Current ENSO Information

Technical ENSO Update

19 May 2003

>	Current conditions
>	Expected conditions

Current Conditions

Although SST anomalies were nearly cool enough in the NINO3.4 region to constitute El Niño conditions in mid-May, they have since moved back into neutral territory as a significant westerly wind event occurred in the equatorial Pacific. This wind event caused SSTs near the dateline to warm to about one-half degree C above normal, while anomalies in the NINO3 and NINO1+2 regions remained fairly strongly negative in spite of some weakening. Subsurface sea temperature, which had been weakly but consistently below normal one month ago, have become near to slightly above normal at most longitudes along the equator. In mid-June, equatorial SSTs were at below-average levels eastward of approximately 150W, although the negative anomalies had weakened compared with those of mid-May. Trade winds that had been been somewhat stronger than average across most of the basin in early May became weaker than normal in early June and were returning toward average by mid-June. The equatorial Southern Oscillation Index was slightly positive for the month of May.

Before the late-May to early-June anomalous westerly wind event described above, anomalously shallow thermocline 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).

The westerly wind anomalies in the western Pacific during late May and June
were associated with strong MJO variability that originated in the Indian
Ocean region. This constituted weather noise that worked against the more slowly acting physical processes that were in progress, that were moving the climate state toward La Nina conditions. This anomalous westerly wind event was particularly strong in the western Pacific, but did extend across the entire equatorial basin which is an unusually large fetch for this type of "weather" event.

Expected Conditions

The anomalous westerly wind event described above has the potential to neutralize the more slowly acting signal that would encourage the development of La Nina in the coming one to two months. In effect, the wind event would kill the La Nina event before it matures. The westerly wind anomalies have already forced downwelling Kelvin waves that have nearly completely eroded the shallow anomalies reinforcing the La Nina development. Without substantial coupling between the east Pacific cold SST anomalies and zonal wind anomalies in the next couple weeks (which would spread cold SST anomaly westward and amplify the cold signal at surface and sub-surface), the La Nina is not likely to materialize. This situation lends itself to greater uncertainty, and to a decrease in the probability of La Nina as compared with that estimated one month ago.

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 Jun-Jul-Aug period, 2003, is 1 out of a total of 17, or 6%. The number of models that predict La Niña conditions is 4 (24%). 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 Jun-Jul-Aug period, while 4 (31%) predict SSTs cold enough to be considered a La Niña. (Note 1). Overall, most of the models indicate that the now neutral to cool SST conditions in the NINO3.4 region will remain in the lower half of the neutral range. For the longer lead forecast for Oct-Nov-Dec 2003, none of the 15 models (0%) forecasts El Niño conditions, 14 models (87%) forecast neutral conditions, and 2 models (13%) forecast La Niña conditions. If only the 12 models that use subsurface ocean temperature data are included, these figures become 0 (0%), 11 (92%), and 1 (8%). 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 somewhat 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.