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Technical ENSO Update

16 July 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-July 2008 SSTs in the central equatorial Pacific are only very slightly below average, and only in a fairly small and diminishing pocket very close to the dateline. The La Niña episode that endured from August 2007 to late May 2008 has ended as of early June, and ENSO-neutral conditions have prevailed since that time. The equatorial Southern Oscillation index (SOI) was slightly positive during May and June, while the traditional Tahiti-Darwin SOI has been close to zero throughout May and June. The deep thermocline anomalies in the western Pacific have adjusted onto the equator and contributed to the dissipation of the La Niña conditions first in the far eastern Pacific, and now extending farther toward the central Pacific. The low-level wind anomalies related to the La Niña conditions have been weak or nonexistent during June and the first half of July. The equatorial heat content is above average throughout most of the equatorial Pacific, this having begun in May. Taken together, the current anomalous ocean-atmosphere conditions indicate the return to neutral conditions as of early June, with no tendency to revert to La Niña conditions during later June or the first half of July .

For June 2008, the SSTs in the NINO3.4 region were slightly below average (-0.31 degrees C anomaly), with near-zero weekly values for the first two weeks of July. For the Mar-Apr-May season they were -0.58 degrees C from average, down from the previous recent running 3-month periods and indicating that the La Niña event was in the process of ending. 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 Jul-Aug-Sep and Aug-Sep-Oct seasons are approximately 0.5C, but will exceed 0.5C in Sep-Oct-Nov and later seasons.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at 0.1 C, is the first positive anomaly since August 2007, and indicates that neutral conditions have returned to the tropical Pacific. How long will conditions remain neutral, and what are the probabilities for developing an El Niño or re-developing a La Niña later in this year? The low-level easterly wind anomalies in the central and western Pacific that were reinforcing the shallow thermocline anomalies have been near average for more than one month. The easterly wind anomalies from several months ago associated with the La Niña event had left behind a build-up of warm water in the western equatorial Pacific that has adjusted onto the equator over the last 2-3 months. As the deep thermocline anomalies adjusted they brought the heat content up, which has now been positive along most of the equatorial Pacific for about two months. There is some possibility that the positive heat content anomalies, although not very strong presently, could help initiate El Niño conditions. However, most of the ocean-atmosphere indicators indicate that ENSO-neutral conditions are likely to continue not only during the Jul-Aug-Sep season in progress, but through most of the remainder of 2008.

Currently, the models see the development of El Niño conditions just slightly more likely than the re-emergence of La Niña conditions, although the probabilities for either reach only about 20% through the forecast period. For the most part, the sub-set of the models that do see the possibility of developing El Niño conditions are the dynamical models, rather than the statistical models. All things considered, there is an approximately 10% likelihood for La Niña conditions to re-emerge during the Jul-Aug-Sep season, a 15% likelihood for El Niño conditions to develop, and a 75% probability for ENSO-neutral conditions.

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 0.40 degrees C in northern late-spring/early-summer 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.

We are now largely through the [boreal] "Spring Barrier" to ENSO prediction. Therefore, the models are showing fair agreement in their ENSO forecasts through the 10-month forecast period. For the current Jul-Aug-Sep season, the majority of the predictions indicate ENSO-neutral conditions. The majority of models also remain ENSO-neutral through 2008, while a couple models develop weak El Niño conditions and a couple re-develop weak La Niña conditions. For the ASO 2008 season, only 2 of 23 models (9%) predict El Niño conditions to develop, and none predict La Niña to re-emerge. 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, none indicate the existence of La Niña or El Niño conditions for the Nov-Dec-Jan season, i.e. 16 of 16 (100%) predict that SSTs will be ENSO-neutral (Note 1). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

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 slightly higher probabilities for El Niño than for Niña conditions for the next several seasons, although probabilities for either one are low. For example, for ASO and SON seasons, El Niño and La Niña probabilities are about 9% and 19%, respectively. Toward the beginning of 2009, the chances of either non-neutral ENSO condition become more equal but remain small, at approximately 15% each. This method shows probabilities for ENSO-neutral beginning as the most likely possibility at just over 80% probability, and holding at 70-75% likely through the end of 2008. With the approach of the next predictability barrier in early 2009, probabilities begin veering toward their climatological values of 25%-50%-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. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.

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 a 75% probability for ENSO-neutral conditions in the coming season.