--beginning at about 1% for Feb-Mar-Apr, rising to 25% by mid-2006. For La Niña the probabilities begin at 65% for Feb-Mar-Apr and decrease to 20% in mid-2006. 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.

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 much higher than average probabilities for La Niña conditions and much lower than climatological probabilities for El Niño through the next season. Those probabilities move towards climatological values at longer leads.

In terms of climate impacts, it must be noted that the atmosphere responds to changing patterns of SST anomalies. The fact that the western tropical Pacific has been so anomalously warm means that the zonal gradient of SST along the equator in the Pacific has been notably strengthened sine mid/late-2005 even though the SST anomalies in the eastern Pacific were weak until the end of 2005. The result of the enhanced SST gradient has been a La Niña-like precipitation response of the tropical Indo-Pacific region, and thus La Niña-like modification of the tropical heating of the atmosphere. As a result the recent monthly, and even seasonal, precipitation anomalies have appeared La Niña-like, particularly in tropical regions.

See also: 

• Summary of model forecasts