Current ENSO Information

Technical ENSO Update

21 August 2003

>	Current conditions
>	Expected conditions

Current Conditions

SST conditions are near neutral over much of the tropical Pacific. From late April through June 2003 eastern equatorial 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 have returned to normal. Observations from mid-August now show some warmer than average SST anomalies in the central Pacific, cooler than average SST anomalies in the eastern equatorial Pacific, and very weak anomalies in the sub-surface temperature (or thermocline depth). Trade winds across much of the basin have been near normal over the last 1 to 2 months. The equatorial (and standard) Southern Oscillation Index was near zero for the month of July.

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 Niñna conditions.

Expected Conditions

Presently in mid-August, the potential for La Niña is lower than its historical, climatological probability of 25%. There is now an estimated 10-15% probability for La Niña, compared with 75% for continued neutral conditions during the remainder of 2003.

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 Sep-Oct-Nov period, 2003, is 1 out of a total of 17, or 6%. 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, 1 out of 13 (8%) call for SSTs of at least a minimum El Niño level for the Sep-Oct-Nov 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 neutral SST conditions in the NINO3.4 region will remain neutral, and many of these predict the upper half of the neutral range. For the longer lead forecast for Dec-Jan-Feb 2003-04, 2 of the 16 models (12%) forecasts El Niño conditions, 14 models (88%) 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 2 (17%), 10 (83%), 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. The IRI's probabilistic ENSO forecast takes into account factors in addition to this set of models, and indicates a higher probability for neutral conditions, with El Niño or La Niña onset during 2003 being equally as unlikely. Some of the reasoning behind this was mentioned in the preceding subsection.