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ENSO Quick
Look IRI ENSO Update / Forecast IRI Probabilistic ENSO Forecast Technical ENSO Update ENSO Prediction Plume Monthly archive Technical ENSO Update17 August 2006
Current ConditionsThe current conditions, as of mid-August 2006, are ENSO-neutral. However, SSTs are now approximately 0.5C above average throughout the equatorial Pacific, and approximately 1.0C near the dateline. SSTs are also above average in the far eastern equatorial Pacific. There has been a slow evolution of the equatorial Pacific towards warmer conditions over the last several months. Deep perturbations exist across the equatorial thermocline, particularly near the dateline and in the east. These perturbations to the sub-surface temperature structure were generated by anomalous westerly winds that have persisted over the western Pacific since late June. These deep thermocline anomalies, although not large, have contributed to the increasing heat content. The increasing SST anomalies are also a result of the westerly wind anomalies, through zonal temperature advection, in the central Pacific. In the east the SST increases are related in part to the equatorial adjustment of thermocline anomalies generated by the wind anomalies, and in part have resulted from convergence of off-equatorial anomalies onto the equator.For July 2006, the SSTs in the NINO3.4 region were 0.28 degrees C above average, and for the May-June-July season were 0.26 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 Aug-Sep-Oct and Sep-Oct-Nov seasons is approximately 0.50C and 0.70C, respectively, away from average. Expected ConditionsSome indications, which suggest the potential for El
Niño development, exist in the surface conditions. These include
persistent westerly wind anomalies in the western Pacific, increasing
equatorial heat content and appearance of warm SST anomalies,
particularly in the eastern equatorial Pacific. However, the
sub-surface indicators, such as large-scale deep perturbations to the
equatorial thermocline and in the western Pacific, whose adjustment
could provide reinenforcement of surface warming, are weak at
best. Given the apparent inability for the tropical Pacific environment
to support coupled ocean-atmosphere growth over the last several years,
even when provided with stronger perturbations to the ocean state than
what is currently observed, it is unclear whether the current state can
initiate and sustain an El Niño event. If that situation
remains, then the future variability of the tropical Pacific air-sea
system will be dominated by less deteministic atmospheric
perturbations, the effects of which are much more difficult to
foretell.
As a result, the maintenance of ENSO-neutral
conditions remains the most likely outcome for the
coming 3-month season. There is an estimated 60%
likelihood for neutral conditions, approximately 2% probability for La
Niña conditions to return, leaving 38% probability that El
Niño conditions will develop during the ASO 2006 season. 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 to weak El Niño conditions for the longer-lead seasons. No model is forecasting La Niña conditions to occur through the forecast period. For the ASO 2006 season, 4 out of 18 models (22%) 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, 4 of 11 models (36%) call for El Niño conditions by the Dec-Jan-Feb 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 remain near-normal throughout the forecast period: ASO 2006 (78%) to AMJ 2007 (69%). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the coming several months. The expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the above probability estimate from the true probability. 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%)-- fluctuating between 33% and 40% from Aug-Sep-Oct 2006, through the end of the year. For La Niña the probabilities do not exceed 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 higher than average probabilities
for ENSO-neutral conditions, somewhat higher than average probabilities
for El Niño, and much lower than climatological
probabilities for La Niña conditions through the end of
2006 and into early 2007. |