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Technical ENSO Update

19 September 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-September 2007 SSTs in the eastern equatorial Pacific are below average, and their longitudinal extent has tended to increase since their initial appearance in February and March, despite some month-to-month fluctuations. The western flank of the region of below average anomalies has expanded westward to the dateline. Easterly wind anomalies have increased dramatically during September. The easterly anomalies have a large fetch, and their placement relative to the SST anomalies suggests active air-sea coupling. The wind anomalies produced the westward expansion of the SST anomalies and are reinforcing the shallow thermocline anomalies in the east-central Pacific. All these elements taken together indicate the continuing evolution toward a La Niña in the equatorial Pacific.
For August 2007, the SSTs in the NINO3.4 region were below average (-0.5 degrees C anomaly), and for the Jun-Jul-Aug season were -0.24 degrees C 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 magnitudes of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Sep-Oct-Nov and Oct-Nov-Dec seasons are approximately 0.75C.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at -0.9 C, qualifies as La Niña conditions. The issue at this point is whether persistence, or further amplification, of this condition is likely. Factors of relevance are the enhanced easterly winds, the increasingly shallow thermocline anomalies extending across much of the equatorial Pacific over the last several months, and the negative SST anomalies in the eastern and now central equatorial Pacific.

Many of the features in the tropical Pacific air-sea system are consistent with the development of La Niña conditions -- the enhanced easterly winds along the equator have contributed greatly to shoaling of the thermocline, and since they are still strong, particularly between about 160E-160W, they will continue to bring the thermocline up. The negative anomalies in the subsurface ocean should be efficiently communicated to the surface at this time of year, when equatorial upwelling is strong. Thus, SST anomalies in the eastern and east-central Pacific will likely get even colder. Some uncertainty nonetheless remains concerning the duration and magnitude of these La Niña conditions. Continuation of enhanced easterly Trade Winds will be required to increase/maintain coupled air-sea growth. The strength and persistence of the Trade Wind anomalies in the coming several months are likely to determine whether this event will develop into an episode of weak or moderate intensity. The current state of the air-sea system indicates at least weak La Niña conditions, and portends some additional growth in the coming month, at least. A good probability exists that the system will continue as at least weak La Niña conditions over the coming seasons. Thus, a 65% probability of weak to moderate La Niña conditions is suggested.

Development of El Niño conditions is very unlikely, with probabilities not exceeding 10% through the Mar-Apr-May of 2008. There is an approximately 65% likelihood for La Niña conditions during the Sep-Oct-Nov season, and a 34% probability for ENSO-neutral conditions. The probabilities for La Niña stay over 60% throughout 2007, and then decline in 2008.

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. Over the coming several months, more than half of the predictions indicate weak to moderate La Niña conditions; the rest predict ENSO-neutral conditions. Just one model predicts El Niño conditions beginning in early 2008. For the OND 2007 season, 14 of 22 models (64%) predict La Niña conditions to exist. 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, 9 of 14 (64%) indicate La Niña conditions will have continued into the Jan-Feb-Mar season, and 5 of 14 (36%) 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 somewhat lower probabilities for La Niña, due to the El Niño-like outlier. For those probabilities, La Niña is 55% likely from the Oct-Nov-Dec season through the beginning of 2008, The probabilities for La Niña then decline going into 2008. This method shows probabilities for ENSO-neutral beginning slightly below the climatologically expected 50%, and then rising just above that level in the beginning of 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 65% probability for La Niña development in the coming season.