Technical ENSO Update

17 March 2005

Current Conditions

From late January through February 2005, above-average SST anomalies declined in the central and eastern equatorial Pacific following the weak El Niño event of late 2004. During February 2005, easterly wind anomalies in the eastern equatoral Pacific created a shallower than normal thermocline and led to below average temperatures locally, but those have largely returned to near-average in recent weeks. Observations in mid-March 2005 indicate that SST anomalies are generally above average across the equatorial Pacific, exceeding 0.5 degrees C from 160E eastward to 140W with a maximum anomaly of just over 1 degree C in the vicinity of the dateline.

Positive anomalies exceeding 4 degrees C are observed in the sub-surface ocean near the thermocline depth in the central Pacific. The positive anomalies in the central Pacific sub-surface temperature are the result of a substantial and persistent westerly wind event observed in the western-central Pacific from late January-early March 2005. This wind event was associated with a strong and large-scale region of anomalous convection that moved into the central Pacific during the February-March 2005 period. Also related, the Southern Oscillation Index (SOI) reached a 3 standard deviation low in February.  The westerly wind events that occurred in 2004 in the western Pacific were instrumental in maintaining the warm SST conditions in the central equatorial Pacific associated with the weak El Niño as they advected the eastern edge of the West Pacific warm pool eastward. The 2004 low-level westerly wind anomalies showed some persistence over the last half of the year, but were weak and remained in the far western Pacific, mainly west of the dateline. The westerly winds observed in late January-early March 2005 were centered at about 175E, and thus were further east than the westerly events of 2004; they were also longer lived and stronger than most of  the 2004 wind events.

The existence of warm SST anomalies in the central equatorial Pacific are likely to 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 teleconnections, while still possible, are not favored.
 

Expected Conditions

Presently in mid-March the potential for maintaining neutral SST conditions through the Apr-May-Jun period carries a probability of 65%, which represents a slight enhancement of the odds relative to the historical climatological likelihood of 50%. There is virtually a 0% probability for La Niña SST conditions, leaving an approximately 35% 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 Apr-May-Jun period of 2005 is 5 out of a total of 20, or 25 %. 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, 5 out of 18 (28%) call for SSTs of at least a minimum El Niño level for the Apr-May-Jun period, while 0 (0%) predict SSTs cold enough to be considered a La Niña. (Note 1).    Overall, most of the models indicate that the current above-average SST conditions in the NINO3.4 region will continue to constitue only near-normal conditions in the next couple seasons. For the longer lead forecast for Jul-Aug-Sep 2005, 9 of the 18 models (50%) forecast El Niño conditions, 9 models (50%) forecast neutral conditions, and no models (0%) forecast La Niña conditions. If only the 16 models that use subsurface ocean temperature data are included, these figures become 9 (56%), 7 (44%), and 0 (0%). This would imply a higher probability for El Niño to develop during the transition phase of the ENSO cycle (centered on May 2005) than the climatological likelihood, implying that re-emergence of warm conditions into the next ENSO cycle (2005-06) is possible. 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 35% through the  Apr-May-Jun season, increasing to about 50% beginning in the Jul-Aug-Sep 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. This suggests that the odds for maintaining neutral conditions or developing El Niño conditions in 2005 are very similar, with neutral conditions being more likely through northern hemisphere spring. 
See also: 

• Summary of model forecasts