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Technical ENSO Update

18 June 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-June 2009 SSTs are somewhat above-average across much of the equatorial Pacific, indicative of borderline El Niño conditions. Between early April and the first 3 weeks of June, SSTs in the east-central tropical Pacific have warmed from weak La Niña conditions to the threshold of weak El Niño levels. Although slightly above-average SSTs and convection related to the weak La Niña earlier this year linger in the western tropical Pacific, westerly wind anomalies have recently developed in this same region. The equatorial Southern Oscillation Index (SOI) is still slightly positive, but the traditional SOI has been slightly negative for about one month. Equatorial heat content has increased to above-average levels over the last several months, and with the ocean adjustment the SST is in the process of becoming above average by greater amounts in the east-central than in the western equatorial Pacific.

For May 2009, the SSTs in the NINO3.4 region were in the neutral range with anomaly of 0.27 C, and for the Mar-Apr-May season they were -0.13 degrees C from 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 Jun-Jul-Aug and the Sep-Oct-Nov seasons are approximately (-0.50C, 0.45) and (-0.75, 0.70), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.5 C, indicating borderline El Niño conditions in the tropical Pacific. What are the probabilities for reverting to neutral conditions, or developing more unambiguously El Niño conditions? Wind anomalies along the equator in the western-to-central Pacific are currently westerly, but in the central and eastern portions of the basin winds remain more weak and random. The thermocline, contituting the main element of the tropical Pacific system imparting ENSO predictability, has become somewhat deeper than average in the central and eastern part of the Pacific. However, positive depth anomalies in the west from several months ago have not yet returned to average, so that a significant zonal gradient of depth anomaly is not yet present.
June is past the middle of the so-called "predictability barrier" of boreal spring. It is typically easier to predict from this point forward than it was a few months ago for forecasts going through the boreal spring. The perturbations to the upper ocean structure, which impact predictability to ENSO, have featured anomalously warm subsurface waters in the west and central Pacific, and increasingly more anomalously warm subsurface wasters in the eastern Pacific. This developing pattern is conducive to El Niño development, as the warm waters at depth in the east are poised to rise to the surface in response to the easterly low-level Trade winds. This process is already underway, and SST anomalies during the most recently observed week exceed 1.0 C in some pockets in the far eastern tropical Pacific. Large-scale coupling between a warmed central and eastern Pacific and the overlying atmosphere has not yet shown signs of beginning, aside from the still small region of anomalous low-level westerly equatorial winds in the west. The spatial distribution of anomalous convection is currently weak, with a pocket of positive anomaly west of the dateline and little anomaly from the dateline eastward. Atmospheric aspects of El Niño typically are not manifest this early in the ENSO seasonal cycle except during the onset of very strong El Niños, which appears improbable for the current likely event now in its initial stage.

Presently, the models indicate probabilities of about 56% for developing El Niño conditions, and about 43% for ENSO-neutral conditions for the Jun-Jul-Aug season in progress. Going forward, probabilities for El Niño increase to 60 to 65% for the remaining 3-month periods in 2009, decreasing toward the end of the year and falling below 50% by northern spring 2010. Probabilities for La Niña conditions are predicted to be negligible through the remainder of 2009.

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 northern late-spring to 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.

The models somewhat varied in their ENSO forecasts through the 10-month forecast period. The statistical models, which were notably reluctant to predicting El Niño as recently as last month, have now begun calling for warmer conditions in the face of appreciably warmer initial conditions. Many of the dynamical models have continued predicting El Niño conditions of varying intensities, a majority showing somewhat greater warmth in current forecasts than those from last month. For the current Jun-Jul-Aug season, no models are predicting La Niña conditions, about half are predicting ENSO-neutral conditions, and half El Niño conditions. For Jul-Aug-Sep and the two subsequent running 3-month periods, the distribution becomes about one-third for ENSO-neutral and two-thirds for El Niño 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, none indicates the existence of La Niña, 11 of 16 (69%) indicate El Niño conditions for the Sep-Oct-Nov season, and 5 of 16 (31%) predict that SSTs will be ENSO-neutral (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 favoring El Niño near 56% for Jun-Jul-Aug, increasing to about 67% for Jul-Aug-Sep and Aug-Sep-Oct, and decreasing slowly but remaining at or above 50% into early 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 56% probability for El Niño conditions in the Jun-Jul-Aug season in progress, increasing to 60 to 65% through Nov-Dec-Jan 2009/10.