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Technical ENSO Update

18 April 2006


>	Current conditions
>	Expected conditions

Current Conditions

The current conditions, as of mid-April 2006, are ENSO-neutral. Little to no anomalous large-scale structure exists in SSTs, sub-surface ocean temperatures or low-level winds. SSTs are very close to their climatological average value for this time of year across the tropical Pacific. The one region of exception is immediately near the coast of South America, where SSTs are close to 3C below-average.

For the month of March, SST conditions showed approximately 0.5C below-average anomalies in the central equatorial Pacific, from 160W to 90W, and 1C below-average anomalies over 140W-110W. At the end of March/beginning of April 2006 the below-average anomalies were quickly erased, largely due to the influence of westerly wind anomalies in the central-eastern Pacific. However, deep thermocline anomalies from the western Pacific that were generated during the La Niña conditions had also been moving eastward, with noticeable equatorial adjustment occuring during the late March/early April timeframe. A portion of the sub-surface thermocline anomalies that were associated with the cold SST anomalies in the eastern Pacific are believed to have continued propagating eastward, and are are now reinforcing the localized cold anomalies off the South American coast. The remainder of the shallow thermocline anomalies were 'pushed' off the equator by the westerly wind anomalies and effectively dissipated. The return to ENSO-neutral conditions from the La Nina conditions that had been in place for the last several months was more rapid than predicted by the ENSO forecast models.

For March 2006, the SSTs in the NINO3.4 region were 0.61 degrees C below average, and for the Jan-Feb-Mar season were 0.70 degrees C below average. As of mid-April, however, the NINO3.4 anomaly is 0.1 degrees C above average. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Apr-May-Jun and May-Jun-Jul seasons is approximately 0.45C away from average.

Expected Conditions

With the lack of strong anomalies in the surface or sub-surface temperature fields, the maintenance of ENSO-neutral conditions remains the most likely outcome for the coming 3-month season. There are, however, some deep thermocline anomalies remaining in the western Pacific, south of the equator. To what extent these will influence future SST variability in the central-eastern Pacific is currently unclear. There is an estimated 80% likelihood for neutral conditions, approximately 10% probability for La Niña conditions to return, leaving 10% probability that El Niño conditions will develop over the AMJ 2006 season.

<>The low-level easterly wind anomalies resulting from the La Niña of the past several months have generated deep thermocline anomalies on and off the equator in the western equatorial Pacific. The off-equatorial thermocline anomalies have converged towards the equator where they propagated eastward as Kelvin waves, helping to nullify the shallow thermocline/cold SST anomalies that had existed in the central/eastern equatorial Pacific. Deep anomalies still remain in the western Pacific, south of the equator, and these have the potential to further increase the equatorial heat content over the next few months if they were to move onto the equator. On the other hand, we do not completely dismiss the possibility of the cold anomalies in the far eastern Pacific being advected westward, either, similar to the process by which the La Niña conditions were initiated. Finally, weather phenomena in the Indo-Pacific region could also influence the near-term evolution of the tropical Pacific, but high frequency variability, such as the MJO, is not well captured by ENSO prediction models. The state of the tropical Pacific is very near average right now; a finite possibility exists for the system to be pushed in either direction, with slightly more potential suggested for future warming.

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. 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 northern spring season and as high as 0.75 degrees C in late northern autumn. 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, mainly for the longer-lead seasons. No models are forecasting El Niño conditions to occur through the May-Jun-Jul period, but 5 out of 14 models (36%) predict El Niño conditions by the end of 2006. 3 of the 20 models (15%) are suggesting La Niña SST conditions for into Jun-Jul-Aug, but these models are forecasting a persistence of last month's conditions rather than a return to La Niña from the current neutral SSTs. 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, 4 of 13 models (31%) call for El Niño conditions by the Jul-Aug-Sep period; 2 out of 13 (15%) indicate SSTs exceeding the threshold for La Niña (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will remain near-normal through the end of the [northern hemisphere] "spring barrier" in MJJ (80%). 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 a probability of El Niño that is much lower than the climatological value (25%)--beginning at about 0% for Apr-May-Jun, rising to 25% by mid-2006. For La Niña the probabilities begin at 20% for Apr-May-Jun and decrease to 10% in mid-2006. 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 much higher than average probabilities for ENSO-neutral conditions and lower than climatological probabilities for El Niño and La Niña through the next season. The probabilities for El Niño move to just above climatological values at longer leads.