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Technical ENSO Update

17 September 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-September 2009, SSTs are above-average across much of the equatorial Pacific, indicative of weak-to-moderate El Niño conditions. Between mid-June and mid-July, SSTs in the east-central tropical Pacific warmed to weak El Niño conditions. Spells of westerly wind anomalies have been observed in the western Pacific, and a sizeable one exists currently. The traditional SOI is negative, although the equatorial Southern Oscillation Index (SOI) has remained weakly positive. Positive convection anomalies emerged in the vicinity of the dateline again recently. On the oceanic side, equatorial heat content has been above-average over the last several months, and the equatorial SST anomalies which had weakened between July and August, are again strengthening, consistent with the recent wind anomalies. The deepened thermocline in the eastern part of the basin, and associated positive subsurface sea temperature anomalies, have also been reinforced by recent westerly wind anomalies.

For August 2009, the SST anomaly in the NINO3.4 region was 0.82 C, sufficient to be classified as indicative of weak El Niño conditions, and very close to the anomaly reported for July. For the Jun-Jul-Aug season they were 0.77 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 NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Sep-Oct-Nov and the Oct-Nov-Dec seasons are approximately (-0.75C, 0.70) and (-0.75, 0.75), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.9 C, indicating weak-to-moderate El Niño conditions in the tropical Pacific. What is the outlook for the ENSO status going forward? Wind anomalies along the equator in the western Pacific are currently westerly, but in the central and eastern portions of the basin winds remain more random. These westerly wind anomalies can create more downwelling Kelvin waves that could enhance the strength of the current El Niño conditions. The spatial pattern of SST anomalies remains fairly unstructured, however, which is not favorable for encouraging a strong atmospheric response that would lead to substantial air-sea coupling of the type observed in strong El Niño events. It is possible that well timed, but independent, intra-seasonal variability in the atmosphere could provide such a kick to the ocean, but such variability is largely beyond the scope of current ENSO prediction models.

August is well past the so-called "predictability barrier" of boreal spring, typically making ENSO predictions easier. This is a time when existing ENSO events typically persist through the end of the year. The emergence of weak El Niño conditions during June, July and August is often, but not always, followed by continued El Niño conditions of at least weak, if not greater, magnitude for the remainder of the calendar year and often into the initial months of the next calendar year. There is also a finite possibility that independent intra-seasonal variability may act against the current El Niño conditions, leading to a return to ENSO-neutral conditions.

Coupling between the warmed central and eastern Pacific and the overlying atmosphere has so far been limited, but not absent. More sustained coupling will be necessary if the current El Niño conditions are to grow further. Although it still seems most likely that El Niño conditions will at least persist through the end of the year, it does not appear likely to become a strong event, based on the timing of its development and the current subsurface heat content.

Presently, the models and observations taken together indicate probabilities of about 89% for maintaining El Niño conditions and about 10% for dissipation to ENSO-neutral conditions for the Sep-Oct-Nov season in progress. Going forward, probabilities for El Niño stay near 85% for the remaining 3-month periods in 2009, decreasing to approximately 50% by early 2010, and falling to climatological probabilities of 25% by mid-2010. Probabilities for La Niña conditions are predicted to be negligible, not rising to above 10% until the middle of boreal spring 2010.

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 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal 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.

The models are somewhat varied in their ENSO forecasts through the 10-month forecast period. The statistical and dynamical models are in agreement in predicting El Niño conditions through the end of the year, though with varying intensities. The warmest NINO3.4 forecasts come from the dynamical models. For the current Sep-Oct-Nov season, most (86%) are predicting El Niño conditions, and 14% predict ENSO-neutral conditions. At lead times of 4 or more 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, 13 of 15 (87%) indicate El Niño conditions for the Jan-Feb-Mar season, and 2 of 15 (13%) predict ENSO-neutral SSTs. (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. This method shows probabilities for El Niño at 86-87% for Sep-Oct-Nov and Oct-Nov-Dec, holding at about 85% throughout the rest of 2009, and then declining to near 50% during boreal spring 2010. 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, and not the range across the models, nor the ensemble range within individual models.

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 89% probability for El Niño conditions in the Sep-Oct-Nov season in progress, remaining near 85% through Dec-Jan-Feb 2009/10 and decreasing rapidly after the first few months of 2010.