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Technical ENSO Update

21 March 2013

Recent and Current Conditions

After a brief period of borderline El Niño SST conditions between July and September 2012, the SST anomaly in the Nino3.4 region returned to neutral levels during October and has remained neutral through mid-March 2013. For February 2013 the Nino3.4 SST anomaly was -0.40 C, indicative of cool-neutral ENSO conditions, and for December-February the anomaly was -0.31 C. Since late 2011, the IRI's definition of El Niño conditions has become the same as that of NOAA/Climate Prediction Center, in which the SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) exceeds 0.5 C. Similarly, for La Niña, the anomaly must be -0.5 C or less. The climatological probabilities for La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly SST anomaly in the NINO3.4 region was -0.3 C, indicating neutral ENSO conditions in the tropical Pacific; this is similar to the -0.40 C level observed in February.

Expected Conditions


What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it called for a high likelihood of neutral ENSO conditions enduring through the second quarter of 2013, with probabilities of El Niño or La Niña each less than 25% through northern summer 2013. The latest set of model ENSO predictions, from mid-February, is now available in the IRI/CPC ENSO prediction plume, discussed below. Currently, SSTs are in the cool half of the ENSO-neutral range (anomaly of 0 to -0.5 C), and SST is above average in the western part of the basin. Subsurface temperatures across the equatorial Pacific average close to the climatological average, but are above average in the western half of the basin and slightly below average in the east-central tropical Pacific. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), has been mainly near average but the low-level zonal winds have slightly favored enhanced trade winds. Anomalous convection (as measured by OLR) has generally been below average in the central tropical Pacific, and above average in the far western part of the basin. Together, although these last features have gradients similar to those found during weak or borderline La Niña, all of the the features collectively reflect only cool-leaning ENSO-neutral conditions.

As of mid-March, 21% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Mar-May 2013 season, none predicts El Niño conditions, and 79% indicates neutral ENSO. At lead times of 3 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. For the Jun-Aug season, among models that do use subsurface temperature information, 84% predicts ENSO-neutral SSTs, 11% predicts El Niño conditions and 5% predicts La Niña contitions. For all model types, the probability for neutral ENSO conditions is 65% or greater from Mar-May to the end of the forecast period in northern autumn 2013. (Note 1). Caution is advised in interpreting the distribution of model predictions 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 more quantitatively precise and less vulnerable to sampling errors than the categorical tallying method used above. This alternative method uses the mean of the predictions of all models on the plume, equally weighted, and constructs a standard error function centered on that mean. The standard error is Gaussian in shape, and has its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill results in a relatively narrower error distribution, while low skill results in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at 20% for Mar-May 2013, 27% for Apr-Jun, and also 27% for May-Jul 2013, remaining between 25% and 30% through northern autumn. Model probabilities for ENSO-neutral conditions are 80% for Mar-May 2013, 71% for Apr-Jun, and 64% for May-Jul 2013, decreasing to between 55% and 60% from Jun-Aug through autumn 2013. Probabilities for El Niño are near 0% for Mar-May 2013, 2% for Apr-Jun, rising to the 10-15% range from Jun-Aug through northern autumn. In words, the models collectively favor neutral ENSO conditions straight through to the fourth quarter of 2013; La Niña is slightly favored over El Niño during all of the period. A plot of the probabilities generated from this most recent IRI/CPC ENSO prediction plume using the multi-model mean and the Gaussian standard error method summarizes the model consensus out to about 10 months into the future. The same cautions mentioned above for the distributional count of model predictions apply to this Gaussian standard error 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 total range across the models, nor the ensemble range within individual models.

The probabilities derived from the 24 or more models on the IRI/CPC plume describe, on average, maintenance of neutral ENSO conditions during the coming months, continuing through into the latter half of 2013. Uncertainty increases greatly from around the Apr-Jun 2013 season onward, when the probablilities for neutral ENSO settle into the 50s and probabilities for non-neutral ENSO are approximately one-in-four for La Niña and one-in-six for El Niño. Following this latest model-based ENSO plume prediction, factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the next official outlook to be generated and issued in early January by CPC and IRI, which will include some human judgement in combination with the model guidance.

Using the 0.5 C thresholds, the climatological probabilities of La Nina, neutral, and El Nino conditions for each 3-month season are as follows:

Climatological Probabilities
SeasonLa NiñaNeutralEl Niño

See also: 

Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.

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