Current ENSO Information

Technical ENSO Update

17 July 2003

>	Current conditions
>	Expected conditions

Current Conditions

SST conditions have returned to neutral over much of the tropical Pacific following their excursion into the below normal range during May and early June. The return to normal came as a result of a significant MJO-related westerly wind event in late May and June. This event was noteworthy not so much for its strength, but its great longitudinal reach, extending nearly to the west coast of South Amereica. Never during the previous two years, including the life cycle of the moderate El Niño of 2002-03, was there a time when westerly wind anomalies stretched across nearly all of the basin. This wind event has caused SSTs near the dateline to warm to more than one-half degree C above normal, SSTs in the NINO3.4 region to warm to about one-quarter degree above normal, and NINO3 to near its average. SST in the NINO1+2 region in the far eastern Pacific warmed to about 1 degree C below normal from its earlier anomaly of nearly 2 degrees C below normal. By mid-July, subsurface sea temperatures had risen to slightly above average at most longitudes along the equator, with a pool of water at approximately 4 degrees C above normal near 120W and 60 to 100 m below the surface. In mid-July, equatorial SSTs were at below-average levels eastward of approximately 120W. Weaker than normal trade winds across much of the basin from late May to late June in association with the MJO event have been near normal during July. However, some anomalous westerly winds continued in the western Pacific. The equatorial Southern Oscillation Index remained slightly positive for the month of June.

Before the late-May to early-June anomalous westerly wind event described above, anomalously shallow themoclline anomalies were in place due to a typical ocean adjustment following the El Nino of 2002-03. This was characterized by rapid cooling in the eastern tropical Pacific from mean upwelling across the anomalously shallow thermocline, and enhanced meridional divergence of low-level winds away
from the area of negative SST anomalies in the east that reinforced the shallow
thermocline anomalies locally (local air-sea coupling). However, the anomalous westerly wind event neutralized the more slowly acting physical processes that were in progress, that had been moving the climate state toward La Nina conditions.

Expected Conditions

The anomalous westerly wind event described above counteracted and largely "killed" the more slowly acting signal that appeared likely to have brought about development of La Niña in the May to July period. The westerly wind anomalies forced downwelling Kelvin waves that have eroded the shallow anomalies that were reinforcing La Nina development. Presently in mid-July, the potential for Niña is no longer significantly higher than its historical, climatological probability of 25%. There is now an estimated 30% probability for La Niña, compared with 52% 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 Aug-Sep-Oct period, 2003, is 0 out of a total of 17, or 0%. The number of models that predict La Niña conditions is 3 (17%). 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 tperature information, 0 out of 13 (0%) call for SSTs of at least a minimum El Niño level for the Aug-Sep-Oct period, while 2 (15%) 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 most of these predict lower half of the neutral range. For the longer lead forecast for Nov-Dec-Jan 2003-04, none of the 16 models (0%) forecasts El Niño conditions, 13 models (81%) forecast neutral conditions, and 3 models (19%) forecast La Niña conditions. If only the 12 models that use subsurface ocean temperature data are included, these figures become 0 (0%), 10 (83%), and 2 (17%). 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 slightly higher probability for La Niña onset than that seen from the tally of model forecasts discussed here. Some of the reasoning behind this was mentioned in the preceding subsection.