Current ENSO Information

Technical ENSO Update

18 December 2003

>	Current conditions
>	Expected conditions

Current Conditions

SST conditions are slighty above average, but ENSO-neutral, over much of the tropical Pacific. From late April through June 2003 eastern equatorial sea surface temperatures were in the below-normal range, suggesting the potential development of La Niña conditions. However, as a result of significant MJO-related westerly wind anomalies in May and June, conditions returned to normal. Observations from late-November to mid-December now show some warmer than average SST anomalies from the dateline through the western tropical Pacific, and near to slightly above average SSTs (e.g. near or just below 0.5 degrees C above average) in most of the east-central and eastern part of the basin. 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. The equatorial (and standard) Southern Oscillation Index has been near zero since July 2003. As a result, trade winds across much of the basin have been near normal over the last 5 months. Significant further warming is believed to be unlikely, based on lack of support from low level equatorial winds and the small amplitude of sub-surface sea temperature anomalies.

Before the May to early-June westerly wind event, anomalously shallow themocline anomalies occupied the eastern equatorial Pacific as a result of typical ocean adjustment following the El Niño of 2002-03. These anomalies led to a rapid decrease in SST during April in the eastern equatorial Pacific due to mean upwelling across the anomalously shallow thermocline and enhanced meridional divergence of low-level winds away from the area of negative SST anomalies, reinforcing the shallow thermocline anomalies through local air-sea coupling. However, the strong and persistent westerly wind anomalies forced downwelling Kelvin waves, eroding the shallow thermocline anomalies that were reinforcing La Niña development. This neutralized the more slowly acting physical processes that were in the process of moving the climate state toward La Niña conditions. Consequently from mid-June to mid-September the ENSO condition was neutral. From late September to the present, SST anomalies have been slightly above normal (by approximately one-half degree C) in the east-central part of the basin, consistent with the sub-surface temperature anomalies.

Expected Conditions

Presently in mid-December the potential for El Niño or La Niña in the remainder of 2003 is believed to be lower than its historical, climatological probability of 25%. There is a negligible probability for La Niña, less than a 20% probability for El Niño, and more than an 80% probability for continued neutral conditions during the remainder of 2003. During the period of February through May 2004, there is a slightly enhanced probability (up to 40%) for development of El Niño conditions as compared with the average probability of 25%. Uncertainty in longer-lead outlooks results in the return to the climatological probability with the approach of mid-2004, while the probability of La Niña is also expected to increase toward its climatological level.

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 Jan-Feb-Mar period of 2003-04 is 10 out of a total of 19, or 53%. 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, 7 out of 14 (50%) call for SSTs of at least a minimum El Niño level for the Jan-Feb-Mar 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 to straddle the neutral/El Nino borderline. For the longer lead forecast for Apr-May-Jun 2004, 7 of the 17 models (41%) forecasts El Niño conditions, 10 models (50%) 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 6 (50%), 6 (50%), 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 56% for the FMA period, thereafter dropping below 50% but remaining above the historical average of 25%. Across all periods, the average probability for neutral conditions is highest, and exceeds 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.

The IRI's probabilistic ENSO forecast takes into account factors in addition to this set of models. It indicates highest probability for neutral conditions, 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 in DJF. From JFM 2004 onward, chances for El Niño increase to levels above those of an average year, particularly during the periods of FMA and MAM 2004. The probability for El Niño then weakens toward its historical average level by the middle of 2004. 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 in early northern spring of 2004. However, despite a higher than average probability for development of El Nino conditions in early 2004, that probability remains less than 50%.