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Technical ENSO Update

19 July 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-July 2007 SSTs in the eastern equatorial Pacific are below average, and their longitudinal extent has increased as compared with last month. Below average anomalies have expanded westward to approximately 165W, following an eastward contraction one month ago. Westerly wind anomalies that occurred in late May were followed by an episode of easterly anomalies in June, and the latter has reinforced a tendency toward shoaling of the thermocline that had been observed over the last several months, but had weakened in June in response to late May's westerly wind anomalies. The shallow thermocline anomalies currently exist in the central Pacific and are no longer overlaid with warm anomalies in the mixed layer due to the wind-driven advection of warmer water from the west due to the westerly wind anomalies that occurred in May.

Although thermocline depth anomalies have oscillated on a 2 to 4-week time-scale in response to alternations of zonal wind anomalies on that intraseasonal time-scale, there has been a more general tendency toward shallow thermocline anomalies since early 2007, indicative of slowly evolving changes in the equatorial Pacific typically associated with developing ENSO events. This suggests the possibility for a La Nina event to develop over the next few months.

For June 2007, the SSTs in the NINO3.4 region were very near average (0.1 degrees C anomaly), and for the March-April-May season were 0.00 degrees C away from 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 (El Niño) conditions for the Jun-Jul-Aug and Jul-Aug-Sep seasons is approximately 0.50C away from average. This required magnitude will increase to approximately 0.70C for the following northern autumn seasons.

Expected Conditions

Although currently the SST anomalies are not of sufficient magnitude to qualify as La Niña conditions, development into a La Niña is possible. Relevant aspects of this include the persistent shallow thermocline anomalies extending across much of the equatorial Pacific, the cold SST anomalies in the eastern equatorial Pacific, and persistence of the general enhancement of easterly Trade Winds across much of the central and eastern tropical Pacific despite interruptions on a shorter time-scale due to intraseasonal episodes.

While many of the features in the tropical Pacific air-sea system are consistent with the potential development of La Niña conditions -- shallow thermocline in the east, cold eastern Pacific SST anomalies, and easterly wind anomalies -- some uncertainty remains over whether such conditions will attain the magnitude of anomalies necessary to be called a La Niña event. In particular, continuation of enhanced easterly Trade Winds will be required to draw the subsurface temperature anomalies to the surface and initiate/maintain coupled air-sea growth. The strength and persistence of these Trade Wind anomalies in coming weeks are likely to determine whether a La Nina event develops. The slower (i.e. monthly and greater timescale) evolution of the air-sea system looks indicative of La Nina development, but the higher-frequency variability has been exerting noticeable influence. Given the relative persistence of the coupled air-sea system over the last several months, there is a moderate probability that the system could reach La Niña conditions within the coming seasons. Probabilities suggest that weak La Niña conditions have slightly higher than an even chance of occurring through late 2007.

Development of El Niño conditions is very unlikely, with probabilities not exceeding 10% through the Mar-Apr-May of 2008. There is an approximately 55% likelihood for reaching La Niña conditions during the Jul-Aug-Sep season, and a 44% probability for ENSO-neutral conditions. The probabilities for La Niña begin to decline slowly at the end of 2007.

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 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.

There is some variation among ENSO model forecasts through the 10-month forecast period. Approximately half of the predictions indicate ENSO-neutral conditions; the rest predict weak to moderate La Niña conditions. No models predict El Niño conditions at any point through the remainder of 2007. For the JJA 2007 season, 10 of 22 models (45%) predict La Niña conditions to develop. 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, 7 of 14 (50%) indicate La Niña conditions will have developed by the Nov-Dec-Jan season, and 7 of 14 (50%) predict that SSTs will remain 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 similar probabilities for La Niña, ranging from 60% likely in the Jul-Aug-Sep and Aug-Sep-Oct seasons, dropping to approximately 40% likely by the beginning of 2008. The probabilities for ENSO-neutral hover fairly close to the climatologically expected 50% throughout the forecast period, being slightly less than 50% in the coming seasons, and increasing to 60% by early 2008. 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 not their range.

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 that a 55% probability for La Niña development in the coming season.