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Technical ENSO Update

20 May 2004

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> Expected conditions

Current Conditions

SST conditions are near average, and therefore ENSO-neutral, over much of the central and eastern tropical Pacific. Observations from late-April to mid-May indicate warmer than average SST anomalies from the dateline through the western tropical Pacific, mainly near average SSTs (e.g. between -0.5 and 0.5 degrees C from average) from the dateline to 120W longitude, and below normal SST from 110W eastward to the South American coast. Weakly positive anomalies of  1 to 2 degrees C are observed in the sub-surface temperature (or equivalently themocline depth anomalies of about 20-40m) in a portion of the east-central equatorial Pacific, with weakly negative sub-surface anomalies farther to the east. The equatorial (and standard) Southern Oscillation Index (SOI) has been near zero since July 2003 when smoothed with a running mean of 3-months or greater, although individual months have shown noteworthy excursions in either direction. For example, a strongly negative standard SOI was observed in April; this has already returned to near zero. Trade winds across much of the basin have been near normal over the last 6 months, with some spells of both weaker and stronger than normal trades on the intraseasonal time-scale. Overall, conditions have not consistently indicated a tendency for either El Niño or La Niña development.

Expected Conditions

Presently in mid-May the potential for El Niño for the May-Jun-July period is believed to be lower than its historical, climatological probability of 25%. There is a 5% probability for La Niña and a 15% probability for El Niño, leaving an approximately 80% probability for continued neutral conditions. During the period of July through the remainder of 2004, there is a slightly enhanced probability for development of El Niño conditions (approximately 40% from late northern summer to northern fall), compared with the average probability of 25%. Because the ENSO state in July, and even in June, has some tendency to persist throughout the remainder of the calendar year (it "sets the stage" for the July-through-March ENSO condition), the slight enhancement of the chances for El Niño carries through to the longer leads of the current forecast. 

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models. 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 0.4 degrees C in Mar-Apr-May and as high as 0.75 degrees C in Oct-Nov-Dec. 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 for the coming several seasons. The number of models that are forecasting El Niño conditions to be occurring in the Jun-Jul-Aug period of 2004 is 6 out of a total of 20, or 30%. The number of models that predict La Niña conditions is 0 (0%). 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, 6 out of 17 (35%) call for SSTs of at least a minimum El Niño level for the Jun-Jul-Aug period, while 0 (0%) predict SSTs cold enough to be considered a La Niña. (Note 1). Overall, most of the models indicate that the now average SST conditions in the NINO3.4 region may warm somewhat, but not rise to levels sufficient to represent a weak El Niño. For the longer lead forecast for Sep-Oct-Nov 2004, 4 of the 18 models (22%) forecasts El Niño conditions, 14 models (78%) forecast neutral conditions, and no models (0%) forecast La Niña conditions. If only the 15 models that use subsurface ocean temperature data are included, these figures become 4 (27%), 11 (73%), and 0 (0%). 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 it's 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 an enhancecd probability of El Nino that maximizes at near 40% during the seasons of JAS and ASO, thereafter dropping to near 35%. For all periods the probability for La Niña is well below its climatological value of 25%. 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 higher than average probabilities for neutral conditions throughout the outlook period, with maximum probability for neutral between now (May) and July. The chances for La Niña onset are less than for an average year through all of 2004, and for El Niño onset slightly higher than that of an average year from northern summer onward. Still, chances for El Niño never attain 50%. This suggests continuation of the currently near neutral conditions over the coming several months and throughout the rest of 2004.
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Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
 
 

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