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Technical ENSO Update

15 November 2006


>	Current conditions
>	Expected conditions

Current Conditions

The current conditions, as of mid-Norvember 2006, indicate that El Niño conditions are in place. SSTs now exceed 1.0C above average throughout the equatorial Pacific, and are more than 1.5C near the dateline and in the far eastern equatorial Pacific. The SST anomalies in the eastern Pacific have been increasing since mid-2006. Consistent with this, there has been a slow evolution of the equatorial Pacific towards warmer conditions. Deep, or downward, perturbations exist across the equatorial thermocline, particularly in the east where the anomalies exceed 30m (or equivalently, sub-surface temperature anomalies of 4.0C). Perturbations to the surface and sub-surface temperatures were initially generated by anomalous westerly winds over the western Pacific during July and August. Those temperature anomalies have been reinforced over the last couple months by westerly wind anomalies with a relatively large magnitude and fetch, now extending from the western Pacific to about 140W. The deep thermocline anomalies have strengthened and become more large-scale over the last month. The increasing SST anomalies, particularly those in the central Pacific, result from the westerly wind anomalies, through zonal temperature advection. In the east the SST increases are related largely to the equatorial adjustment of thermocline anomalies generated by the wind anomalies, which weakens the vertical advection of colder subsurface water to the surface.

For October 2006, the SSTs in the NINO3.4 region were 0.85 degrees C above average, and for the August-September-October season were 0.69 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 Oct-Nov-Dec and Nov-Dec-Jan seasons is approximately 0.75C away from average.

Expected Conditions

El Niño conditions exist in the surface and sub-surface tropical Pacific. These include the warm SST anomalies in the central and eastern equatorial Pacific, persistent westerly wind anomalies in the western Pacific and central Pacific, above-normal equatorial heat content, and large-scale deep perturbations to the equatorial thermocline. The current westerly wind anomalies will likely enhance the subsurface anomalies, and further expand the positive SST anomalies. The localized maximum in anomalous SST in the central Pacific has been expanding eastward and may soon unite with that in the eastern Pacific to form a more coherent and larger-scale pattern of SST anomalies. This could initiate a stronger atmospheric response and greater positive feedback to sustain growth. It should be noted, however, that over the last several years the tropical Pacific environment has not readily supported strong coupled ocean-atmosphere growth, even when provided with perturbations to the ocean state as strong, or stronger, than what is currently observed. If that continues to be the case, it would limit the peak strength of the current El Niño conditions. Although the southern oscillation index (SOI) is negative, the precipitation anomalies have shown only minor response to the SST anomalies.

The tropical Pacific ocean-atmosphere system has been slowly evolving towards warmer conditions since early-to-mid 2006, and the model predictions of El Niño have been suggesting progressively warmer outlooks. As a result, El Niño is the most likely outcome for the remainder of 2006 and early 2007. There is an estimated 92% likelihood for El Niño, approximately 1% probability for La Niña conditions to return, leaving just under 7% 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 0.45 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, mainly ranging from neutral but warm to moderate El Niño conditions occuring near the end of the year. No model is forecasting La Niña conditions to occur through the forecast period. For the NDJ 2006/07 season, 17 out of 20 models (85%) predict El Niño conditions. 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, 12 of 13 models (92%) call for El Niño conditions extending into the Mar-Apr-May period; none 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 exhibit El Niño conditions throughout the forecast period: NDJ 2006/07 (80%) to JAS 2007 (67%). 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 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 also indicate a probability of El Niño that is higher than the climatological value (25%)-- approximately 85%-95% from Nov-Dec-Jan 2006/07 to FMA 2007. For La Niña the probabilities stay below 10% throughout the forecast period. 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 a very high confidence for the continuation of El Niño conditions, much lower than expected probabilities for ENSO-neutral, and negligible probability of La Niña conditions through the end of 2006 and into early 2007.