Current ENSO Information

Technical ENSO Update

15 January 2004

>	Current conditions
>	Expected conditions

Current Conditions

SST conditions are slighty above average, but ENSO-neutral, over much of the tropical Pacific. Observations from late-November to mid-January have shown some warmer than average SST anomalies from the dateline through the western tropical Pacific, and near to slightly above average SSTs (e.g. between zero and 0.5 degrees C above average) in most of the east-central and eastern part of the basin. SST anomalies in the east-central part of the basin attained the 0.5 C level during October and November, followed by some cooling to below 0.5 C in December and early January. Weakly positive anomalies of approximately 2 degrees C are observed in the sub-surface temperature (or equivalently themocline depth anomalies of about 20m) in the far eastern equatorial Pacific, while negative anomalies of about 3 degrees C are located near 100 meters depth at about 135W longitude. In the last several weeks, positive anomalies have increased in strength to about 3 to 4 degrees C near the dateline from 100 to 200 meters depth. The equatorial (and standard) Southern Oscillation Index has been near zero since July 2003, except for December when it became somewhat positive (indicative of La Niña). This may be a brief perturbation; it warrants watching over the coming month. Trade winds across much of the basin have been near normal over the last 5 months, although a strong westerly wind episode near and just west of the dateline occurred in the first half of January. This event is expected to spawn a downwelling Kelvin wave that would reach the South American coast in March, and could potentially induce a local rise in SST that may trigger positive rainfall anomalies along the coast of Ecuador and northern Peru. While it is possible that such a "classical" El Niño event could lead to a more basin-wide, multi-season El Niño, this is not considered very likely, based on the lack of more general, larger-scale support from low level equatorial winds and the fairly small amplitude of the current sub-surface sea temperature anomalies. However, the overall state of the tropical Pacific climate leans somewhat more in the direction of El Niño than La Niña, as evidenced by model forecasts as discussed below.

Expected Conditions

Presently in mid-January the potential for El Niño or La Niña for the Jan-Feb-Mar period is believed to be lower than its historical, climatological probability of 25%. There is a near-zero probability for La Niña and only a 20% probability for El Niño, leaving an approximately 80% probability for continued neutral conditions. During the period of February through July 2004, there is a slightly enhanced probability (40 to 45%) for development of El Niño conditions, compared with the average probability of 25%. Uncertainty in the longer-lead outlooks (later seasons of 2004) results in a small degree of return toward the climatological probability by late northern summer of 2004. However, because the ENSO state in June has some tendency to persist throughout the remainder of the calendar year and "set the stage" for the July-through-March ENSO condition, some enhancement of the chances for El Niño carry 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 Feb-Mar-Apr period of 2004 is 8 out of a total of 20, or 40%. 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 15 (40%) call for SSTs of at least a minimum El Niño level for the Feb-Mar-Apr 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 slightly above average SST conditions in the NINO3.4 region will continue, but not attain levels sufficient to represent a weak El Niño. For the longer lead forecast for May-Jun-Jul 2004, 7 of the 17 models (41%) forecasts El Niño conditions, 10 models (59%) forecast neutral conditions, and no models (0%) forecast La Niña conditions. If only the 12 models that use subsurface ocean temperature data are included, these figures become 5 (42%), 7 (58%), 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 45% over a broad period from FMA to JAS, thereafter dropping to just below 40%. From FMA onward, the probability for neutral conditions is near its climatological value of 50%, and 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 highest probability for neutral conditions throughout the period, with La Niña onset being less likely than for an average year through mid-2004, and El Niño onset less likely than for an average year only in JFM. From FMA 2004 onward, chances for El Niño increase to levels of 40 to 45% which are above those of an average year. This would suggest continuation of the currently near neutral (but slightly warmer than average) conditions over the coming several months, with some chance of SSTs attaining weak El Nino levels by northern spring of 2004. However, despite a higher than average probability for development of El Nino conditions in early or mid-2004, that probability remains less than 50%.