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Technical ENSO Update

19 February 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-February 2009 SSTs are below-average across the central and eastern Pacific. These SSTs, indicative of weak La Niña conditions, developed in early December 2008. They are the result of persistent easterly wind anomalies -- in particular, a large fetch of easterly wind anomalies observed from the central to the west-central Pacific since October 2008, corresponding with both the traditional and equatorial Southern Oscillation Indices (SOI) having been consistently positive. Their affect on the sub-surface ocean structure became notable in November, making for an increasely negative net heat content. During early December the La Niña conditions of the atmosphere finally began encompassing the upper ocean as well. However, given the timing of the seasonal cycle, which has passed its coldest months in the eastern Pacific, it appears that La Niña conditions are unlikely to become strong or to endure for multiple consecutive seasons. Such brief cool conditions occurred during the boreal winters of 2000-2001 and also 2005-2006, but even in those two cases the SST crossed into La Niña conditions earlier than mid-December.

For January 2009, the SSTs in the NINO3.4 region were more than minimally classifiable as La Niña conditions (-0.99 degrees C anomaly), and for the Oct-Nov-Dec season they were -0.65 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 magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Feb-Mar-Apr and the Mar-Apr-May seasons are approximately 0.45C and 0.40, respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.7 C, indicating weak La Niña conditions in the tropical Pacific. What are the probabilities for achieving a La Niña event (i.e. an "event" refers to La Niña conditions persisting for approximately 5 months) in early 2009? Persistent and large-scale easterly wind anomalies along the equator in the western-to-central Pacific since early October now have become coupled to changes in the upper ocean, but only after two to three months of relative oceanic unresponsiveness from September to November 2008. The thermocline, which contitutes the main element of the tropical Pacific system imparting ENSO predictability, is shallow in the eastern part of the Pacific, but deep anomalies in the west loom ready to reverse the current cooling.

It is possible that the negative anomalies in the east-central Pacific could persist or even strengthen slightly as the recently stronger sub-surface anomaly structure infuences the surface in the coming month. However, the intensity of enhancement of the Trade Winds has not increased, and even appears to have slightly weakened, since its late-November maximum and recent late January secondary maximum. Additionally, as the ITCZ moves closer to the equator, the Trade Winds weaken and the seasonal warming of the eastern Pacific ensues, it will become more difficult for the atmosphere to maintain cold oceanic conditions. The models, both dynamical and statistical, suggest a tendency towards neutral conditions beginning from the first season.

Currently, the models indicate probabilities greater than 50% for La Niña conditions, and very little possbility of developing El Niña conditions for the Feb-Mar-Apr season in progress. All things considered, during the Feb-Mar-Apr season there is very nearly a 60% probability for La Niña conditions, 40% probability for ENSO-neutral conditions, and a near-zero probability for El Niño conditions.

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 showing fair agreement in their ENSO forecasts through the 10-month forecast period. For the current Feb-Mar-Apr season, more than half of the models are predicting La Niña conditions, and the rest predict weakly below-average ENSO-neutral conditions. However, as early as Mar-Apr-May 2009, and beyond, the models favor ENSO-neutral conditions. For the Mar-Apr-May 2009 season, 8 of 21 models (38%) predict La Niña conditions and none predicts El Niño development. 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, only 1 of 16 (6%) indicates the existence of La Niña, while 5 of 16 (31%) indicate El Niño conditions for the Jun-Jul-Aug season; 10 of 16 (63%) 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 La Niña near 65% for Feb-Mar-Apr and 45% for Mar-Apr-May, and favoring ENSO-neutral as being consistently most likely from Mar-Apr-May onward. 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 59% probability for La Niña conditions in the Feb-Mar-Apr season in progress, declining to 48% for Mar-Apr-May, and to 25% by Jun-Jul-Aug and beyond.