Technical ENSO Update

19 May 2005

Current Conditions

In early-to-mid April, a downwelling Kelvin wave arrived in the far eastern equatorial Pacific, bringing positive subsurface anomalies in excess of 3 degrees C.. The subsurface anomalies, once located where the mean thermocline is shallow and upwelling currents are strong, began to influence local SSTs. By late April positive SST anomalies had increased to over 1 degree C in the eastern equatorial Pacific, extending to approximately 120W. The subsurface anomalies were not reinforced by additional downwelling Kelvin waves, and have decayed over the first half of May 2005. As a result, and due to a lack of air-sea coupling (requiring anomalous convection) associated with the eastern SST increase, the SSTs are also returning to average climatological values. The passage of the Kelvin wave during March and April has left in its wake a shallower than normal thermocline, and negative heat content anomalies are now observed in the central Pacific.

The existence of warm SST anomalies in the equatorial Pacific may influence local climate patterns, even if the NINO3.4 region is not currently warm enough to be categorized as El Niño conditions. However, the NINO3.4 region is implicated as the region of the tropical Pacific where SST anomalies are most associated with global ENSO teleconnections. The fact that the NINO3.4 index is not in the warmest 25%-ile of the historical distribution suggests that remote (ENSO-associated) teleconnections are not favored.
 

Expected Conditions

Presently in mid-May the potential for maintaining neutral SST conditions through the Jun-Jul-Aug period carries a probability of 65%, which represents a slight enhancement of the odds relative to the historical climatological likelihood of 50%. There is a mere 2% probability for La Niña SST conditions, leaving approximately 33% probability for El Niño SST conditions. 

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 0.45 degrees C in Apr-May-Jun and as high as 0.75 degrees C in Nov-Dec-Jan. 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 considerable 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 2005 is 5 out of a total of 19, or 26%. The number of models that predict La Niña conditions is 1 (5%). 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, 5 out of 14 (36%) call for SSTs of at least a minimum El Niño level for the Jun-Jul-Aug period, while 1 (7%) predict SSTs cold enough to be considered a La Niña. (Note 1).    Roughly half of the models indicate that the current above-average SST conditions in the NINO3.4 region will continue to constitute only near-normal conditions in the next couple seasons. For the longer lead forecast for Sep-Oct-Nov 2005, 6 of the 17 models (35%) forecast El Niño conditions, 11 models (65%) 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 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 an enhanced probability of El Niño that is approximately 34% through the Jun-Jul-Aug season, increasing to about 42% beginning in the Aug-Sep-Oct season. 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 both neutral and El Niño conditions, and lower than average probabilities for La Niña, throughout the outlook period, even into the second half of 2005. Throughout the forecast period, however, neutral conditions are indicated as most likely.  
See also: 

• Summary of model forecasts