Technical ENSO Update

17 February 2005

Current Conditions

Observations from late-January to mid-February indicate SST anomalies exceeding 0.5 degrees C from 160E eastward to 160W with a maximum anomaly of just over 1 degree C in the vicinity of the dateline. Although the pattern of above-average SST anomalies is large-scale, it is focused near and even west of the dateline rather than in the eastern or central-eastern Pacific, and thus differs from previous El Niños.SSTs in the eastern Pacific, which is usually instrumental in the development of an El Niño event, have remained largely neutral throughout 2004 and early 2005, and are currently below average. Positive anomalies of  2 to 4 degrees C are observed in the sub-surface ocean near the thermocline depth in the central Pacific. The positive anomalies in SST and in the sub-surface temperature in the western-central Pacific are the result of a substantial westerly wind event that has been observed in the western central Pacific since late January 2005. Previous westerly wind events, which occurred during June, August, October again in December in the western Pacific, have been instrumental in maintaining the current warm SST conditions in the central equatorial Pacific as they advect the eastern edge of the West Pacific warm pool further eastward. Low-level westerly wind anomalies have shown some persistence since May, but they have been weak and have remained mainly west of approximately 170E in the western Pacific. The westerly winds observed in late January-February 2005 are centered at about 175E, and thus are further east than the westerly events of 2004.

The large-scale atmospheric changes expected during an El Niño event, such as enhanced convection in the central Pacific, have been conspicuously absent during this episode. Although the convective precipitation normally present over Indonesia and the Maritime Continent at this time of year is shifted eastward, and drier than normal conditions have been present there since mid-2004, the eastward shift in the precipitation is small. The above-average convection has remained west of the dateline, while nearly all previous El Nino events have been accompanied by shifts in the convective precipitation that reached at least to the dateline  More consistent with previous El Nino events, the Southern Oscillation Index (SOI) was negative in the last half of 2004. However, the SOI has fluctuated considerably with monthly values that are only weakly negative, and the SOI was currently positive in January 2005. Furthermore, the standard SOI (which is a difference between Tahiti and Darwin SLP) this year has been negative primarily because of the high pressure pattern over Indonesia and northern Australia, with little contribution from the SLP at Tahiti. The anomalous conditions of late 2004 and January 2005 are likely to be categorized as an El Niño event as measured by NINO3.4 SST index, but many features typical of an evolving event are not manifest. 

The existence of warm SST anomalies in the central equatorial Pacific are likely to influence local climate patterns. 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 (and has been since mid-2004) in the warmest 25%-ile of the historical distribution suggests that remote teleconnections are possible. However, such teleconnection responses are unlikely as long as the convection anomalies that connect SST to the overlying atmospheric circulation are absent.
 

Expected Conditions

Presently in mid-February the potential for maintaining El Niño SST conditions through the Mar-Apr-May period carries a probability of 65%, which represents more than a doubling of the odds relative to the historical climatological likelihood. There is virtually a 0% probability for La Niña SST conditions, leaving an approximately 35% probability for neutral 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 0.4 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 Mar-Apr-May period of 2005 is 12 out of a total of 17, or 71%. 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, 11 out of 15 (73%) call for SSTs of at least a minimum El Niño level for the Mar-Apr-May 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 remain warm enough to represent at least a weak El Niño. For the longer lead forecast for Jun-Jul-Aug 2005, 8 of the 16 models (50%) forecast El Niño conditions, 8 models (50%) forecast neutral conditions, and no models (0%) forecast La Niña conditions. If only the 14 models that use subsurface ocean temperature data are included, these figures become 7 (50%), 7 (50%), and 0 (0%). This would imply a higher probability for El Niño to persist through the transition phase of the ENSO cycle (centered on May 2005) than the climatological likelihood, implying that continuation 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 varies between 60% and 70% through the Mar-Apr-May season, dropping to below 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 El Niño conditions, and lower than average probabilities for La Niña, throughout the outlook period, even into the second half of 2005. Probabilities for neutral ENSO conditions are low until the end of northern spring 2005, when they return toward their normal of approximately 50%. This suggests that the greatest likelihood is for continuation of weak El Niño conditions through at least April. 
See also: 

• Summary of model forecasts