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ENSO Quick
Look IRI ENSO Update / Forecast Technical ENSO Update Summary of Model Forecasts Monthly archive Technical ENSO Update17 May 2006
Current ConditionsThe current conditions, as of mid-May 2006, are ENSO-neutral. Little to no anomalous large-scale structure exists in SSTs, sub-surface ocean temperatures or low-level winds. SSTs are very close to their climatological average value for this time of year across the tropical Pacific.At the end of March/beginning of April 2006 the below-average anomalies observed in early 2006 were quickly erased, largely due to the influence of westerly wind anomalies in the central-eastern Pacific. However, deep thermocline anomalies from the western Pacific that were generated during the La Niña conditions had also been moving eastward, with noticeable equatorial adjustment occuring during the late March/early April timeframe. The return to ENSO-neutral conditions from the La Nina conditions that had been in place for several months was more rapid than predicted by the ENSO forecast models. For April 2006, the SSTs in the NINO3.4 region were 0.11 degrees C below average, and for the Feb-Mar-Apr season were 0.43 degrees C below average. As of mid-May, however, the weekly NINO3.4 anomaly is 0.0. 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 May-Jun-Jul and Jun-Jul-Aug seasons is approximately 0.45C away from average. Expected ConditionsWith the lack of strong anomalies in the surface or
sub-surface temperature fields, the maintenance of ENSO-neutral
conditions remains the most likely outcome for the
coming 3-month season. There are, however, some deep thermocline
anomalies remaining in the western Pacific, south of the equator. To
what extent these will influence future SST variability in the
central-eastern Pacific is currently unclear. There is an estimated 90%
likelihood for neutral conditions, approximately 5% probability for La
Niña conditions to return, leaving 5% probability that El
Niño conditions will develop over the MJJ 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/searly-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 for the longer-lead seasons. No models are forecasting El Niño conditions to occur through the May-Jun-Jul period, but 4 out of 14 models (29%) predict El Niño conditions by JFM 2007. 2 of the 20 models (10%) are suggesting La Niña SST conditions for into Jul-Aug-Sep, but these models are forecasting a persistence of last month's conditions rather than a return to La Niña from the current neutral SSTs. 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, 2 of 13 models (15%) call for El Niño conditions by the Aug-Sep-Oct period; 2 out of 13 (15%) 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 through the forecast period: JJA 2006 (85%) to JFM 2007 (64%). 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 indicate a probability of El Niño that is much lower than the climatological value (25%)--beginning at about 2% for May-Jun-Jul, rising to 30% by the beginning of 2007. For La Niña the probabilities begin at 7% for May-Jun-Jul and increase to 14% in Sep-Oct-Nov 2006. 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 much higher than average probabilities
for ENSO-neutral conditions and lower than climatological
probabilities for El Niño and La Niña through the end of
2006. |