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Technical ENSO Update

17 May 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-May 2007 SSTs in the eastern equatorial Pacific are below average. The pattern of anomalous SSTs resembles that of developing La Nina conditions. The cold eastern Pacific anomalies were caused by upwelling across a shallow equatorial thermocline that resulted from equatorial adjustment following the 2006 El Niño. Anomalously easterly Trade Winds brought the subsurface cold anomalies to the surface, which were then reinforced by the divergence of local winds away from the cold SST anomaly. Locally some of the cold anomalies have reached below -3.0C since March in the far eastern Pacific. In recent weeks the cold anomalies have amplified slightly and spread in spatial extent, such that the below-average SSTs now extend approximately to the dateline.

Shallow thermocline anomalies are observed along the equatorial Pacific, east of 160E. They have been fairly persistent since February, reinforced by low-level wind anomalies. Peak values of over 30m shallower than average (more than 3C colder than average at the level of the thermocline) exist near 140W. One of the most relevant aspects to this ENSO outlook is the persistence of these shallow thermocline anomalies over the last several months, indicating the type of slowly evolving changes in the equatorial Pacific typically associated with developing ENSO events. This strongly suggests the possibility for a La Nina event to develop over the next few months. However, the Southern Oscillation Index (SOI), which is another key indicator of ENSO events, is still slightly negative and thus does not yet indicate development of La Nina conditions.

For April 2007, the SSTs in the NINO3.4 region were very near average (0.1 degrees C anomaly), and for the February-March-April season were 0.08 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 (El Niño) conditions for the May-Jun-Jul and Jun-Jul-Aug seasons is approximately 0.50C (0.45C) away from average, respectively.

Expected Conditions

The tropical Pacific air-sea system has transitioned out of the 2006/07 El Niño event, leaving shallow equatorial thermocline anomalies in its wake. The oceanic adjustment resulted in eastward propagation of shallow thermocline anomalies into the central-eastern Pacific by January. By February, substantial shoaling of the thermocline was observed throughout the eastern and central equatorial Pacific. Enhanced easterly Trade Winds and divergent meridional winds have maintained the anomalous shoaling of the thermocline in the east, and some deepening in the far western Pacific -- typical of a developing La Niña event. Although currently the SST anomalies are not of sufficient magnitude to qualify as La Niña conditions, it is possible that the tropical Pacific may transition all the way into La Niña conditions. Relevant aspects of this include the sizable shallow thermocline anomalies extending across the central and eastern equatorial Pacific, the emerging and growing pattern of cold SST anomalies in the eastern equatorial Pacific, and enhancement of the easterly Trade Winds across much of the central and eastern tropical Pacific.

While many of the features in the tropical Pacific air-sea system are consistent with the initiation of La Niña conditions -- shallow thermocline in the east, cold eastern Pacific SST anomalies, and mild 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, enhanced easterly Trade Winds will be required to draw the subsurface temperature anomalies to the surface and initiate/maintain coupled air-sea growth. That growth, in magnitude of cold SST anomalies and their spatial extent, has been observed over the last month. In the last week, however, westerly wind anomalies developed in the far western Pacific, and these are converging near the dateline with the easterly Trade Wind anomalies. The convergence of the low-level winds will create localized deepening of the thermocline, which can then propagate eastward and weaken the existing shallow thermocline anomalies. The extent of impact will depend on the strength and duration of the westerly wind anomalies. On the other hand, in recent weeks, meridional wind anomalies diverging from the equator have enhanced the shallow thermocline anomalies, and this may offset any weakening due to the westerly anomalies. Given the strength and persistence of the coupled air-sea system over the last several months, there is a finite possibility that the system may reach La Niña conditions within the coming seasons. Probabilities suggest that weak La Niña conditions are most likely through late 2007.

Development of El Niño conditions is very unlikely, with probabilities not exceeding 5% throughout the forecast period. There is an approximately 55% likelihood for reaching La Niña conditions during the Jun-Jul-Aug season, and a 44% probability for ENSO-neutral conditions. The probabilities for La Niña begin to decline slightly at the turn of 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. More than 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 forecast period. For the MJJ 2007 season, 8 of 22 models (36%) 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, 8 of 14 (57%) indicate SSTs will remain ENSO-neutral through the Sep-Oct-Nov season, and 6 of 14 (43%) predict that SSTs will exceed the threshold for La Niña (Note 1). A recent noted shortcoming of most of the models is that they did not capture the rate at which the tropical Pacific returned to average conditions in early 2007. 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 a similar probability for the ENSO-neutral category, ranging from 64% likely in the May-June-July season, dropping to approximately 45% likely by mid-2007. The probabilities for La Niña remain above the climatologically expected 25% throughout the forecast period, and peak at approximately 55% in mid-2007. 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.