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Technical ENSO Update

21 May 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-May 2009 SSTs are near-average across much of the equatorial Pacific, indicative of neutral ENSO conditions. During April and the first 3 weeks of May, SSTs in the east-central tropical Pacific have warmed from the cool half of the near-average range to the warm half of that range. Enhanced easterly low-level winds that had been responsible for the cold SST anomalies earlier this year, led to a deepening of the thermocline in the western Pacific and concentration of the warmest waters in the far western Pacific. Recent relaxation of the easterly wind anomalies along the equator allowed warmer water in the west to move eastward. The equatorial Southern Oscillation Index (SOI) is still slightly positive, but the traditional SOI has now become slightly negative. Equatorial heat content has been increasing during the last several months, and with the ocean adjustment it has now become above average across much of the equatorial Pacific.

For April 2009, the SSTs in the NINO3.4 region were in the neutral range with anomaly of -0.18 C, having risen from being in the La Niña range during the four previous months; and for the Feb-Mar-Apr season they were -0.47 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 May-Jun-Jul and the Aug-Sep-Oct seasons are approximately (-0.50C, 0.45) and (-0.55, 0.50), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.3 C, indicating ENSO-neutral conditions in the tropical Pacific. What are the probabilities for maintaining neutral conditions, returning to La Niña conditions, or developing El Niño conditions? Persistent and large-scale easterly wind anomalies along the equator in the western-to-central Pacific two months ago have weakened over the last 1-2 months, and are currently largely weak and random. The thermocline, which contitutes the main element of the tropical Pacific system imparting ENSO predictability, has become slightly deeper than its long-time mean depth in the central and eastern part of the Pacific. Deep anomalies in the west 1-2 months ago have edged eastward, making it unlikely for Niña conditions to return in the coming few months.

May is near 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 and going through the boreal spring. The perturbations to the upper ocean structure, which impact predictability to ENSO, are currently quite weak. However, dynamical models are often more sensitive to these small perturbations than are statistical models that consider the sub-surface ocean, which may be why the dynamical models are all producing notably warmer ENSO forecasts than their statistical counterparts.

Currently, the models indicate probabilities of about 80% for ENSO-neutral conditions, about 5% for the return of La Niña conditions, and about 15% for developing El Niño conditions for the May-Jun-Jul season in progress. Going forward the models indicate that maintenance of ENSO-neutral conditions is the most likely scenario, but with the possibility of developing El Niño conditions only slightly less likely, and the possibility of returning to La Niña conditions much smaller than either of the above.

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/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 are somewhat split in their ENSO forecasts through the 10-month forecast period, with the statistical models mostly predicting maintenance of ENSO-neutral conditions and the dynamical models predicting warm ENSO-neutral to El Niño conditions. For the current May-Jun-Jul season, no models are predicting La Niña conditions, while more than three-quarters are predicting ENSO-neutral conditions. However, this distribution shifts during the second half of 2009, when the models continue to favor ENSO-neutral conditions, but with development of El Niño conditions not far behind. For the Apr-May-Jun 2009 season, 3 of 22 models (14%) predict El Niño developement, while 19 models (86%) predict 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, none indicates the existence of La Niña, 8 of 16 (50%) indicate El Niño conditions for the Aug-Sep-Oct season, and 8 of 16 (50%) predict that SSTs will remain 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 ENSO-neutral near 83% for May-Jun-Jul, decreasing to 49% for Aug-Sep-Oct. ENSO-neutral is favored as being consistently most likely throughout the forecast period, but with El Niño probabilities averaging only 10 to 15% less likely and La Niña probabilities less that 10%. 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 70% probability for ENSO-neutral conditions in the May-Jun-Jul season in progress, declining to just below 50% by Jul-Aug-Sep and beyond as El Niño probabilities rise to a nearly matching 45%.